De novo design and directed folding of disulfide-bridged peptide heterodimers

Motif-Directed Oxidative Folding to Design and Discover Multicyclic Peptides for Protein Recognition

ConspectusMulticyclic peptides that are constrained through covalent cross-linkers can usually maintain stable three-dimensional (3D) structures without the necessity of incorporating noncovalently interacting cores. This configuration allows for a greater utilization of residues for functional purposes compared to larger proteins, rendering multicyclic peptides attractive molecular modalities for the development of chemical tools and therapeutic agents. Even smaller multicyclic peptides, which may lack stable 3D structures due to limited sequence-driven folding capabilities, can still benefit from the specific conformations stabilized by covalent cross-linkers to facilitate target binding. Disulfide-rich peptides (DRPs) are a class of particularly significant multicyclic peptides that are primarily composed of disulfide bonds in their interior. However, the structural diversity of DRPs is limited to a few naturally occurring and designer scaffolds, which significantly impedes the development of multicyclic peptide ligands and therapeutics. To address this issue, we developed a novel method that utilizes disulfide-directing motifs to design and discover DRPs with new structures and functions in random sequence space. Compared with traditional DRPs, these new DRPs that incorporate disulfide-directing motifs exhibit more precise oxidative folding regarding disulfide pairing and demonstrate greater tolerance to sequence manipulations. Thus, we designated these peptides as disulfide-directed multicyclic peptides (DDMPs).Over the past decade, we have developed a new class of multicyclic peptides by leveraging disulfide-directing motifs, including biscysteine motifs such as CPXXC, CPPC, and CXC (C: cysteine; P: proline; X: any amino acid), as well as triscysteine motifs that rationally combine two biscysteine motifs (e.g., CPPCXC and CPXXCXC) to direct the oxidative folding of peptides. This leads to the introduction of a novel concept known as motif-directed oxidative folding, which is valuable for the construction of peptides with multiple disulfide bonds. A large diversity of DDMPs have been designed by simply altering the disulfide-directing motifs, the arrangement of cysteine residues (i.e., cysteine patterns), and the number of random residues separating them. As the oxidative folding of DDMPs is primarily determined by disulfide-directing motifs, these peptides are intrinsically more tolerant of extensive sequence manipulations compared to traditional DRPs. Consequently, multicyclic peptide libraries with an unprecedented high degree of sequence randomization have been developed by utilizing commonly used biological display systems such as phage display. We have validated the applicability of these libraries by successfully discovering DDMPs with unique protein-like 3D structures and high affinity and specificity to various cell-surface receptors, including tumor-associated antigens, immune costimulatory receptors, and G protein-coupled receptors (GPCRs). Currently, multicyclic peptides used in clinical settings are of natural origin or derived from natural DRPs. Our studies have opened up the possibility of developing multicyclic peptides without relying on natural scaffolds, representing a pivotal breakthrough in the field of peptide ligand and drug discovery. Further investigations will facilitate the application of our DDMPs in broader fields such as bioanalysis, chemical biology, and biomedicine.

Biomimetic Folding Strategies for Chemical Synthesis of Disulfide-Bonded Peptides and Proteins

Disulfide-bonded peptides and proteins, including hormones, toxins, growth factors, and others, are abundant in living organisms. These molecules play crucial physiological roles such as regulating cell and organism growth, development, and metabolism. They have also found widespread applications as drugs or tool molecules in biomedical and pharmaceutical research. However, the chemical synthesis of disulfide-bonded proteins is complicated by the challenges associated with their folding. This review focuses on the latest advancements in disulfide-bonded peptide and protein folding technologies. Particularly, it highlights biomimetic folding strategies that emulate the naturally occurring oxidative folding processes in nature. These strategies include chaperone-assisted folding, glycosylation-assisted folding, and organic-based oxidative folding methods. The review also anticipates future directions in folding technology. Such research offers innovative approaches for the chemical synthesis of complex proteins that are otherwise difficult to fold.

