Controlling the SARS-CoV-2 spike glycoprotein conformation

Virus-like particles of retroviral origin in protein aggregation and neurodegenerative diseases

A wide range of human diseases are associated with protein misfolding and amyloid aggregates. Recent studies suggest that in certain neurological disorders, including Amyotrophic Lateral Sclerosis (ALS), Frontotemporal Dementia (FTD) and various tauopathies, protein aggregation may be promoted by virus-like particles (VLPs) formed by endogenous retroviruses (ERVs). The molecular mechanisms by which these VLPs contribute to protein aggregation, however, remain enigmatic. Here, we discuss possible molecular mechanisms of ERV-derived VLPs in the formation and spread of protein aggregates. An intriguing possibility is that liquid-like condensates may facilitate the formation of both protein aggregates and ERV-derived VLPs. We also describe how RNA chaperoning, and the encapsulation and trafficking of misfolded proteins, may contribute to protein homeostasis through the elimination of protein aggregates from cells. Based on these insights, we discuss future potential therapeutic opportunities.

Production and cryo-electron microscopy structure of an internally tagged SARS-CoV-2 spike ecto-domain construct

The SARS-CoV-2 spike protein is synthesized in the endoplasmic reticulum of host cells, from where it undergoes export to the Golgi and the plasma membrane or retrieval from the Golgi to the endoplasmic reticulum. Elucidating the fundamental principles of this bidirectional secretion are pivotal to understanding virus assembly and designing the next generation of spike genetic vaccine with enhanced export properties. However, the widely used strategy of C-terminal affinity tagging of the spike cytosolic tail interferes with proper bidirectional trafficking. Hence, the structural and biophysical investigations of spike protein trafficking have been hindered by a lack of appropriate spike constructs. Here we describe a strategy for the internal tagging of the spike protein. Using sequence analyses and AlphaFold modeling, we identified a site down-stream of the signal sequence for the insertion of a twin-strep-tag, which facilitates purification of an ecto-domain construct from the extra-cellular medium of mammalian Expi293F cells. Mass spectrometry analyses show that the internal tag has minimal impact on N-glycan modifications, which are pivotal for spike-host interactions. Single particle cryo-electron microscopy reconstructions of the spike ecto-domain reveal conformational states compatible for ACE2 receptor interactions, further solidifying the feasibility of the internal tagging strategy. Collectively, these results present a substantial advance towards reagent development for the investigations of spike protein trafficking during coronavirus infection and genetic vaccination.

Infectivity and structure of SARS-CoV-2 after hydrogen peroxide treatment

Hydrogen peroxide (H2O2) exhibits broad-spectrum antiviral activity and is commonly used as an over-the-counter disinfecting agent. However, its potential activities against SARS-CoV-2 have not been systematically evaluated, and mechanisms of action are not well understood. In this study, we investigate H2O2's antiviral activity against SARS-CoV-2 infection and its impact on the virion's structural integrity as compared to the commonly used fixative agent paraformaldehyde (PFA). We show that H2O2 rapidly and directly inactivates SARS-CoV-2 with a half-maximal inhibitory concentration (IC50) of 0.0015%. Cryogenic electron tomography (cryo-ET) with subtomogram averaging reveals that treatment with PFA induced the viral trimeric spike protein (S) to adopt a post-fusion conformation, and treatment of viral particles with H2O2 locked S in its pre-fusion conformation. Therefore, H2O2 treatment likely has induced modifications, such as oxidation of cysteine residues within the S subunits of the spike trimer that locked them in their pre-fusion conformation. Locking of the meta-stable pre-fusion trimer prevents its transition to the post-fusion conformation, a process essential for viral fusion with host cells and entry into host cells. Together, our cellular, biochemical, and structural studies established that hydrogen peroxide can inactivate SARS-CoV-2 in tissue culture and uncovered its underlying molecular mechanism.IMPORTANCEHydrogen peroxide (H2O2) is the commonly used, over-the-counter antiseptic solution available in pharmacies, but its effect against the SARS-CoV-2 virus has not been evaluated systematically. In this study, we show that H2O2 inactivates the SARS-CoV-2 infectivity and establish the effective concentration of this activity. Cryogenic electron tomography and sub-tomogram averaging reveal a detailed structural understanding of how H2O2 affects the SARS-CoV-2 spike in comparison with that of the commonly used fixative PFA under identical conditions. We found that PFA promoted a post-fusion conformation of the viral spike protein, while H2O2 could potentially lock the spike in its pre-fusion state. Our findings not only substantiate the disinfectant efficacy of H2O2 as a potent agent against SARS-CoV-2 but also lay the groundwork for future investigations into targeted antiviral therapies that may leverage the virus' structural susceptibilities. In addition, this study may have significant implications for developing new antiviral strategies and improving existing disinfection protocols.

Targeting the early life stages of SARS-CoV-2 using a multi-peptide conjugate vaccine

The spike glycoprotein is a key factor in the infection cycle of SARS-CoV-2, as it mediates both receptor recognition and membrane fusion by the virus. Therefore, in this study, we aimed to design a multi-peptide conjugate vaccine against SARS-CoV-2, targeting the early stages of the virus's life cycle. We used iBoost technology, which is designed to induce immune responses against low- or non-immunogenic epitopes. We selected six peptide sequences, each representing a key domain of the spike protein (i.e., receptor binding domain (RBM), subdomain 1 (SD1), subdomain 2 (SD2), S1/S2, fusion peptide and the S2' sequences (FP + S2'), heptad repeat 1 (HR1)). Immunization studies in mice displayed targeted humoral and cellular immune responses against specific peptides of the spike protein simultaneously, while inducing cross-protection against the Delta and Omicron coronavirus variants. Moreover, vaccinated hamsters challenged with SARS-CoV-2 elicited high antibody levels against key peptides, induced early neutralizing antibody responses and resulted in less weight loss compared to controls. This highlights the potential for improving viral control and disease outcomes when utilizing this strategy. Therefore, by using iBoost technology in conjunction with our peptide design strategy, we were able to successfully target non-immunodominant regions in the spike protein while activating both arms of the adaptive immune system.

Galectin-3 modulates cellular infectivity and inflammatory response mediated by spike protein of SARS-CoV2

We report that the recombinantly produced galectin-3 (Gal-3) not only reduces the infectivity of a pseudotyped lentivirus expressing SARS-CoV2-S protein i.e., LV(CoV2-S) in the susceptible cells but also dampens the inflammatory response of innate immune cells. Glycan moieties of the CoV2-S protein promote cellular infectivity of LV(CoV2-S). Exogenously added Gal-3, acting via its carbohydrate recognition domain (CRD), prevents LV(CoV2-S) infection of the susceptible cells. Accordingly, Gal-3 mediated LV(CoV2-S) neutralization is inhibited when Gal-3 is pre-incubated with either α-lactose or a single domain antibody specific to the CRD of Gal-3. BMDCs from Gal-3KO as compared to those from WT mice generate significantly higher cytokine response and the exogenously added Gal-3 reduces cytokine levels following stimulation with the derivates of CoV2-S protein. Therefore, modifying the interaction of Gal-3 and glycans of the viral CoV2-S protein might represent a strategy that reduces the infectivity of SARS-CoV2 and mitigates immunopathology caused by the virus infection.

Exploring Diverse Binding Mechanisms of Broadly Neutralizing Antibodies S309, S304, CYFN-1006 and VIR-7229 Targeting SARS-CoV-2 Spike Omicron Variants: Integrative Computational Modeling Reveals Balance of Evolutionary and Dynamic Adaptability in Shaping Molecular Determinants of Immune Escape

Evolution of SARS-CoV-2 has led to the emergence of variants with increased immune evasion capabilities, posing significant challenges to antibody-based therapeutics and vaccines. The cross-neutralization activity of antibodies against Omicron variants is governed by a complex and delicate interplay of multiple energetic factors and interaction contributions. In this study, we conducted a comprehensive analysis of the interactions between the receptor-binding domain (RBD) of the SARS-CoV-2 spike protein and four neutralizing antibodies S309, S304, CYFN1006, and VIR-7229. Using integrative computational modeling that combined all-atom molecular dynamics (MD) simulations, mutational scanning, and MM-GBSA binding free energy calculations, we elucidated the structural, energetic, and dynamic determinants of antibody binding. Our findings reveal distinct dynamic binding mechanisms and evolutionary adaptation driving broad neutralization effect of these antibodies. We show that S309 targets conserved residues near the ACE2 interface, leveraging synergistic van der Waals and electrostatic interactions, while S304 focuses on fewer but sensitive residues, making it more susceptible to escape mutations. The analysis of CYFN-1006.1 and CYFN-1006.2 antibody binding highlights broad epitope coverage with critical anchors at T345, K440, and T346, enhancing its efficacy against variants carrying the K356T mutation which caused escape from S309 binding. Our analysis of broadly potent VIR-7229 antibody binding to XBB.1.5 and EG.5 Omicron variants emphasized a large and structurally complex epitope, demonstrating certain adaptability and compensatory effects to F456L and L455S mutations. Mutational profiling identified key residues crucial for antibody binding, including T345, P337, and R346 for S309, and T385 and K386 for S304, underscoring their roles as evolutionary "weak spots" that balance viral fitness and immune evasion. The results of this energetic analysis demonstrate a good agreement between the predicted binding hotspots and critical mutations with respect to the latest experiments on average antibody escape scores. The results of this study dissect distinct energetic mechanisms of binding and importance of targeting conserved residues and diverse epitopes to counteract viral resistance. Broad-spectrum antibodies CYFN1006 and VIR-7229 maintain efficacy across multiple variants and achieve neutralization by targeting convergent evolution hotspots while enabling tolerance to mutations in these positions through structural adaptability and compensatory interactions at the binding interface. The results of this study underscore the diversity of binding mechanisms employed by different antibodies and molecular basis for high affinity and excellent neutralization activity of the latest generation of antibodies.

