Principles of Virus Structure

Resolving viral structural complexity by super-resolution microscopy

In this review, we discuss different super-resolution microscopy (SRM) techniques employed to study viral structures and virus composition with nanometric resolution. We describe the basic principles of the different microscopy methods utilized to break the light diffraction limit, enabling the study of protein composition in viral structures. Finally, we demonstrate for the first time the differential spatial distribution of two structural proteins in an individual baculovirus using single-molecule super-resolution microscopy. We discuss the future of these powerful methods for virology, medicine, and biotechnology applications.

Dynamics, allostery, and stabilities of whole virus particles by amide hydrogen/deuterium exchange mass spectrometry (HDXMS)

X-ray crystallography and cryo-electron microscopy have enabled the determination of structures of numerous viruses at high resolution and have greatly advanced the field of structural virology. These structures represent only a subset of snapshot end-state conformations, without describing all conformational transitions that virus particles undergo. Allostery plays a critical role in relaying the effects of varied perturbations both on the surface through environmental changes and protein (receptor/antibody) interactions into the genomic core of the virus. Correspondingly, allostery carries implications for communicating changes in genome packaging to the overall stability of the virus particle. Amide hydrogen/deuterium exchange mass spectrometry (HDXMS) of whole viruses is a powerful probe for uncovering virus allostery. Here we critically discuss advancements in understanding virus dynamics by HDXMS with single particle cryo-EM and computational approaches.

Delineating organizational principles of the endogenous L-A virus by cryo-EM and computational analysis of native cell extracts

The high abundance of most viruses in infected host cells benefits their structural characterization. However, endogenous viruses are present in low copy numbers and are therefore challenging to investigate. Here, we retrieve cell extracts enriched with an endogenous virus, the yeast L-A virus. The determined cryo-EM structure discloses capsid-stabilizing cation-π stacking, widespread across viruses and within the Totiviridae, and an interplay of non-covalent interactions from ten distinct capsomere interfaces. The capsid-embedded mRNA decapping active site trench is supported by a constricting movement of two flexible opposite-facing loops. tRNA-loaded polysomes and other biomacromolecules, presumably mRNA, are found in virus proximity within the cell extract. Mature viruses participate in larger viral communities resembling their rare in-cell equivalents in terms of size, composition, and inter-virus distances. Our results collectively describe a 3D-architecture of a viral milieu, opening the door to cell-extract-based high-resolution structural virology.

Structural highlights of macromolecular complexes and assemblies

The structures of macromolecular assemblies have given us deep insights into cellular processes and have profoundly impacted biological research and drug discovery. We highlight the structures of macromolecular assemblies that have been modeled using integrative and computational methods and describe how open access to these structures from structural archives has empowered the research community. The arsenal of experimental and computational methods for structure determination ensures a future where whole organelles and cells can be modeled.

Design of Ligand-Operable Protein-Cages That Open Upon Specific Protein Binding

Protein nanocages have diverse applications in medicine and biotechnology, including molecular delivery. However, although numerous studies have demonstrated the ability of protein nanocages to encapsulate various molecular species, limited methods are available for subsequently opening a nanocage for cargo release under specific conditions. A modular platform with a specific protein-target-based mechanism of nanocage opening is notably lacking. To address this important technology gap, we present a new class of designed protein cages, the Ligand-Operable Cage (LOC). LOCs primarily comprise a protein nanocage core and a fused surface binding adaptor. The geometry of the LOC is designed so that binding of a target protein ligand (or multiple copies thereof) to the surface binder is sterically incompatible with retention of the assembled state of the cage. Therefore, the tight binding of a target ligand drives cage disassembly by mass action, subsequently exposing the encapsulated cargo. LOCs are modular; direct substitution of the surface binder sequence can reprogram the nanocage to open in response to any target protein ligand of interest. We demonstrate these design principles using both a natural and a designed protein cage as the core, with different proteins acting as the triggering ligand and with different reporter readouts─fluorescence unquenching and luminescence─for cage disassembly. These developments advance the critical problem of targeted molecular delivery and detection.

Preparation of Bunyavirus-Infected Cells for Electron Cryo-Tomography

Cellular electron cryo-tomography (cryoET) produces high-resolution three-dimensional images of subcellular structures in a near-native frozen-hydrated state. These three-dimensional images are obtained by recording a series of two-dimensional tilt images on a transmission electron cryo-microscope that are subsequently back-projected to form a tomogram. Key to a successful experiment is however a high-quality sample. This chapter outlines a basic workflow for the preparation of cellular cryoET samples. It covers the preparation of infected cells on electron cryo-microscopy grids and the vitrification by plunge-freezing and clipping of grids into AutoGrid rims. It also provides a general overview of the workflow for thinning the vitrified cells by focused ion beam (FIB) milling. Although this book is dedicated to Rift Valley fever virus research, the present protocol may also be applied to any other research subject where high-resolution structural insight into intracellular processes is desired.

Protein-based Nanoparticles: From Drug Delivery to Imaging, Nanocatalysis and Protein Therapy

Proteins and enzymes are versatile biomaterials for a wide range of medical applications due to their high specificity for receptors and substrates, high degradability, low toxicity, and overall good biocompatibility. Protein nanoparticles are formed by the arrangement of several native or modified proteins into nanometer-sized assemblies. In this review, we will focus on artificial nanoparticle systems, where proteins are the main structural element and not just an encapsulated payload. While under natural conditions, only certain proteins form defined aggregates and nanoparticles, chemical modifications or a change in the physical environment can further extend the pool of available building blocks. This allows the assembly of many globular proteins and even enzymes. These advances in preparation methods led to the emergence of new generations of nanosystems that extend beyond transport vehicles to diverse applications, from multifunctional drug delivery to imaging, nanocatalysis and protein therapy.