ProBID-Net: a deep learning model for protein-protein binding interface design

Protein-protein interactions are pivotal in numerous biological processes. The computational design of these interactions facilitates the creation of novel binding proteins, crucial for advancing biopharmaceutical products. With the evolution of artificial intelligence (AI), protein design tools have swiftly transitioned from scoring-function-based to AI-based models. However, many AI models for protein design are constrained by assuming complete unfamiliarity with the amino acid sequence of the input protein, a feature most suited for de novo design but posing challenges in designing protein-protein interactions when the receptor sequence is known. To bridge this gap in computational protein design, we introduce ProBID-Net. Trained using natural protein-protein complex structures and protein domain-domain interface structures, ProBID-Net can discern features from known target protein structures to design specific binding proteins based on their binding sites. In independent tests, ProBID-Net achieved interface sequence recovery rates of 52.7%, 43.9%, and 37.6%, surpassing or being on par with ProteinMPNN in binding protein design. Validated using AlphaFold-Multimer, the sequences designed by ProBID-Net demonstrated a close correspondence between the design target and the predicted structure. Moreover, the model's output can predict changes in binding affinity upon mutations in protein complexes, even in scenarios where no data on such mutations were provided during training (zero-shot prediction). In summary, the ProBID-Net model is poised to significantly advance the design of protein-protein interactions.

Mechanical Regulation of Polymer Gels

The mechanical properties of polymer gels devote to emerging devices and machines in fields such as biomedical engineering, flexible bioelectronics, biomimetic actuators, and energy harvesters. Coupling network architectures and interactions has been explored to regulate supportive mechanical characteristics of polymer gels; however, systematic reviews correlating mechanics to interaction forces at the molecular and structural levels remain absent in the field. This review highlights the molecular engineering and structural engineering of polymer gel mechanics and a comprehensive mechanistic understanding of mechanical regulation. Molecular engineering alters molecular architecture and manipulates functional groups/moieties at the molecular level, introducing various interactions and permanent or reversible dynamic bonds as the dissipative energy. Molecular engineering usually uses monomers, cross-linkers, chains, and other additives. Structural engineering utilizes casting methods, solvent phase regulation, mechanochemistry, macromolecule chemical reactions, and biomanufacturing technology to construct and tailor the topological network structures, or heterogeneous modulus compositions. We envision that the perfect combination of molecular and structural engineering may provide a fresh view to extend exciting new perspectives of this burgeoning field. This review also summarizes recent representative applications of polymer gels with excellent mechanical properties. Conclusions and perspectives are also provided from five aspects of concise summary, mechanical mechanism, biofabrication methods, upgraded applications, and synergistic methodology.

Triscysteine disulfide-directing motifs enabling design and discovery of multicyclic peptide binders

Peptides are valuable for therapeutic development, with multicyclic peptides showing promise in mimicking antigen-binding potency of antibodies. However, our capability to engineer multicyclic peptide scaffolds, particularly for the construction of large combinatorial libraries, is still limited. Here, we study the interplay of disulfide pairing between three biscysteine motifs, and designed a range of triscysteine motifs with unique disulfide-directing capability for regulating the oxidative folding of multicyclic peptides. We demonstrate that incorporating these motifs into random sequences allows the design of disulfide-directed multicyclic peptide (DDMP) libraries with up to four disulfide bonds, which have been applied for the successful discovery of peptide binders with nanomolar affinity to several challenging targets. This study encourages the use of more diverse disulfide-directing motifs for creating multicyclic peptide libraries and opens an avenue for discovering functional peptides in sequence and structural space beyond existing peptide scaffolds, potentially advancing the field of peptide drug discovery.

Strategies of rational and structure-driven vaccine design for Arenaviruses

The COVID-19 outbreak has highlighted the importance of pandemic preparedness for the prevention of future health crises. One virus family with high pandemic potential are Arenaviruses, which have been detected almost worldwide, particularly in Africa and the Americas. These viruses are highly understudied and many questions regarding their structure, replication and tropism remain unanswered, making the design of an efficacious and molecularly-defined vaccine challenging. We propose that structure-driven computational vaccine design will contribute to overcome these challenges. Computational methods for stabilization of viral glycoproteins or epitope focusing have made progress during the last decades and particularly during the COVID-19 pandemic, and have proven useful for rational vaccine design and the establishment of novel diagnostic tools. In this review, we summarize gaps in our understanding of Arenavirus molecular biology, highlight challenges in vaccine design and discuss how structure-driven and computationally informed strategies will aid in overcoming these obstacles.