Development of the TSR-based computational method to investigate spike and monoclonal antibody interactions

Introduction

Monoclonal antibody (mAb) drug treatments have proven effective in reducing COVID-19-related hospitalizations or fatalities, particularly among high-risk patients. Numerous experimental studies have explored the structures of spike proteins and their complexes with ACE2 or mAbs. These 3D structures provide crucial insights into the interactions between spike proteins and ACE2 or mAb, forming a basis for the development of diagnostic tools and therapeutics. However, the field of computational biology has faced substantial challenges due to the lack of methods for precise protein structural comparisons and accurate prediction of molecular interactions. In our previous studies, we introduced the Triangular Spatial Relationship (TSR)-based algorithm, which represents a protein's 3D structure using a vector of integers (keys). These earlier studies, however, were limited to individual proteins.

Purpose

This study introduces new extensions of the TSR-based algorithm, enhancing its ability to study interactions between two molecules. We apply these extensions to gain a mechanistic understanding of spike - mAb interactions.

Method

We expanded the basic TSR method in three novel ways: (1) TSR keys encompassing all atoms, (2) cross keys for interactions between two molecules, and (3) intra-residual keys for amino acids. This TSR-based representation of 3D structures offers a unique advantage by simplifying the search for similar substructures within structural datasets.

Results

The study's key findings include: (i) The method effectively quantified and interpreted conformational changes and steric effects using the newly introduced TSR keys. (ii) Six clusters for CDRH3 and three clusters for CDRL3 were identified using all-atom keys. (iii) We constructed the TSR-STRSUM (TSR-STRucture SUbstitution Matrix), a matrix that represents pairwise similarities between amino acid structures, providing valuable applications in protein sequence and structure comparison. (iv) Intra-residual keys revealed two distinct Tyr clusters characterized by specific triangle geometries.

Conclusion

This study presents an advanced computational approach that not only quantifies and interprets conformational changes in protein backbones, entire structures, or individual amino acids, but also facilitates the search for substructures induced by molecular binding across protein datasets. In some instances, a direct correlation between structures and functions was successfully established.

Stabilizing Prefusion SARS-CoV-2 Spike by Destabilizing the Postfusion Conformation

Background/Objectives: As with many viral fusion proteins, the native conformation of SARS-CoV-2 Spike is metastable. Most COVID-19 vaccines utilize a stabilized Spike (Spike-2P) containing two proline substitutions, and subsequently, a further stabilized variant with four additional proline substitutions, Spike-6P, has been developed. In an alternative approach, we introduced two aspartic acid residues (2D) in the HR1 region of Spike at positions that are exposed and buried in the pre- and postfusion states, respectively, to destabilize the postfusion conformation. Methods: The recombinant protein constructs were expressed in a mammalian cell culture and characterized for their yield and antigenicity, and the formulations were then used to immunize hamsters. After two immunizations, the hamsters were challenged with live B.1.351 SARS-CoV-2 virus for an evaluation of the protective efficacy. Results: The introduction of the two aspartic acid mutations resulted in an approximately six-fold increase in expression, comparable to that in Spike-2P. When the 2D mutations were combined with the above four proline mutations (Spike-4P-2D), this led to a further three- to four-fold enhancement of protein expression, similar to that seen in Spike-6P. When formulated with the oil-in-water emulsion adjuvant Sepivac SWE, the 2P, 2D, 6P, and 4P-2D Spike variants all protected female hamsters against heterologous challenge with the B.1.351 SARS-CoV-2 virus and elicited high titers of neutralizing antibodies. Conclusions: We suggest that destabilization of the postfusion conformation through the introduction of charged amino acids at sites that are exposed in the pre- and buried in the postfusion conformation offers a general strategy to enhance the yield and stability of the native, prefusion conformation of viral surface proteins.

Allosteric Control and Glycan Shielding Adaptations in the SARS-CoV-2 Spike from Early to Peak Virulence

The SARS-CoV-2 Spike glycoprotein is central to viral infectivity and immune evasion, making it a key target for vaccine and therapeutic design. This trimeric peplomer undergoes dynamic conformational changes, particularly in its Receptor Binding Domain (RBD), which transitions between closed (down) and ACE2-accessible (up) states relative to the rest of the protein, to facilitate host cell entry. Structural understanding of such critical inter-domain motions, as well as epitope exposure quantification, is essential for obtaining an effective molecular handle over this protein and, in turn, exploiting it towards improved immunogen development. Focusing on the early circulating D614G form and the later emerging Delta (B.1.617.2) variant with higher virulence, we performed large-scale molecular dynamics simulations of the soluble form of the Spike in both 'down' and 'up' conformations of the RBD. Guided by differences in overall fluctuations, we described reaction coordinates based on domain rotations and tilting to extract features that distinguish D614G versus Delta structural behavior of the N-terminal Domain (NTD) and RBD. Using reaction coordinate analysis and Principal Component Analysis (PCA), we identify allosteric coupling between the N-terminal Domain (NTD) and RBD, where NTD tilting influences RBD gating. While some of these motions are conserved across variants, Delta exhibits an optimized RBD-gating mechanism that enhances ACE2 accessibility. Additionally, glycan remodeling in Delta enhances shielding at the NTD supersite, contributing to reduced sensitivity to neutralizing antibodies. Finally, we uncover the impact of the D950N mutation in the HR1 region, which modulates downstream Spike dynamics and immune evasion. Together, our findings reveal variant-specific and conserved structural determinants of SARS-CoV-2 Spike function, providing a mechanistic basis for allosteric modulation, glycan-mediated immune evasion, and viral adaptation. These insights offer valuable guidance for rational vaccine and therapeutic design against SARS-CoV-2 and emerging variants.

Conformational and Stability Analysis of SARS-CoV-2 Spike Protein Variants by Molecular Simulation

We performed a comprehensive structural analysis of the conformational space of several spike (S) protein variants using molecular dynamics (MD) simulations. Specifically, we examined four well-known variants (Delta, BA.1, XBB.1.5, and JN.1) alongside the wild-type (WT) form of SARS-CoV-2. The conformational states of each variant were characterized by analyzing their distributions within a selected space of collective variables (CVs), such as inter-domain distances between the receptor-binding domain (RBD) and the N-terminal domain (NTD). Our primary focus was to identify conformational states relevant to potential structural transitions and to determine the set of native contacts (NCs) that stabilize these conformations. The results reveal that genetically more distant variants, such as XBB.1.5, BA.1, and JN.1, tend to adopt more compact conformational states compared to the WT. Additionally, these variants exhibit novel NC profiles, characterized by an increased number of specific contacts distributed among ionic, polar, and nonpolar residues. We further analyzed the impact of specific mutations, including T478K, N500Y, and Y504H. These mutations not only enhance interactions with the human host receptor but also alter inter-chain stability by introducing additional NCs compared to the WT. Consequently, these mutations may influence the accessibility of certain protein regions to neutralizing antibodies. Overall, these findings contribute to a deeper understanding of the structural and functional variations among S protein variants.

Exploring the effects of N234 and N343 linked glycans to SARS CoV 2 spike protein pocket accessibility using Gaussian accelerated molecular dynamics simulations

The N234 and N343-linked glycans of the SARS-CoV 2 spike protein are known to stabilize the up-conformation of its receptor-binding domains (RBDs), enabling human angiotensin enzyme 2 (hACE2) receptor binding. However, the effect of spike-hACE2 binding on these important glycans remains poorly understood, and these changes could have implications in the development of drugs that inhibit viral entry. In this study, Gaussian accelerated molecular dynamics (GaMD) simulations of the hACE2-free and hACE2-bound spike protein are performed. Biophysical analyses were focused on the accessibility of three previously suggested druggable pockets underneath the three RBD subunits. A shielding effect by N234-linked glycans on the components of their adjacent pockets was observed. Although deshielding of central scaffold residues was observed in the hACE2-bound state, pocket A's accessibility was reduced due to an increase in NTDB-RBDB contacts, restricting entry into the pocket. For pocket B, changes in N234C and N343C expose the central scaffold residues in the bound state, increasing accessibility. In Pocket C, increased shielding due to N234A was found in the bound state, reducing accessibility. Despite these changes, the pockets remain accessible to ligands in both states and are still valid targets for drug development studies.

The effect of cysteine oxidation on conformational changes of SARS-CoV-2 spike protein using atomistic simulations

The SARS-CoV-2 Spike (S) protein plays a central role in viral entry into host cells, making it a key target for therapeutic interventions. Oxidative stress, often triggered during viral infections, can cause oxidation of cysteine in this protein. Here we investigate the impact of cysteine oxidation, specifically the formation of cysteic acid, on the conformational dynamics of the SARS-CoV-2 S protein using atomistic simulations. In particular, we examine how cysteine oxidation influences the transitions of the S protein's receptor-binding domain (RBD) between "down" (inaccessible) and "up" (accessible) states, which are critical for host cell receptor engagement. Using solvent-accessible surface area (SASA) analysis, we identify key cysteine residues susceptible to oxidation. The results of targeted molecular dynamics (TMD) and umbrella sampling (US) simulations reveal that oxidation reduces the energy barrier for RBD transitions by approximately 30 kJ mol-1, facilitating conformational changes and potentially enhancing viral infectivity. Furthermore, we analyze the interactions between oxidized cysteine residues and glycans, as well as alterations in hydrogen bonds and salt bridges. Our results show that oxidation disrupts normal RBD dynamics, influencing the energy landscape of conformational transitions. Our work provides novel insights into the role of cysteine oxidation in modulating the structural dynamics of the SARS-CoV-2 S protein, highlighting potential targets for antiviral strategies aimed at reducing oxidative stress or modifying post-translational changes. These findings contribute to a deeper understanding of viral infectivity and pathogenesis under oxidative conditions.