Analyzing the Geometry and Dynamics of Viral Structures: A Review of Computational Approaches Based on Alpha Shape Theory, Normal Mode Analysis, and Poisson-Boltzmann Theories

The current SARS-CoV-2 pandemic highlights our fragility when we are exposed to emergent viruses either directly or through zoonotic diseases. Fortunately, our knowledge of the biology of those viruses is improving. In particular, we have more and more structural information on virions, i.e., the infective form of a virus that includes its genomic material and surrounding protective capsid, and on their gene products. It is important to have methods that enable the analyses of structural information on such large macromolecular systems. We review some of those methods in this paper. We focus on understanding the geometry of virions and viral structural proteins, their dynamics, and their energetics, with the ambition that this understanding can help design antiviral agents. We discuss those methods in light of the specificities of those structures, mainly that they are huge. We focus on three of our own methods based on the alpha shape theory for computing geometry, normal mode analyses to study dynamics, and modified Poisson-Boltzmann theories to study the organization of ions and co-solvent and solvent molecules around biomacromolecules. The corresponding software has computing times that are compatible with the use of regular desktop computers. We show examples of their applications on some outer shells and structural proteins of the West Nile Virus.

Polymersomes as virus-surrogate particles for evaluating the performance of air filter materials

The development of antivirus air filter materials has attracted considerable interests due to the pandemic of coronavirus disease 2019 (COVID-19). Filtration efficiency (FE) of these materials against virus is critical in the assessment of their use in disease prevention. Due to the high cost and biosafety laboratory required for conducting research using actual virus samples, surrogates for virus are commonly used in the filtration test. Here, we explore the employment of polymersomes (polymeric vesicles) as a new type of surrogate. The polymersomes are hollow shell nanoparticles with amphiphilic bilayer membranes, which can be fabricated in nanosized, and possess similar size and structural features to virus. The performance of commercial KN95 mask and surgical mask with micro-sized fibers, and electrospun polyvinylidene fluoride (PVDF) and polyacrylonitrile (PAN) nanofibers were chosen to be evaluated. The filtration tests against fluorescent-labeled virus-surrogate particles (VSPs), i.e. polymersomes, allowed the determination of the FE of the multilayered filter materials in a layer-specific manner. The results suggested the importance of hydrophobicity in designing the nanofibrous filter materials. The employment of VSPs in filtration performance evaluation allows a cost-effective way to estimate the FE against virus, providing guidance on future development of air filter materials.

Spindle-shaped archaeal viruses evolved from rod-shaped ancestors to package a larger genome

Spindle- or lemon-shaped viruses infect archaea in diverse environments. Due to the highly pleomorphic nature of these virions, which can be found with cylindrical tails emanating from the spindle-shaped body, structural studies of these capsids have been challenging. We have determined the atomic structure of the capsid of Sulfolobus monocaudavirus 1, a virus that infects hosts living in nearly boiling acid. A highly hydrophobic protein, likely integrated into the host membrane before the virions assemble, forms 7 strands that slide past each other in both the tails and the spindle body. We observe the discrete steps that occur as the tail tubes expand, and these are due to highly conserved quasiequivalent interactions with neighboring subunits maintained despite significant diameter changes. Our results show how helical assemblies can vary their diameters, becoming nearly spherical to package a larger genome and suggest how all spindle-shaped viruses have evolved from archaeal rod-like viruses.

Broad-Spectrum Antiviral Peptides and Polymers

As the human cost of the pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is still being witnessed worldwide, the development of broad-spectrum antiviral agents against emerging and re-emerging viruses is seen as a necessity to hamper the spread of infections. Various targets during the viral life-cycle can be considered to inhibit viral infection, from viral attachment to viral fusion or replication. Macromolecules represent a particularly attractive class of therapeutics due to their multivalency and versatility. Although several antiviral macromolecules hold great promise in clinical applications, the emergence of resistance after prolonged exposure urges the need for improved solutions. In the present article, the recent advancement in the discovery of antiviral peptides and polymers with diverse structural features and antiviral mechanisms is reviewed. Future perspectives, such as, the development of virucidal peptides/polymers and their coatings against SARS-CoV-2 infection, standardization of antiviral testing protocols, and use of artificial intelligence or machine learning as a tool to accelerate the discovery of antiviral macromolecules, are discussed.

Crystal structure of the potato leafroll virus coat protein and implications for viral assembly

Luteoviruses, poleroviruses, and enamoviruses are insect-transmitted, agricultural pathogens that infect a wide array of plants, including staple food crops. Previous cryo-electron microscopy studies of virus-like particles show that luteovirid viral capsids are built from a structural coat protein that organizes with T = 3 icosahedral symmetry. Here, we present the crystal structure of a truncated version of the coat protein monomer from potato leafroll virus at 1.80-Å resolution. In the crystal lattice, monomers pack into flat sheets that preserve the two-fold and three-fold axes of icosahedral symmetry and show minimal structural deviations when compared to the full-length subunits of the assembled virus-like particle. These observations have important implications in viral assembly and maturation and suggest that the CP N-terminus and its interactions with RNA play an important role in generating capsid curvature.