Structural Characterization of Disulfide-Linked p53-Derived Peptide Dimers

Disulfide bonds provide a convenient method for chemoselective alteration of peptide and protein structure and function. We previously reported that mild oxidation of a p53-derived bisthiol peptide (CTFANLWRLLAQNC) under dilute non-denaturing conditions led to unexpected disulfide-linked dimers as the exclusive product. The dimers were antiparallel, significantly α-helical, resistant to protease degradation, and easily reduced back to the original bisthiol peptide. Here we examine the intrinsic factors influencing peptide dimerization using a combination of amino acid substitution, circular dichroism (CD) spectroscopy, and X-ray crystallography. CD analysis of peptide variants suggests critical roles for Leu6 and Leu10 in the formation of stable disulfide-linked dimers. The 1.0 Å resolution crystal structure of the peptide dimer supports these data, revealing a leucine-rich LxxLL dimer interface with canonical knobs-into-holes packing. Two levels of higher-order oligomerization are also observed in the crystal: an antiparallel "dimer of dimers" mediated by Phe3 and Trp7 residues in the asymmetric unit and a tetramer of dimers mediated by Trp7 and Leu10. In CD spectra of Trp-containing peptide variants, minima at 227 nm provide evidence for the dimer of dimers in dilute aqueous solution. Importantly, and in contrast to the original dimer model, the canonical leucine-rich core and robust dimerization of most peptide variants suggests a tunable molecular architecture to target various proteins and evaluate how folding and oligomerization impact various properties, such as cell permeability.

De novo design of proteins housing excitonically coupled chlorophyll special pairs

Natural photosystems couple light harvesting to charge separation using a 'special pair' of chlorophyll molecules that accepts excitation energy from the antenna and initiates an electron-transfer cascade. To investigate the photophysics of special pairs independently of the complexities of native photosynthetic proteins, and as a first step toward creating synthetic photosystems for new energy conversion technologies, we designed C2-symmetric proteins that hold two chlorophyll molecules in closely juxtaposed arrangements. X-ray crystallography confirmed that one designed protein binds two chlorophylls in the same orientation as native special pairs, whereas a second designed protein positions them in a previously unseen geometry. Spectroscopy revealed that the chlorophylls are excitonically coupled, and fluorescence lifetime imaging demonstrated energy transfer. The cryo-electron microscopy structure of a designed 24-chlorophyll octahedral nanocage with a special pair on each edge closely matched the design model. The results suggest that the de novo design of artificial photosynthetic systems is within reach of current computational methods.

Reconstructing dynamics correlation network to simultaneously improve activity and stability of 2,3-butanediol dehydrogenase by design of distal interchain disulfide bonds

The complete enzyme catalytic cycle includes substrate binding, chemical reaction and product release, in which different dynamic conformations are adopted. Due to the complex relationship among enzyme activity, stability and dynamics, the directed evolution of enzymes for improved activity or stability commonly leads to a trade-off in stability or activity. It hence remains a challenge to engineer an enzyme to have both enhanced activity and stability. Here, we have attempted to reconstruct the dynamics correlation network involved with active center to improve both activity and stability of a 2,3-butanediol dehydrogenase (2,3-BDH) by introducing inter-chain disulfide bonds. A computational strategy was first applied to evaluate the effect of introducing inter-chain disulfide bond on activity and stability of three 2,3-BDHs, and the N258C mutation of 2,3-BDH from Corynebacterium glutamicum (CgBDH) was proved to be effective in improving both activity and stability. In the results, CgBDH-N258C showed a different unfolding curve from the wild type, with two melting temperatures (Tm) of 68.3 °C and 50.8 °C, 19.7 °C and 2 °C higher than 48.6 °C of the wild type. Its half-life was also improved by 14.8-fold compared to the wild type. Catalytic efficiency (kcat/Km) of the mutant was increased by 7.9-fold toward native substrate diacetyl and 8.8-fold toward non-native substrate 2,5-hexanedione compared to the wild type. Molecular dynamics simulations revealed that an interaction network formed by Cys258, Arg162, Ala144 and the catalytic residues was reconstructed in the mutant and the dynamics change caused by the disulfide bond could be propagated through the interactions network. This improved the enzyme stability and activity by decreasing the flexibility and locking more "reactive" pose, respectively. Further construction of mutations including A144G showing a 44-fold improvement in catalytic efficiency toward meso-2,3-BD confirmed the role of modifying dynamics correlation network in tunning enzyme activity and selectivity. This study provided important insights into the relationship among dynamics, enzyme catalysis and stability, and will be useful in the designing new enzymes with co-evolution of stability, activity and selectivity.