Mutational Scanning and Binding Free Energy Computations of the SARS-CoV-2 Spike Complexes with Distinct Groups of Neutralizing Antibodies: Energetic Drivers of Convergent Evolution of Binding Affinity and Immune Escape Hotspots

The rapid evolution of SARS-CoV-2 has led to the emergence of variants with increased immune evasion capabilities, posing significant challenges to antibody-based therapeutics and vaccines. In this study, we conducted a comprehensive structural and energetic analysis of SARS-CoV-2 spike receptor-binding domain (RBD) complexes with neutralizing antibodies from four distinct groups (A-D), including group A LY-CoV016, group B AZD8895 and REGN10933, group C LY-CoV555, and group D antibodies AZD1061, REGN10987, and LY-CoV1404. Using coarse-grained simplified simulation models, rapid energy-based mutational scanning, and rigorous MM-GBSA binding free energy calculations, we elucidated the molecular mechanisms of antibody binding and escape mechanisms, identified key binding hotspots, and explored the evolutionary strategies employed by the virus to evade neutralization. The residue-based decomposition analysis revealed energetic mechanisms and thermodynamic factors underlying the effect of mutations on antibody binding. The results demonstrate excellent qualitative agreement between the predicted binding hotspots and the latest experiments on antibody escape. These findings provide valuable insights into the molecular determinants of antibody binding and viral escape, highlighting the importance of targeting conserved epitopes and leveraging combination therapies to mitigate the risk of immune evasion.

Quantitative Characterization and Prediction of the Binding Determinants and Immune Escape Hotspots for Groups of Broadly Neutralizing Antibodies Against Omicron Variants: Atomistic Modeling of the SARS-CoV-2 Spike Complexes with Antibodies

A growing body of experimental and computational studies suggests that the cross-neutralization antibody activity against Omicron variants may be driven by the balance and tradeoff between multiple energetic factors and interaction contributions of the evolving escape hotspots involved in antigenic drift and convergent evolution. However, the dynamic and energetic details quantifying the balance and contribution of these factors, particularly the balancing nature of specific interactions formed by antibodies with epitope residues, remain largely uncharacterized. In this study, we performed molecular dynamics simulations, an ensemble-based deep mutational scanning of SARS-CoV-2 spike residues, and binding free energy computations for two distinct groups of broadly neutralizing antibodies: the E1 group (BD55-3152, BD55-3546, and BD5-5840) and the F3 group (BD55-3372, BD55-4637, and BD55-5514). Using these approaches, we examined the energetic determinants by which broadly potent antibodies can largely evade immune resistance. Our analysis revealed the emergence of a small number of immune escape positions for E1 group antibodies that correspond to the R346 and K444 positions in which the strong van der Waals and interactions act synchronously, leading to the large binding contribution. According to our results, the E1 and F3 groups of Abs effectively exploit binding hotspot clusters of hydrophobic sites that are critical for spike functions along with the selective complementary targeting of positively charged sites that are important for ACE2 binding. Together with targeting conserved epitopes, these groups of antibodies can lead expand the breadth and resilience of neutralization to the antigenic shifts associated with viral evolution. The results of this study and the energetic analysis demonstrate excellent qualitative agreement between the predicted binding hotspots and critical mutations with respect to the latest experiments on average antibody escape scores. We argue that the E1 and F3 groups of antibodies targeting binding epitopes may leverage strong hydrophobic interactions with the binding epitope hotspots that are critical for the spike stability and ACE2 binding, while escape mutations tend to emerge in sites associated with synergistically strong hydrophobic and electrostatic interactions.

Engineering Protein Dynamics through Mutational Energy Landscape Traps

Protein dynamics is essential for various biological processes, influencing functions such as enzyme activity, molecular recognition, and signal transduction. However, traditional protein engineering methods often focus on static structures, lacking tools to precisely manipulate dynamic behaviors. Here, we developed Mutational Energy Landscape Trap (MELT), a novel method designed to control protein dynamics by combining Normal Mode Analysis (NMA) and in silico mutagenesis. MELT works by displacing protein structures along low-frequency normal modes and introducing mutations to either lock proteins in these conformations or increase dynamics along the chosen normal modes. We tested MELT using hen-egg lysozyme as a model system. The method was validated by monitoring relevant collective coordinates during molecular dynamics simulations and evaluation of the collective movements of each construct. Our experiments showed that MELT was able to consistently create new protein sequences with the desired dynamical behavior in simulations. It demonstrates its potential for applications in the field of protein engineering, being an unprecedented way of manipulating protein features.

SARS-CoV-2 vaccine based on ferritin complexes with screened immunogenic sequences from the Fv-antibody library

In this study, the vaccine against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was developed using ferritin complexes with the immunogenic sequences screened against the SARS-CoV-2 spike protein (SP) from the Fv-antibody library. The Fv-antibody library was prepared on the outer membrane of E. coli by the expression of the VH region of immunoglobulin G (IgG) with a randomized complementarity-determining region 3 (CDR3). Four Fv-antibodies to the receptor-binding domain (RBD) were screened from the Fv-antibody library, which had a comparable binding constant (KD) between SARS-CoV-2 SP and the angiotensin-converting enzyme 2 (ACE2) receptor. The binding sites of screened Fv-antibodies on the RBD were analyzed using a docking analysis, and these binding sites were used as immunogenic sequences for the vaccine. The four immunogenic sequences were modified and co-expressed as a part of ferritin which was assembled into a ferritin complex. After the vaccination of ferritin complexes to mice, the anti-sera were analyzed to have a high enough titer. Additionally, the immune responses were found to be activated by vaccination, such as the expression of IgG subclasses and the increased level of cytokines. The neutralizing activity of the anti-sera was estimated using a cell-based infection assay based on pseudo-virus expressing the SP of SARS-CoV-2 variants.

Quantitative Characterization and Prediction of the Binding Determinants and Immune Escape Hotspots for Groups of Broadly Neutralizing Antibodies Against Omicron Variants: Atomistic Modeling of the SARS-CoV-2 Spike Complexes with Antibodies

The growing body of experimental and computational studies suggested that the cross-neutralization antibody activity against Omicron variants may be driven by balance and tradeoff of multiple energetic factors and interaction contributions of the evolving escape hotspots involved in antigenic drift and convergent evolution. However, the dynamic and energetic details quantifying the balance and contribution of these factors, particularly the balancing nature of specific interactions formed by antibodies with the epitope residues remain scarcely characterized. In this study, we performed molecular dynamics simulations, ensemble-based deep mutational scanning of SARS-CoV-2 spike residues and binding free energy computations for two distinct groups of broadly neutralizing antibodies : E1 group (BD55-3152, BD55-3546 and BD5-5840) and F3 group (BD55-3372, BD55-4637 and BD55-5514). Using these approaches, we examine the energetic determinants by which broadly potent antibodies can largely evade immune resistance. Our analysis revealed the emergence of a small number of immune escape positions for E1 group antibodies that correspond to R346 and K444 positions in which the strong van der Waals and interactions act synchronously leading to the large binding contribution. According to our results, E1 and F3 groups of Abs effectively exploit binding hotspot clusters of hydrophobic sites critical for spike functions along with selective complementary targeting of positively charged sites that are important for ACE2 binding. Together with targeting conserved epitopes, these groups of antibodies can lead to the expanded neutralization breadth and resilience to antigenic shift associated with viral evolution. The results of this study and the energetic analysis demonstrate excellent qualitative agreement between the predicted binding hotspots and critical mutations with respect to the latest experiments on average antibody escape scores. We argue that E1 and F3 groups of antibodies targeting binding epitopes may leverage strong hydrophobic interactions with the binding epitope hotspots critical for the spike stability and ACE2 binding, while escape mutations tend to emerge in sites associated with synergistically strong hydrophobic and electrostatic interactions.

Harnessing traditional Chinese medicine polysaccharides for combatting COVID-19

With the global spread of COVID-19 posing ongoing challenges to public health systems, there is an ever-increasing demand for effective therapeutics that can mitigate both viral transmission and disease severity. This review surveys the landscape of polysaccharides derived from traditional Chinese medicine, acclaimed for their medicinal properties and potential to contribute to the COVID-19 response. We specifically focus on the capability of these polysaccharides to thwart SARS-CoV-2 entry into host cells, a pivotal step in the viral life cycle that informs transmission and pathogenicity. Moreover, we delve into the concept of trained immunity, an innate immune system feature that polysaccharides may potentiate, offering an avenue for a more moderated yet efficacious immune response against various pathogens, including SARS-CoV-2. Our comprehensive overview aims to bolster understanding of the possible integration of these substances within anti-COVID-19 measures, emphasizing the need for rigorous investigation into their potential applications and underlying mechanisms. The insights provided here strongly support ongoing investigations into the adjunctive use of polysaccharides in the management of COVID-19, with the anticipation that such findings could lead to a deeper appreciation and clearer elucidation of the antiviral potentials inherent in complex Chinese herbal remedies.

Interfacial subregions of SARS-CoV-2 spike RBD to hACE2 affect intermolecular affinity by their distinct roles played in association and dissociation kinetics

SARS-CoV-2's rapid global transmission depends on spike RBD's strong affinity to hACE2. In the context of binding hot spots well defined, the work investigated how interfacial subregions of SARS-CoV-2 spike RBD to hACE2 affect intermolecular affinity and their potential distinct roles involved in association and dissociation kinetics due to their local structural characteristics. Three spatially consecutive subregions of SARS-CoV-2 RBD were structurally partitioned across RBD's receptor binding motif (RBM). Their impacts on binding affinity and kinetics were differentiated through a comprehensive SPR measurement of hACE2 binding by chimeric swap mutants of respective subdomains from SARS-CoV-2 VOCs & phylogenetically close sarbecoviruses, and further compared with those of included single mutations across RBM and around the RBD core. The data supports that the intermediate interfacial subregion of RBD involving key residue at 417 is the rate-limiting effector of association kinetics and the subregion encompassing residues at 501/498/449 is the key binding energy contributor dictating dissociation kinetics, both of which relate to SARS-CoV-2's adaptive mutational evolution and host tropism closely. The kinetic data and structural analysis of local mutations' impact on spike RBD's binding and thermal stability provide a new perspective in evaluating SARS-CoV-2 evolution and other sarbecoviruses' evolvable binding to hACE2. The inherent binding mode offers direct clues of valid epitope in designing new antibodies that the coronavirus can't elude.

Virion morphology and on-virus spike protein structures of diverse SARS-CoV-2 variants

The evolution of SARS-CoV-2 variants with increased fitness has been accompanied by structural changes in the spike (S) proteins, which are the major target for the adaptive immune response. Single-particle cryo-EM analysis of soluble S protein from SARS-CoV-2 variants has revealed this structural adaptation at high resolution. The analysis of S trimers in situ on intact virions has the potential to provide more functionally relevant insights into S structure and virion morphology. Here, we characterized B.1, Alpha, Beta, Gamma, Delta, Kappa, and Mu variants by cryo-electron microscopy and tomography, assessing S cleavage, virion morphology, S incorporation, "in-situ" high-resolution S structures, and the range of S conformational states. We found no evidence for adaptive changes in virion morphology, but describe multiple different positions in the S protein where amino acid changes alter local protein structure. Taken together, our data are consistent with a model where amino acid changes at multiple positions from the top to the base of the spike cause structural changes that can modulate the conformational dynamics of the S protein.

Dynamic expedition of leading mutations in SARS-CoV-2 spike glycoproteins

The continuous evolution of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which caused the recent pandemic, has generated countless new variants with varying fitness. Mutations of the spike glycoprotein play a particularly vital role in shaping its evolutionary trajectory, as they have the capability to alter its infectivity and antigenicity. We present a time-resolved statistical method, Dynamic Expedition of Leading Mutations (deLemus), to analyze the evolutionary dynamics of the SARS-CoV-2 spike glycoprotein. The proposed L -index of the deLemus method is effective in quantifying the mutation strength of each amino acid site and outlining evolutionarily significant sites, allowing the comprehensive characterization of the evolutionary mutation pattern of the spike glycoprotein.

Discovery of broad-spectrum high-affinity peptide ligands of spike protein for the vaccine purification of SARS-CoV-2 and Omicron variants

To combat with emerging SARS-CoV-2 variants of concern (VOCs), we report the identification of a set of unique HWK-motif peptide ligands for the receptor-binding domain (RBD) of the SARS-CoV-2 spike (S) protein from a phage-displayed peptide library. These HWK-motif peptides exhibited nanomolar affinity for RBD. Among them, the peptide, HWKAVNWLKPWT (SP-HWK), had not only the highest affinities for RBD and trimer S protein, but also broad-spectrum affinities for RBDs from VOCs. Molecular dynamics simulations and competitive ELISA revealed a conserved pocket between the cryptic and the outer faces of RBD for SP-HWK binding, distinct from the human angiotensin-converting enzyme 2 receptor binding site. By coupling SP-HWK to agarose gel, the as-prepared affinity gel could efficiently capture RBD and trimer S from the ancestral strain and the Omicron variant, and the bound targets could be recovered by mild elution at pH 6.0. More importantly, the affinity gel presented excellent and stable chromatographic performance in the purification of inactivated SARS-CoV-2 and Omicron vaccines, affording high yields and purities, and strong HCP reduction. The results demonstrated the potential of SP-HWK as a broad-spectrum peptide ligand for developing a universal platform for the vaccine purification of SARS-CoV-2 and VOCs.

Preclinical characterization of the Omicron XBB.1.5-adapted BNT162b2 COVID-19 vaccine

As SARS-CoV-2 evolves, increasing in potential for greater transmissibility and immune escape, updated vaccines are needed to boost adaptive immunity to protect against COVID-19 caused by circulating strains. Here, we report features of the monovalent Omicron XBB.1.5-adapted BNT162b2 vaccine, which contains XBB.1.5-specific sequence changes, relative to the original BNT162b2 backbone, in the encoded prefusion-stabilized SARS-CoV-2 spike protein (S(P2)). Biophysical characterization of Omicron XBB.1.5 S(P2) demonstrated that it maintains a prefusion conformation and adopts a flexible, predominantly open, state, with high affinity for the human ACE-2 receptor. When administered as a 4th dose in BNT162b2-experienced mice, the monovalent Omicron XBB.1.5 vaccine elicited substantially higher serum neutralizing titers against pseudotyped viruses of Omicron XBB.1.5, XBB.1.16, XBB.1.16.1, XBB.2.3, EG.5.1 and HV.1 sublineages and phylogenetically distant BA.2.86 lineage than the bivalent Wild Type + Omicron BA.4/5 vaccine. Similar trends were observed against Omicron XBB sublineage pseudoviruses when the vaccine was administered as a 2-dose series in naive mice. Strong S-specific Th1 CD4+ and IFNγ+ CD8+ T cell responses were also observed. These findings, together with real world performance of the XBB.1.5-adapted vaccine, suggest that preclinical data for the monovalent Omicron XBB.1.5-adapted BNT162b2 was predictive of protective immunity against dominant SARS-CoV-2 strains.

A novel neutralizing monoclonal antibody recognizes a linear antigenic epitope of the spike protein of swine acute diarrhoea syndrome coronavirus

Swine acute diarrhoea syndrome coronavirus (SADS-CoV) causes vomiting, severe diarrhoea and death in newborn piglets. The spike (S) protein plays a crucial role in promoting virus invasion and inducing neutralizing antibody production. In this study, the extracellular region of the S protein was used as an immunogen to immunize BALB/c mice. After immunization, B cells were collected, fused with SP2/0 myeloma cells, cultured and subcloned, and a cell line capable of secreting neutralizing antibodies was obtained and named as 5D6. Additionally, it was determined that the 5D6 mAb could be used as the primary antibody for western blotting and indirect immunofluorescence assay (IFA) to detect SADS-CoV. Further studies indicated that the 5D6 mAb binds to the 136STSHAAD142 motif, which located in the N-terminal domain (NTD) of the spike protein. This result suggested that the NTD of the S protein can induce the production of neutralizing antibodies. Amino acid sequence alignment revealed that the epitope of the 5D6 mAb was conserved among SADS-CoV strains. This study helps elucidate the S protein function of SADS-CoV, and the 5D6 mAb may be used to develop diagnostic and treatment tools for detecting SADS-CoV infection.

SARS-CoV-2 S protein harbors furin cleavage site located in a short loop between antiparallel β-strand

Furin cleavage site (FCS) of the SARS-CoV-2 S protein, which connects the S1/S2 junction, is essential for facilitating fusion with the host cells. Wild-type (Wt) SARS-CoV-2 S protein, PDB ID: 6yvb, lacks a sequence of amino acid residues, including the FCS that links the S1/S2 junction. For the first time, we demonstrated that a stretch of 14 amino acid residues (677QTNSPRRARSVASQ689) forms an antiparallel β-sheet comprising of PRRAR sequence in the FCS within a short loop. Upon comparing the loop content of the S1/S2 junction with that of Wt SARS-CoV-2 containing PRRAR in the FCS, we observed a decrease in antiparallel β-sheet content and an increase in loop content in the B.1.1.7 variant with HRRAR in the FCS. This short loop within antiparallel β-sheet can serve as a docking site for various proteases, including TMPRSS2 and α1AT. We performed a 300-ns simulation of the SARS-CoV-2 receptor binding domain (RBD) using several antibacterial and antiviral ligands commonly used to treat various infections. Our findings indicate that the receptor binding domain (RBD) comprising the receptor binding motif (RBM) utilizes β6 and a significant portion of the loop to bind with ligands, suggesting its potential for treating SARS-CoV-2 infections.

Evaluation of an Affinity-Enhanced Anti-SARS-CoV2 Nanobody Design Workflow Using Machine Learning and Molecular Dynamics

In silico optimization of protein binding has received a great deal of attention in the recent years. Since in silico prefiltering of strong binders is fast and cheap compared to in vitro library screening methods, the advent of powerful hardware and advanced machine learning algorithms has made this strategy more accessible and preferred. These advances have already impacted the global response to pandemic threats. In this study, we proposed and tested a workflow for designing nanobodies targeting the SARS-CoV-2 spike protein receptor binding domain (S-RBD) using machine learning techniques complemented by molecular dynamics simulations. We evaluated the feasibility of this workflow using a test set of 3 different nanobodies and 2 different S-RBD variants, from in silico design and bacterial expression to binding assays of the designed nanobody mutants. We successfully designed nanobodies that were subsequently tested against both the wild-type (Wuhan type) and the delta variant S-RBD and found 2 of them to be stronger binders compared to the wild-type nanobody. We use this case study to describe both the strengths and weaknesses of this in silico assisted nanobody design strategy.

Ultrapotent class I neutralizing antibodies post Omicron breakthrough infection overcome broad SARS-CoV-2 escape variants

BACKGROUND

The spread of emerging SARS-CoV-2 immune escape sublineages, especially JN.1 and KP.2, has resulted in new waves of COVID-19 globally. The evolving memory B cell responses elicited by the parental Omicron variants to subvariants with substantial antigenic drift remain incompletely investigated.

METHODS

Using the single B cell antibody cloning technology, we isolated single memory B cells, delineated the B cell receptor repertoire and conducted the pseudovirus-based assay for recovered neutralizing antibodies (NAb) screening. We analyzed the cryo-EM structures of top broadly NAbs (bnAbs) and evaluated their in vivo efficacy (golden Syrian hamster model).