De Novo Discovery of Cysteine Frameworks for Developing Multicyclic Peptide Libraries for Ligand Discovery

Conserved cysteine frameworks are essential components of disulfide-rich peptides (DRPs), which dominantly define the structural diversity of both naturally occurring and de novo-designed DRPs. However, there are only very limited numbers of conserved cysteine frameworks, and general methods enabling de novo discovery of cysteine frameworks with robust foldability are still not available. Here, we devised a "touchstone"-based strategy that relies on chasing oxidative foldability between two individual disulfide-rich folds on the phage surface to discover new cysteine frameworks from random sequences. Unique cysteine frameworks with a high degree of compatibility with phage display systems and broad sequence tolerance were successfully identified, which were subsequently exploited for the development of multicyclic DRP libraries, enabling the rapid discovery of new peptide ligands with low-nanomolar and picomolar binding affinity. This study provides an unprecedented method for exploring and exploiting the sequence and structure space of DRPs that is not readily accessible by existing strategies, holding the potential to revolutionize the study of DRPs and significantly advance the design and discovery of multicyclic peptide ligands and drugs.

NACs, generalist in plant life

Plant-specific NAC proteins constitute a major transcription factor family that is well-known for its roles in plant growth, development, and responses to abiotic and biotic stresses. In recent years, there has been significant progress in understanding the functions of NAC proteins. NAC proteins have a highly conserved DNA-binding domain; however, their functions are diverse. Previous understanding of the structure of NAC transcription factors can be used as the basis for their functional diversity. NAC transcription factors consist of a target-binding domain at the N-terminus and a highly versatile C-terminal domain that interacts with other proteins. A growing body of research on NAC transcription factors helps us comprehend the intricate signalling network and transcriptional reprogramming facilitated by NAC-mediated complexes. However, most studies of NAC proteins have been limited to a single function. Here, we discuss the upstream regulators, regulatory components and targets of NAC in the context of their prospective roles in plant improvement strategies via biotechnology intervention, highlighting the importance of the NAC transcription factor family in plants and the need for further research.

Design of stimulus-responsive two-state hinge proteins

In nature, proteins that switch between two conformations in response to environmental stimuli structurally transduce biochemical information in a manner analogous to how transistors control information flow in computing devices. Designing proteins with two distinct but fully structured conformations is a challenge for protein design as it requires sculpting an energy landscape with two distinct minima. Here we describe the design of "hinge" proteins that populate one designed state in the absence of ligand and a second designed state in the presence of ligand. X-ray crystallography, electron microscopy, double electron-electron resonance spectroscopy, and binding measurements demonstrate that despite the significant structural differences the two states are designed with atomic level accuracy and that the conformational and binding equilibria are closely coupled.

Folding-Assisted Peptide Disulfide Formation and Dimerization

Disulfide bonds form covalent bonds between distal regions of peptides and proteins to dramatically impact their folding, stability, and oligomerization. Given the prevalence of disulfide bonds in many natural products, considerable effort has been invested in site-selective disulfide bond formation approaches to control the folding of chemically synthesized peptides and proteins. Here, we show that the careful choice of thiol oxidation conditions can lead to monomeric or dimeric species from fully deprotected linear bisthiol peptides. Starting from a p53-derived peptide, we found that oxidation under aqueous (nondenaturing) conditions produces antiparallel dimers with enhanced α-helical character, while oxidation under denaturing conditions promotes formation of a nonhelical intramolecular disulfide species. Examination across peptide variants suggests that intramolecular disulfide formation is robust across diverse peptide sequences, while dimerization is sensitive to both the α-helical folding of the linear peptide and aromatic residues at the dimerization interface. All disulfide species are more resistant to protease degradation than the linear peptide but are easily reduced to restore the initial bisthiol peptide. Both disulfide formation approaches are compatible with α-helix-stabilizing cross-linkers. These results provide an approach for using disulfide bonds to control peptide folding and oligomerization to better understand how folding influences interactions with diverse molecular targets.