FINDINGS

By investigating the evolution of human B cell immunity, we discovered a new panel of bnAbs arising from vaccinees after Omicron BA.2/BA.5 breakthrough infections. Two lead bnAbs neutralized major Omicron subvariants including JN.1 and KP.2 with IC50 values less than 10 ng/mL, representing ultrapotent receptor binding domain (RBD)-specific class I bnAbs. They belonged to the IGHV3-53/3-66 clonotypes instead of evolving from the pre-existing vaccine-induced IGHV1-58/IGKV3-20 bnAb ZCB11. Despite sequence diversity, they targeted previously unrecognized, highly conserved conformational epitopes in the receptor binding motif (RBM) for ultrapotent ACE2 blockade. The lead bnAb ZCP3B4 not only protected the lungs of hamsters intranasally challenged with BA.5.2, BQ.1.1 and XBB.1.5 but also prevented their contact transmission.

INTERPRETATION

Our findings demonstrated that class I bnAbs have evolved an ultrapotent mode of action protecting against highly transmissible and broad Omicron escape variants, and their epitopes are potential targets for novel bnAbs and vaccine development.

FUNDING

A full list of funding bodies that contributed to this study can be found in the Acknowledgements section.

Delineating the functional activity of antibodies with cross-reactivity to SARS-CoV-2, SARS-CoV-1 and related sarbecoviruses

The recurring spillover of pathogenic coronaviruses and demonstrated capacity of sarbecoviruses, such SARS-CoV-2, to rapidly evolve in humans underscores the need to better understand immune responses to this virus family. For this purpose, we characterized the functional breadth and potency of antibodies targeting the receptor binding domain (RBD) of the spike glycoprotein that exhibited cross-reactivity against SARS-CoV-2 variants, SARS-CoV-1 and sarbecoviruses from diverse clades and animal origins with spillover potential. One neutralizing antibody, C68.61, showed remarkable neutralization breadth against both SARS-CoV-2 variants and viruses from different sarbecovirus clades. C68.61, which targets a conserved RBD class 5 epitope, did not select for escape variants of SARS-CoV-2 or SARS-CoV-1 in culture nor have predicted escape variants among circulating SARS-CoV-2 strains, suggesting this epitope is functionally constrained. We identified 11 additional SARS-CoV-2/SARS-CoV-1 cross-reactive antibodies that target the more sequence conserved class 4 and class 5 epitopes within RBD that show activity against a subset of diverse sarbecoviruses with one antibody binding every single sarbecovirus RBD tested. A subset of these antibodies exhibited Fc-mediated effector functions as potent as antibodies that impact infection outcome in animal models. Thus, our study identified antibodies targeting conserved regions across SARS-CoV-2 variants and sarbecoviruses that may serve as therapeutics for pandemic preparedness as well as blueprints for the design of immunogens capable of eliciting cross-neutralizing responses.

AlphaFold2 Modeling and Molecular Dynamics Simulations of the Conformational Ensembles for the SARS-CoV-2 Spike Omicron JN.1, KP.2 and KP.3 Variants: Mutational Profiling of Binding Energetics Reveals Epistatic Drivers of the ACE2 Affinity and Escape Hotspots of Antibody Resistance

The most recent wave of SARS-CoV-2 Omicron variants descending from BA.2 and BA.2.86 exhibited improved viral growth and fitness due to convergent evolution of functional hotspots. These hotspots operate in tandem to optimize both receptor binding for effective infection and immune evasion efficiency, thereby maintaining overall viral fitness. The lack of molecular details on structure, dynamics and binding energetics of the latest FLiRT and FLuQE variants with the ACE2 receptor and antibodies provides a considerable challenge that is explored in this study. We combined AlphaFold2-based atomistic predictions of structures and conformational ensembles of the SARS-CoV-2 spike complexes with the host receptor ACE2 for the most dominant Omicron variants JN.1, KP.1, KP.2 and KP.3 to examine the mechanisms underlying the role of convergent evolution hotspots in balancing ACE2 binding and antibody evasion. Using the ensemble-based mutational scanning of the spike protein residues and computations of binding affinities, we identified binding energy hotspots and characterized the molecular basis underlying epistatic couplings between convergent mutational hotspots. The results suggested the existence of epistatic interactions between convergent mutational sites at L455, F456, Q493 positions that protect and restore ACE2-binding affinity while conferring beneficial immune escape. To examine immune escape mechanisms, we performed structure-based mutational profiling of the spike protein binding with several classes of antibodies that displayed impaired neutralization against BA.2.86, JN.1, KP.2 and KP.3. The results confirmed the experimental data that JN.1, KP.2 and KP.3 harboring the L455S and F456L mutations can significantly impair the neutralizing activity of class 1 monoclonal antibodies, while the epistatic effects mediated by F456L can facilitate the subsequent convergence of Q493E changes to rescue ACE2 binding. Structural and energetic analysis provided a rationale to the experimental results showing that BD55-5840 and BD55-5514 antibodies that bind to different binding epitopes can retain neutralizing efficacy against all examined variants BA.2.86, JN.1, KP.2 and KP.3. The results support the notion that evolution of Omicron variants may favor emergence of lineages with beneficial combinations of mutations involving mediators of epistatic couplings that control balance of high ACE2 affinity and immune evasion.

CryoTRANS: predicting high-resolution maps of rare conformations from self-supervised trajectories in cryo-EM

Cryogenic electron microscopy (cryo-EM) has revolutionized structural biology, enabling efficient determination of structures at near-atomic resolutions. However, a common challenge arises from the severe imbalance among various conformations of vitrified particles, leading to low-resolution reconstructions in rare conformations due to a lack of particle images in these quasi-stable states. We introduce CryoTRANS, a method that predicts high-resolution maps of rare conformations by constructing a self-supervised pseudo-trajectory between density maps of varying resolutions. This trajectory is represented by an ordinary differential equation parameterized by a deep neural network, ensuring retention of detailed structures from high-resolution density maps. By leveraging a single high-resolution density map, CryoTRANS significantly improves the reconstruction of rare conformations and has been validated on four real-world datasets: alpha-2-macroglobulin, actin-binding protein complexes, SARS-CoV-2 spike glycoprotein, and the 70S ribosome. CryoTRANS can also predict high-resolution structures in cryogenic electron tomography maps using a high-resolution cryo-EM map.

Hybrid immunity to SARS-CoV-2 arises from serological recall of IgG antibodies distinctly imprinted by infection or vaccination

We describe the molecular-level composition of polyclonal immunoglobulin G (IgG) anti-spike antibodies from ancestral severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, vaccination, or their combination ("hybrid immunity") at monoclonal resolution. Infection primarily triggers S2/N-terminal domain (NTD)-reactive antibodies, whereas vaccination mainly induces anti-receptor-binding domain (RBD) antibodies. This imprint persists after secondary exposures wherein >60% of ensuing hybrid immunity derives from the original IgG pool. Monoclonal constituents of the original IgG pool can increase breadth, affinity, and prevalence upon secondary exposures, as exemplified by the plasma antibody SC27. Following a breakthrough infection, vaccine-induced SC27 gained neutralization breadth and potency against SARS-CoV-2 variants and zoonotic viruses (half-maximal inhibitory concentration [IC50] ∼0.1-1.75 nM) and increased its binding affinity to the protective RBD class 1/4 epitope (dissociation constant [KD] < 5 pM). According to polyclonal escape analysis, SC27-like binding patterns are common in SARS-CoV-2 hybrid immunity. Our findings provide a detailed molecular definition of immunological imprinting and show that vaccination can produce class 1/4 (SC27-like) IgG antibodies circulating in the blood.

Temperature-dependent Spike-ACE2 interaction of Omicron subvariants is associated with viral transmission

The continued evolution of severe acute respiratory syndrome 2 (SARS-CoV-2) requires persistent monitoring of its subvariants. Omicron subvariants are responsible for the vast majority of SARS-CoV-2 infections worldwide, with XBB and BA.2.86 sublineages representing more than 90% of circulating strains as of January 2024. To better understand parameters involved in viral transmission, we characterized the functional properties of Spike glycoproteins from BA.2.75, CH.1.1, DV.7.1, BA.4/5, BQ.1.1, XBB, XBB.1, XBB.1.16, XBB.1.5, FD.1.1, EG.5.1, HK.3, BA.2.86 and JN.1. We tested their capacity to evade plasma-mediated recognition and neutralization, binding to angiotensin-converting enzyme 2 (ACE2), their susceptibility to cold inactivation, Spike processing, as well as the impact of temperature on Spike-ACE2 interaction. We found that compared to the early wild-type (D614G) strain, most Omicron subvariants' Spike glycoproteins evolved to escape recognition and neutralization by plasma from individuals who received a fifth dose of bivalent (BA.1 or BA.4/5) mRNA vaccine and improve ACE2 binding, particularly at low temperatures. Moreover, BA.2.86 had the best affinity for ACE2 at all temperatures tested. We found that Omicron subvariants' Spike processing is associated with their susceptibility to cold inactivation. Intriguingly, we found that Spike-ACE2 binding at low temperature was significantly associated with growth rates of Omicron subvariants in humans. Overall, we report that Spikes from newly emerged Omicron subvariants are relatively more stable and resistant to plasma-mediated neutralization, present improved affinity for ACE2 which is associated, particularly at low temperatures, with their growth rates.IMPORTANCEThe persistent evolution of SARS-CoV-2 gave rise to a wide range of variants harboring new mutations in their Spike glycoproteins. Several factors have been associated with viral transmission and fitness such as plasma-neutralization escape and ACE2 interaction. To better understand whether additional factors could be of importance in SARS-CoV-2 variants' transmission, we characterize the functional properties of Spike glycoproteins from several Omicron subvariants. We found that the Spike glycoprotein of Omicron subvariants presents an improved escape from plasma-mediated recognition and neutralization, Spike processing, and ACE2 binding which was further improved at low temperature. Intriguingly, Spike-ACE2 interaction at low temperature is strongly associated with viral growth rate, as such, low temperatures could represent another parameter affecting viral transmission.

SARS-CoV-2 Omicron XBB lineage spike structures, conformations, antigenicity, and receptor recognition

A recombinant lineage of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant, named XBB, appeared in late 2022 and evolved descendants that successively swept local and global populations. XBB lineage members were noted for their improved immune evasion and transmissibility. Here, we determine cryoelectron microscopy (cryo-EM) structures of XBB.1.5, XBB.1.16, EG.5, and EG.5.1 spike (S) ectodomains to reveal reinforced 3-receptor binding domain (RBD)-down receptor-inaccessible closed states mediated by interprotomer RBD interactions previously observed in BA.1 and BA.2. Improved XBB.1.5 and XBB.1.16 RBD stability compensated for stability loss caused by early Omicron mutations, while the F456L substitution reduced EG.5 RBD stability. S1 subunit mutations had long-range impacts on conformation and epitope presentation in the S2 subunit. Our results reveal continued S protein evolution via simultaneous optimization of multiple parameters, including stability, receptor binding, and immune evasion, and the dramatic effects of relatively few residue substitutions in altering the S protein conformational landscape.

Cryo-electron microscopy in the study of virus entry and infection

Viruses have been responsible for many epidemics and pandemics that have impacted human life globally. The COVID-19 pandemic highlighted both our vulnerability to viral outbreaks, as well as the mobilization of the scientific community to come together to combat the unprecedented threat to humanity. Cryo-electron microscopy (cryo-EM) played a central role in our understanding of SARS-CoV-2 during the pandemic and continues to inform about this evolving pathogen. Cryo-EM with its two popular imaging modalities, single particle analysis (SPA) and cryo-electron tomography (cryo-ET), has contributed immensely to understanding the structure of viruses and interactions that define their life cycles and pathogenicity. Here, we review how cryo-EM has informed our understanding of three distinct viruses, of which two - HIV-1 and SARS-CoV-2 infect humans, and the third, bacteriophages, infect bacteria. For HIV-1 and SARS-CoV-2 our focus is on the surface glycoproteins that are responsible for mediating host receptor binding, and host and cell membrane fusion, while for bacteriophages, we review their structure, capsid maturation, attachment to the bacterial cell surface and infection initiation mechanism.

Atomistic Prediction of Structures, Conformational Ensembles and Binding Energetics for the SARS-CoV-2 Spike JN.1, KP.2 and KP.3 Variants Using AlphaFold2 and Molecular Dynamics Simulations: Mutational Profiling and Binding Free Energy Analysis Reveal Epistatic Hotspots of the ACE2 Affinity and Immune Escape

The most recent wave of SARS-CoV-2 Omicron variants descending from BA.2 and BA.2.86 exhibited improved viral growth and fitness due to convergent evolution of functional hotspots. These hotspots operate in tandem to optimize both receptor binding for effective infection and immune evasion efficiency, thereby maintaining overall viral fitness. The lack of molecular details on structure, dynamics and binding energetics of the latest FLiRT and FLuQE variants with the ACE2 receptor and antibodies provides a considerable challenge that is explored in this study. We combined AlphaFold2-based atomistic predictions of structures and conformational ensembles of the SARS-CoV-2 Spike complexes with the host receptor ACE2 for the most dominant Omicron variants JN.1, KP.1, KP.2 and KP.3 to examine the mechanisms underlying the role of convergent evolution hotspots in balancing ACE2 binding and antibody evasion. Using the ensemble-based mutational scanning of the spike protein residues and computations of binding affinities, we identified binding energy hotspots and characterized molecular basis underlying epistatic couplings between convergent mutational hotspots. The results suggested that the existence of epistatic interactions between convergent mutational sites at L455, F456, Q493 positions that enable to protect and restore ACE2 binding affinity while conferring beneficial immune escape. To examine immune escape mechanisms, we performed structure-based mutational profiling of the spike protein binding with several classes of antibodies that displayed impaired neutralization against BA.2.86, JN.1, KP.2 and KP.3. The results confirmed the experimental data that JN.1, KP.2 and KP.3 harboring the L455S and F456L mutations can significantly impair the neutralizing activity of class-1 monoclonal antibodies, while the epistatic effects mediated by F456L can facilitate the subsequent convergence of Q493E changes to rescue ACE2 binding. Structural and energetic analysis provided a rationale to the experimental results showing that BD55-5840 and BD55-5514 antibodies that bind to different binding epitopes can retain neutralizing efficacy against all examined variants BA.2.86, JN.1, KP.2 and KP.3. The results support the notion that evolution of Omicron variants may favor emergence of lineages with beneficial combinations of mutations involving mediators of epistatic couplings that control balance of high ACE2 affinity and immune evasion.

Molecular dynamic simulation reveals spider antimicrobial peptide Latarcin-1 and human eosinophil cationic protein as peptide inhibitors of SARS-CoV-2 variants

COVID-19 has rapidly proliferated around 180 countries, and new cases are reported frequently. No peptide medication has been developed that can reliably block SARS-CoV-2 infection. The investigation focuses on the crucial host receptors angiotensin-converting enzyme 2 (ACE2) , which can bind receptor-binding domain (RBD) on the SARS-CoV-2 spike protein (S). To investigate the inhibitory effects of human Eosinophil Cationic Protein (hECP) and Latarcin-1 (L1)on SARS-CoV-2 infection, we have selected them as research subjects. Further, we ran extensive molecular dynamics simulations to bring the docked peptide-ACE2 complex into its equilibrium state. The outcomes were then evaluated with g_MMPBSA and interaction analysis. We have also considered the Delta and Omicron variants to examine these peptides' inhibitory effects. The experimental findings revealed an enhanced capability of L1 and hECP as SARS-CoV-2 inhibitors, occupying hot spots and numerous key residues in ACE2. These include ASP30, ASP38, GLU35 and GLU75, which significantly inhibit the binding of RBD and ACE2 and are effective against two common variants in a similar manner. In addition, this study can serve as a springboard for future research on SARS-CoV-2 inhibitors.Communicated by Ramaswamy H. Sarma.

Spike deep mutational scanning helps predict success of SARS-CoV-2 clades

SARS-CoV-2 variants acquire mutations in the spike protein that promote immune evasion1 and affect other properties that contribute to viral fitness, such as ACE2 receptor binding and cell entry2,3. Knowledge of how mutations affect these spike phenotypes can provide insight into the current and potential future evolution of the virus. Here we use pseudovirus deep mutational scanning4 to measure how more than 9,000 mutations across the full XBB.1.5 and BA.2 spikes affect ACE2 binding, cell entry or escape from human sera. We find that mutations outside the receptor-binding domain (RBD) have meaningfully affected ACE2 binding during SARS-CoV-2 evolution. We also measure how mutations to the XBB.1.5 spike affect neutralization by serum from individuals who recently had SARS-CoV-2 infections. The strongest serum escape mutations are in the RBD at sites 357, 420, 440, 456 and 473; however, the antigenic effects of these mutations vary across individuals. We also identify strong escape mutations outside the RBD; however, many of them decrease ACE2 binding, suggesting they act by modulating RBD conformation. Notably, the growth rates of human SARS-CoV-2 clades can be explained in substantial part by the measured effects of mutations on spike phenotypes, suggesting our data could enable better prediction of viral evolution.

GENESIS 2.1: High-Performance Molecular Dynamics Software for Enhanced Sampling and Free-Energy Calculations for Atomistic, Coarse-Grained, and Quantum Mechanics/Molecular Mechanics Models

GENeralized-Ensemble SImulation System (GENESIS) is a molecular dynamics (MD) software developed to simulate the conformational dynamics of a single biomolecule, as well as molecular interactions in large biomolecular assemblies and between multiple biomolecules in cellular environments. To achieve the latter purpose, the earlier versions of GENESIS emphasized high performance in atomistic MD simulations on massively parallel supercomputers, with or without graphics processing units (GPUs). Here, we implemented multiscale MD simulations that include atomistic, coarse-grained, and hybrid quantum mechanics/molecular mechanics (QM/MM) calculations. They demonstrate high performance and are integrated with enhanced conformational sampling algorithms and free-energy calculations without using external programs except for the QM programs. In this article, we review new functions, molecular models, and other essential features in GENESIS version 2.1 and discuss ongoing developments for future releases.

Exploring conformational landscapes and binding mechanisms of convergent evolution for the SARS-CoV-2 spike Omicron variant complexes with the ACE2 receptor using AlphaFold2-based structural ensembles and molecular dynamics simulations

In this study, we combined AlphaFold-based approaches for atomistic modeling of multiple protein states and microsecond molecular simulations to accurately characterize conformational ensembles evolution and binding mechanisms of convergent evolution for the SARS-CoV-2 spike Omicron variants BA.1, BA.2, BA.2.75, BA.3, BA.4/BA.5 and BQ.1.1. We employed and validated several different adaptations of the AlphaFold methodology for modeling of conformational ensembles including the introduced randomized full sequence scanning for manipulation of sequence variations to systematically explore conformational dynamics of Omicron spike protein complexes with the ACE2 receptor. Microsecond atomistic molecular dynamics (MD) simulations provide a detailed characterization of the conformational landscapes and thermodynamic stability of the Omicron variant complexes. By integrating the predictions of conformational ensembles from different AlphaFold adaptations and applying statistical confidence metrics we can expand characterization of the conformational ensembles and identify functional protein conformations that determine the equilibrium dynamics for the Omicron spike complexes with the ACE2. Conformational ensembles of the Omicron RBD-ACE2 complexes obtained using AlphaFold-based approaches for modeling protein states and MD simulations are employed for accurate comparative prediction of the binding energetics revealing an excellent agreement with the experimental data. In particular, the results demonstrated that AlphaFold-generated extended conformational ensembles can produce accurate binding energies for the Omicron RBD-ACE2 complexes. The results of this study suggested complementarities and potential synergies between AlphaFold predictions of protein conformational ensembles and MD simulations showing that integrating information from both methods can potentially yield a more adequate characterization of the conformational landscapes for the Omicron RBD-ACE2 complexes. This study provides insights in the interplay between conformational dynamics and binding, showing that evolution of Omicron variants through acquisition of convergent mutational sites may leverage conformational adaptability and dynamic couplings between key binding energy hotspots to optimize ACE2 binding affinity and enable immune evasion.