De novo design of energy transfer proteins housing excitonically coupled chlorophyll special pairs

Natural photosystems couple light harvesting to charge separation using a "special pair" of chlorophyll molecules that accepts excitation energy from the antenna and initiates an electron-transfer cascade. To investigate the photophysics of special pairs independent of complexities of native photosynthetic proteins, and as a first step towards synthetic photosystems for new energy conversion technologies, we designed C2-symmetric proteins that precisely position chlorophyll dimers. X-ray crystallography shows that one designed protein binds two chlorophylls in a binding orientation matching native special pairs, while a second positions them in a previously unseen geometry. Spectroscopy reveals excitonic coupling, and fluorescence lifetime imaging demonstrates energy transfer. We designed special pair proteins to assemble into 24-chlorophyll octahedral nanocages; the design model and cryo-EM structure are nearly identical. The design accuracy and energy transfer function of these special pair proteins suggest that de novo design of artificial photosynthetic systems is within reach of current computational methods.

Machine learning for evolutionary-based and physics-inspired protein design: Current and future synergies

Computational protein design facilitates the discovery of novel proteins with prescribed structure and functionality. Exciting designs were recently reported using novel data-driven methodologies that can be roughly divided into two categories: evolutionary-based and physics-inspired approaches. The former infer characteristic sequence features shared by sets of evolutionary-related proteins, such as conserved or coevolving positions, and recombine them to generate candidates with similar structure and function. The latter approaches estimate key biochemical properties, such as structure free energy, conformational entropy, or binding affinities using machine learning surrogates, and optimize them to yield improved designs. Here, we review recent progress along both tracks, discuss their strengths and weaknesses, and highlight opportunities for synergistic approaches.

D-Peptide and D-Protein Technology: Recent Advances, Challenges, and Opportunities

Total chemical protein synthesis provides access to entire D-protein enantiomers enabling unique applications in molecular biology, structural biology, and bioactive compound discovery. Key enzymes involved in the central dogma of molecular biology have been prepared in their D-enantiomeric forms facilitating the development of mirror-image life. Crystallization of a racemic mixture of L- and D-protein enantiomers provides access to high-resolution X-ray structures of polypeptides. Additionally, D-enantiomers of protein drug targets can be used in mirror-image phage display allowing discovery of non-proteolytic D-peptide ligands as lead candidates. This review discusses the unique applications of D-proteins including the synthetic challenges and opportunities.

Disulfide-Directed Multicyclic Peptide Libraries for the Discovery of Peptide Ligands and Drugs

Multicyclic peptides with stable 3D structures are a kind of novel and promising peptide formats for drug design and discovery as they have the potential to combine the best characteristics of small molecules and proteins. However, the development of multicyclic peptides is largely limited to naturally occurring products. It remains a big challenge to develop multicyclic peptides with new structures and functions without recourse to the existing natural scaffolds. Here, we report a general and robust method relying on the utility of new disulfide-directing motifs for designing and discovering diverse multicyclic peptides with potent protein-binding capability. These peptides, referred to as disulfide-directed multicyclic peptides (DDMPs), are tolerant to extensive sequence manipulations and variations of disulfide-pairing frameworks, enabling the development of de novo DDMP libraries useful for ligand and drug discovery. This study opens a new avenue for creating a new generation of multicyclic peptides in sequence and structure space inaccessible by natural scaffolds, thus would greatly benefit the field of peptide drug discovery.

De novo design of immunoglobulin-like domains

Antibodies, and antibody derivatives such as nanobodies, contain immunoglobulin-like (Ig) β-sandwich scaffolds which anchor the hypervariable antigen-binding loops and constitute the largest growing class of drugs. Current engineering strategies for this class of compounds rely on naturally existing Ig frameworks, which can be hard to modify and have limitations in manufacturability, designability and range of action. Here, we develop design rules for the central feature of the Ig fold architecture—the non-local cross-β structure connecting the two β-sheets—and use these to design highly stable Ig domains de novo, confirm their structures through X-ray crystallography, and show they can correctly scaffold functional loops. Our approach opens the door to the design of antibody-like scaffolds with tailored structures and superior biophysical properties.

The immunoglobulin domain framework of antibodies has been a long standing design challenge. Here, the authors describe design rules for tailoring these domains and show they can be accurately designed, de novo, with high stability and the ability to scaffold functional loops.