AlphaFold2 Predictions of Conformational Ensembles and Atomistic Simulations of the SARS-CoV-2 Spike XBB Lineages Reveal Epistatic Couplings between Convergent Mutational Hotspots that Control ACE2 Affinity

In this study, we combined AlphaFold-based atomistic structural modeling, microsecond molecular simulations, mutational profiling, and network analysis to characterize binding mechanisms of the SARS-CoV-2 spike protein with the host receptor ACE2 for a series of Omicron XBB variants including XBB.1.5, XBB.1.5+L455F, XBB.1.5+F456L, and XBB.1.5+L455F+F456L. AlphaFold-based structural and dynamic modeling of SARS-CoV-2 Spike XBB lineages can accurately predict the experimental structures and characterize conformational ensembles of the spike protein complexes with the ACE2. Microsecond molecular dynamics simulations identified important differences in the conformational landscapes and equilibrium ensembles of the XBB variants, suggesting that combining AlphaFold predictions of multiple conformations with molecular dynamics simulations can provide a complementary approach for the characterization of functional protein states and binding mechanisms. Using the ensemble-based mutational profiling of protein residues and physics-based rigorous calculations of binding affinities, we identified binding energy hotspots and characterized the molecular basis underlying epistatic couplings between convergent mutational hotspots. Consistent with the experiments, the results revealed the mediating role of the Q493 hotspot in the synchronization of epistatic couplings between L455F and F456L mutations, providing a quantitative insight into the energetic determinants underlying binding differences between XBB lineages. We also proposed a network-based perturbation approach for mutational profiling of allosteric communications and uncovered the important relationships between allosteric centers mediating long-range communication and binding hotspots of epistatic couplings. The results of this study support a mechanism in which the binding mechanisms of the XBB variants may be determined by epistatic effects between convergent evolutionary hotspots that control ACE2 binding.

Some mechanistic underpinnings of molecular adaptations of SARS-COV-2 spike protein by integrating candidate adaptive polymorphisms with protein dynamics

We integrate evolutionary predictions based on the neutral theory of molecular evolution with protein dynamics to generate mechanistic insight into the molecular adaptations of the SARS-COV-2 spike (S) protein. With this approach, we first identified candidate adaptive polymorphisms (CAPs) of the SARS-CoV-2 S protein and assessed the impact of these CAPs through dynamics analysis. Not only have we found that CAPs frequently overlap with well-known functional sites, but also, using several different dynamics-based metrics, we reveal the critical allosteric interplay between SARS-CoV-2 CAPs and the S protein binding sites with the human ACE2 (hACE2) protein. CAPs interact far differently with the hACE2 binding site residues in the open conformation of the S protein compared to the closed form. In particular, the CAP sites control the dynamics of binding residues in the open state, suggesting an allosteric control of hACE2 binding. We also explored the characteristic mutations of different SARS-CoV-2 strains to find dynamic hallmarks and potential effects of future mutations. Our analyses reveal that Delta strain-specific variants have non-additive (i.e., epistatic) interactions with CAP sites, whereas the less pathogenic Omicron strains have mostly additive mutations. Finally, our dynamics-based analysis suggests that the novel mutations observed in the Omicron strain epistatically interact with the CAP sites to help escape antibody binding.

Variation in structural motifs within SARS-related coronavirus spike proteins

SARS-CoV-2 is the third known coronavirus (CoV) that has crossed the animal-human barrier in the last two decades. However, little structural information exists related to the close genetic species within the SARS-related coronaviruses. Here, we present three novel SARS-related CoV spike protein structures solved by single particle cryo-electron microscopy analysis derived from bat (bat SL-CoV WIV1) and civet (cCoV-SZ3, cCoV-007) hosts. We report complex glycan trees that decorate the glycoproteins and density for water molecules which facilitated modeling of the water molecule coordination networks within structurally important regions. We note structural conservation of the fatty acid binding pocket and presence of a linoleic acid molecule which are associated with stabilization of the receptor binding domains in the "down" conformation. Additionally, the N-terminal biliverdin binding pocket is occupied by a density in all the structures. Finally, we analyzed structural differences in a loop of the receptor binding motif between coronaviruses known to infect humans and the animal coronaviruses described in this study, which regulate binding to the human angiotensin converting enzyme 2 receptor. This study offers a structural framework to evaluate the close relatives of SARS-CoV-2, the ability to inform pandemic prevention, and aid in the development of pan-neutralizing treatments.

Exploring Binding Pockets in the Conformational States of the SARS-CoV-2 Spike Trimers for the Screening of Allosteric Inhibitors Using Molecular Simulations and Ensemble-Based Ligand Docking

Understanding mechanisms of allosteric regulation remains elusive for the SARS-CoV-2 spike protein, despite the increasing interest and effort in discovering allosteric inhibitors of the viral activity and interactions with the host receptor ACE2. The challenges of discovering allosteric modulators of the SARS-CoV-2 spike proteins are associated with the diversity of cryptic allosteric sites and complex molecular mechanisms that can be employed by allosteric ligands, including the alteration of the conformational equilibrium of spike protein and preferential stabilization of specific functional states. In the current study, we combine conformational dynamics analysis of distinct forms of the full-length spike protein trimers and machine-learning-based binding pocket detection with the ensemble-based ligand docking and binding free energy analysis to characterize the potential allosteric binding sites and determine structural and energetic determinants of allosteric inhibition for a series of experimentally validated allosteric molecules. The results demonstrate a good agreement between computational and experimental binding affinities, providing support to the predicted binding modes and suggesting key interactions formed by the allosteric ligands to elicit the experimentally observed inhibition. We establish structural and energetic determinants of allosteric binding for the experimentally known allosteric molecules, indicating a potential mechanism of allosteric modulation by targeting the hinges of the inter-protomer movements and blocking conformational changes between the closed and open spike trimer forms. The results of this study demonstrate that combining ensemble-based ligand docking with conformational states of spike protein and rigorous binding energy analysis enables robust characterization of the ligand binding modes, the identification of allosteric binding hotspots, and the prediction of binding affinities for validated allosteric modulators, which is consistent with the experimental data. This study suggested that the conformational adaptability of the protein allosteric sites and the diversity of ligand bound conformations are both in play to enable efficient targeting of allosteric binding sites and interfere with the conformational changes.

Ensemble-Based Mutational Profiling and Network Analysis of the SARS-CoV-2 Spike Omicron XBB Lineages for Interactions with the ACE2 Receptor and Antibodies: Cooperation of Binding Hotspots in Mediating Epistatic Couplings Underlies Binding Mechanism and Immune Escape

In this study, we performed a computational study of binding mechanisms for the SARS-CoV-2 spike Omicron XBB lineages with the host cell receptor ACE2 and a panel of diverse class one antibodies. The central objective of this investigation was to examine the molecular factors underlying epistatic couplings among convergent evolution hotspots that enable optimal balancing of ACE2 binding and antibody evasion for Omicron variants BA.1, BA2, BA.3, BA.4/BA.5, BQ.1.1, XBB.1, XBB.1.5, and XBB.1.5 + L455F/F456L. By combining evolutionary analysis, molecular dynamics simulations, and ensemble-based mutational scanning of spike protein residues in complexes with ACE2, we identified structural stability and binding affinity hotspots that are consistent with the results of biochemical studies. In agreement with the results of deep mutational scanning experiments, our quantitative analysis correctly reproduced strong and variant-specific epistatic effects in the XBB.1.5 and BA.2 variants. It was shown that Y453W and F456L mutations can enhance ACE2 binding when coupled with Q493 in XBB.1.5, while these mutations become destabilized when coupled with the R493 position in the BA.2 variant. The results provided a molecular rationale of the epistatic mechanism in Omicron variants, showing a central role of the Q493/R493 hotspot in modulating epistatic couplings between convergent mutational sites L455F and F456L in XBB lineages. The results of mutational scanning and binding analysis of the Omicron XBB spike variants with ACE2 receptors and a panel of class one antibodies provide a quantitative rationale for the experimental evidence that epistatic interactions of the physically proximal binding hotspots Y501, R498, Q493, L455F, and F456L can determine strong ACE2 binding, while convergent mutational sites F456L and F486P are instrumental in mediating broad antibody resistance. The study supports a mechanism in which the impact on ACE2 binding affinity is mediated through a small group of universal binding hotspots, while the effect of immune evasion could be more variant-dependent and modulated by convergent mutational sites in the conformationally adaptable spike regions.

Predicting Functional Conformational Ensembles and Binding Mechanisms of Convergent Evolution for SARS-CoV-2 Spike Omicron Variants Using AlphaFold2 Sequence Scanning Adaptations and Molecular Dynamics Simulations

In this study, we combined AlphaFold-based approaches for atomistic modeling of multiple protein states and microsecond molecular simulations to accurately characterize conformational ensembles and binding mechanisms of convergent evolution for the SARS-CoV-2 Spike Omicron variants BA.1, BA.2, BA.2.75, BA.3, BA.4/BA.5 and BQ.1.1. We employed and validated several different adaptations of the AlphaFold methodology for modeling of conformational ensembles including the introduced randomized full sequence scanning for manipulation of sequence variations to systematically explore conformational dynamics of Omicron Spike protein complexes with the ACE2 receptor. Microsecond atomistic molecular dynamic simulations provide a detailed characterization of the conformational landscapes and thermodynamic stability of the Omicron variant complexes. By integrating the predictions of conformational ensembles from different AlphaFold adaptations and applying statistical confidence metrics we can expand characterization of the conformational ensembles and identify functional protein conformations that determine the equilibrium dynamics for the Omicron Spike complexes with the ACE2. Conformational ensembles of the Omicron RBD-ACE2 complexes obtained using AlphaFold-based approaches for modeling protein states and molecular dynamics simulations are employed for accurate comparative prediction of the binding energetics revealing an excellent agreement with the experimental data. In particular, the results demonstrated that AlphaFold-generated extended conformational ensembles can produce accurate binding energies for the Omicron RBD-ACE2 complexes. The results of this study suggested complementarities and potential synergies between AlphaFold predictions of protein conformational ensembles and molecular dynamics simulations showing that integrating information from both methods can potentially yield a more adequate characterization of the conformational landscapes for the Omicron RBD-ACE2 complexes. This study provides insights in the interplay between conformational dynamics and binding, showing that evolution of Omicron variants through acquisition of convergent mutational sites may leverage conformational adaptability and dynamic couplings between key binding energy hotspots to optimize ACE2 binding affinity and enable immune evasion.

Nanobody engineering for SARS-CoV-2 neutralization and detection

In response to the ongoing COVID-19 pandemic, the quest for coronavirus inhibitors has inspired research on a variety of small proteins beyond conventional antibodies, including robust single-domain antibody fragments, i.e., "nanobodies." Here, we explore the potential of nanobody engineering in the development of antivirals and diagnostic tools. Through fusion of nanobody domains that target distinct binding sites, we engineered multimodular nanobody constructs that neutralize wild-type SARS-CoV-2 and the Alpha and Delta variants at high potency, with IC50 values as low as 50 pM. Despite simultaneous binding to distinct epitopes, Beta and Omicron variants were more resistant to neutralization by the multimodular nanobodies, which highlights the importance of accounting for antigenic drift in the design of biologics. To further explore the applications of nanobody engineering in outbreak management, we present an assay based on fusions of nanobodies with fragments of NanoLuc luciferase that can detect sub-nanomolar quantities of the SARS-CoV-2 spike protein in a single step. Our work showcases the potential of nanobody engineering to combat emerging infectious diseases.

IMPORTANCE

Nanobodies, small protein binders derived from the camelid antibody, are highly potent inhibitors of respiratory viruses that offer several advantages over conventional antibodies as candidates for specific therapies, including high stability and low production costs. In this work, we leverage the unique properties of nanobodies and apply them as building blocks for new therapeutic and diagnostic tools. We report ultra-potent SARS-CoV-2 inhibition by engineered nanobodies comprising multiple modules in structure-guided combinations and develop nanobodies that carry signal molecules, allowing rapid detection of the SARS-CoV-2 spike protein. Our results highlight the potential of engineered nanobodies in the development of effective countermeasures, both therapeutic and diagnostic, to manage outbreaks of emerging viruses.

Characterization of a neutralizing antibody that recognizes a loop region adjacent to the receptor-binding interface of the SARS-CoV-2 spike receptor-binding domain

Although the global crisis caused by the coronavirus disease 2019 (COVID-19) pandemic is over, the global epidemic of the disease continues. Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the cause of COVID-19, initiates infection via the binding of the receptor-binding domain (RBD) of its spike protein to the human angiotensin-converting enzyme II (ACE2) receptor, and this interaction has been the primary target for the development of COVID-19 therapeutics. Here, we identified neutralizing antibodies against SARS-CoV-2 by screening mouse monoclonal antibodies and characterized an antibody, CSW1-1805, that targets a narrow region at the RBD ridge of the spike protein. CSW1-1805 neutralized several variants in vitro and completely protected mice from SARS-CoV-2 infection. Cryo-EM and biochemical analyses revealed that this antibody recognizes the loop region adjacent to the ACE2-binding interface with the RBD in both a receptor-inaccessible "down" state and a receptor-accessible "up" state and could stabilize the RBD conformation in the up-state. CSW1-1805 also showed different binding orientations and complementarity determining region properties compared to other RBD ridge-targeting antibodies with similar binding epitopes. It is important to continuously characterize neutralizing antibodies to address new variants that continue to emerge. Our characterization of this antibody that recognizes the RBD ridge of the spike protein will aid in the development of future neutralizing antibodies.IMPORTANCESARS-CoV-2 cell entry is initiated by the interaction of the viral spike protein with the host cell receptor. Therefore, mechanistic findings regarding receptor recognition by the spike protein help uncover the molecular mechanism of SARS-CoV-2 infection and guide neutralizing antibody development. Here, we characterized a SARS-CoV-2 neutralizing antibody that recognizes an epitope, a loop region adjacent to the receptor-binding interface, that may be involved in the conformational transition of the receptor-binding domain (RBD) of the spike protein from a receptor-inaccessible "down" state into a receptor-accessible "up" state, and also stabilizes the RBD in the up-state. Our mechanistic findings provide new insights into SARS-CoV-2 receptor recognition and guidance for neutralizing antibody development.

SARS-CoV-2 Omicron XBB lineage spike structures, conformations, antigenicity, and receptor recognition

A recombinant lineage of the SARS-CoV-2 Omicron variant, named XBB, appeared in late 2022 and evolved descendants that successively swept local and global populations. XBB lineage members were noted for their improved immune evasion and transmissibility. Here, we determine cryo-EM structures of XBB.1.5, XBB.1.16, EG.5 and EG.5.1 spike (S) ectodomains to reveal reinforced 3-RBD-down receptor inaccessible closed states mediated by interprotomer receptor binding domain (RBD) interactions previously observed in BA.1 and BA.2. Improved XBB.1.5 and XBB.1.16 RBD stability compensated for stability loss caused by early Omicron mutations, while the F456L substitution reduced EG.5 RBD stability. S1 subunit mutations had long-range impacts on conformation and epitope presentation in the S2 subunit. Our results reveal continued S protein evolution via simultaneous optimization of multiple parameters including stability, receptor binding and immune evasion, and the dramatic effects of relatively few residue substitutions in altering the S protein conformational landscape.

AlphaFold2-Enabled Atomistic Modeling of Structure, Conformational Ensembles, and Binding Energetics of the SARS-CoV-2 Omicron BA.2.86 Spike Protein with ACE2 Host Receptor and Antibodies: Compensatory Functional Effects of Binding Hotspots in Modulating Mechanisms of Receptor Binding and Immune Escape

The latest wave of SARS-CoV-2 Omicron variants displayed a growth advantage and increased viral fitness through convergent evolution of functional hotspots that work synchronously to balance fitness requirements for productive receptor binding and efficient immune evasion. In this study, we combined AlphaFold2-based structural modeling approaches with atomistic simulations and mutational profiling of binding energetics and stability for prediction and comprehensive analysis of the structure, dynamics, and binding of the SARS-CoV-2 Omicron BA.2.86 spike variant with ACE2 host receptor and distinct classes of antibodies. We adapted several AlphaFold2 approaches to predict both the structure and conformational ensembles of the Omicron BA.2.86 spike protein in the complex with the host receptor. The results showed that the AlphaFold2-predicted structural ensemble of the BA.2.86 spike protein complex with ACE2 can accurately capture the main conformational states of the Omicron variant. Complementary to AlphaFold2 structural predictions, microsecond molecular dynamics simulations reveal the details of the conformational landscape and produced equilibrium ensembles of the BA.2.86 structures that are used to perform mutational scanning of spike residues and characterize structural stability and binding energy hotspots. The ensemble-based mutational profiling of the receptor binding domain residues in the BA.2 and BA.2.86 spike complexes with ACE2 revealed a group of conserved hydrophobic hotspots and critical variant-specific contributions of the BA.2.86 convergent mutational hotspots R403K, F486P, and R493Q. To examine the immune evasion properties of BA.2.86 in atomistic detail, we performed structure-based mutational profiling of the spike protein binding interfaces with distinct classes of antibodies that displayed significantly reduced neutralization against the BA.2.86 variant. The results revealed the molecular basis of compensatory functional effects of the binding hotspots, showing that BA.2.86 lineage may have evolved to outcompete other Omicron subvariants by improving immune evasion while preserving binding affinity with ACE2 via through a compensatory effect of R493Q and F486P convergent mutational hotspots. This study demonstrated that an integrative approach combining AlphaFold2 predictions with complementary atomistic molecular dynamics simulations and robust ensemble-based mutational profiling of spike residues can enable accurate and comprehensive characterization of structure, dynamics, and binding mechanisms of newly emerging Omicron variants.

Antibody targeting of conserved sites of vulnerability on the SARS-CoV-2 spike receptor-binding domain

Given the continuous emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern (VoCs), immunotherapeutics that target conserved epitopes on the spike (S) glycoprotein have therapeutic advantages. Here, we report the crystal structure of the SARS-CoV-2 S receptor-binding domain (RBD) at 1.95 Å and describe flexibility and distinct conformations of the angiotensin-converting enzyme 2 (ACE2)-binding site. We identify a set of SARS-CoV-2-reactive monoclonal antibodies (mAbs) with broad RBD cross-reactivity including SARS-CoV-2 Omicron subvariants, SARS-CoV-1, and other sarbecoviruses and determine the crystal structures of mAb-RBD complexes with Ab246 and CR3022 mAbs targeting the class IV site, WRAIR-2134, which binds the recently designated class V epitope, and WRAIR-2123, the class I ACE2-binding site. The broad reactivity of class IV and V mAbs to conserved regions of SARS-CoV-2 VoCs and other sarbecovirus provides a framework for long-term immunotherapeutic development strategies.