Structural Insights into Reovirus σ1 Interactions with Two Neutralizing Antibodies.

Reovirus attachment protein σ1 engages glycan receptors and junctional adhesion molecule-A (JAM-A) and is thought to undergo a conformational change during the proteolytic disassembly of virions to infectious subvirion particles (ISVPs) that accompanies cell entry. The σ1 protein is also the primary target of neutralizing antibodies. Here, we present a structural and functional characterization of two neutralizing antibodies that target σ1 of serotype 1 (T1) and serotype 3 (T3) reoviruses. The crystal structures revealed that each antibody engages its cognate σ1 protein within the head domain via epitopes distinct from the JAM-A-binding site. Surface plasmon resonance and cell-binding assays indicated that both antibodies likely interfere with JAM-A engagement by steric hindrance. To define the interplay between the carbohydrate receptor and antibody binding, we conducted hemagglutination inhibition assays using virions and ISVPs. The glycan-binding site of T1 σ1 is located in the head domain and is partly occluded by the bound Fab in the crystal structure. The T1-specific antibody inhibited hemagglutination by virions and ISVPs, probably via direct interference with glycan engagement. In contrast to T1 σ1, the carbohydrate-binding site of T3 σ1 is located in the tail domain, distal to the antibody epitope. The T3-specific antibody inhibited hemagglutination by T3 virions but not ISVPs, indicating that the antibody- and glycan-binding sites in σ1 are in closer spatial proximity on virions than on ISVPs. Our results provide direct evidence for a structural rearrangement of σ1 during virion-to-ISVP conversion and contribute new information about the mechanisms of antibody-mediated neutralization of reovirus. IMPORTANCE: Virus attachment proteins mediate binding to host cell receptors, serve critical functions in cell and tissue tropism, and are often targeted by the neutralizing antibody response. The structural investigation of antibody-antigen complexes can provide valuable information for understanding the molecular basis of virus neutralization. Studies with enveloped viruses, such as HIV and influenza virus, have helped to define sites of vulnerability and guide vaccination strategies. By comparison, less is known about antibody binding to nonenveloped viruses. Here, we structurally investigated two neutralizing antibodies that bind the attachment protein σ1 of reovirus. Furthermore, we characterized the neutralization efficiency, the binding affinity for σ1, and the effect of the antibodies on reovirus receptor engagement. Our analysis defines reovirus interactions with two neutralizing antibodies, allows us to propose a mechanism by which they block virus infection, and provides evidence for a conformational change in the σ1 protein during viral cell entry.

Hepatitis B Virus Capsids Have Diverse Structural Responses to Small-Molecule Ligands Bound to the Heteroaryldihydropyrimidine Pocket.

UNLABELLED: Though the hepatitis B virus (HBV) core protein is an important participant in many aspects of the viral life cycle, its best-characterized activity is self-assembly into 240-monomer capsids. Small molecules that target core protein (core protein allosteric modulators [CpAMs]) represent a promising antiviral strategy. To better understand the structural basis of the CpAM mechanism, we determined the crystal structure of the HBV capsid in complex with HAP18. HAP18 accelerates assembly, increases protein-protein association more than 100-fold, and induces assembly of nonicosahedral macrostructures. In a preformed capsid, HAP18 is found at quasiequivalent subunit-subunit interfaces. In a detailed comparison to the two other extant CpAM structures, we find that the HAP18-capsid structure presents a paradox. Whereas the two other structures expanded the capsid diameter by up to 10 Å, HAP18 caused only minor changes in quaternary structure and actually decreased the capsid diameter by ∼3 Å. These results indicate that CpAMs do not have a single allosteric effect on capsid structure. We suggest that HBV capsids present an ensemble of states that can be trapped by CpAMs, indicating a more complex basis for antiviral drug design. IMPORTANCE: Hepatitis B virus core protein has multiple roles in the viral life cycle-assembly, compartment for reverse transcription, intracellular trafficking, and nuclear functions-making it an attractive antiviral target. Core protein allosteric modulators (CpAMs) are an experimental class of antivirals that bind core protein. The most recognized CpAM activity is that they accelerate core protein assembly and strengthen interactions between subunits. In this study, we observe that the CpAM-binding pocket has multiple conformations. We compare structures of capsids cocrystallized with different CpAMs and find that they also affect quaternary structure in different ways. These results suggest that the capsid "breathes" and is trapped in different states by the drug and crystallization. Understanding that the capsid is a moving target will aid drug design and improve our understanding of HBV interaction with its environment.

The hepatitis B virus core protein intradimer interface modulates capsid assembly and stability.

During the hepatitis B virus (HBV) life cycle, capsid assembly and disassembly must ensure correct packaging and release of the viral genome. Here we show that changes in the dynamics of the core protein play an important role in regulating these processes. The HBV capsid assembles from 120 copies of the core protein homodimer. Each monomer contains a conserved cysteine at position 61 that can form an intradimer disulfide that we use as a marker for dimer conformational states. We show that dimers in the context of capsids form intradimer disulfides relatively rapidly. Surprisingly, compared to reduced dimers, fully oxidized dimers assembled slower and into capsids that were morphologically similar but less stable. We hypothesize that oxidized protein adopts a geometry (or constellation of geometries) that is unfavorable for capsid assembly, resulting in weaker dimer-dimer interactions as well as slower assembly kinetics. Our results suggest that structural flexibility at the core protein intradimer interface is essential for regulating capsid assembly and stability. We further suggest that capsid destabilization by the C61-C61 disulfide has a regulatory function to support capsid disassembly and release of the viral genome.

Conformational changes in the hepatitis B virus core protein are consistent with a role for allostery in virus assembly.

In infected cells, virus components must be organized at the right place and time to ensure assembly of infectious virions. From a different perspective, assembly must be prevented until all components are available. Hypothetically, this can be achieved by allosterically controlling assembly. Consistent with this hypothesis, here we show that the structure of the hepatitis B virus (HBV) core protein dimer, which can spontaneously self-assemble, is incompatible with capsid assembly. Systematic differences between core protein dimer and capsid conformations demonstrate linkage between the intradimer interface and interdimer contact surface. These structures also provide explanations for the capsid-dimer selectivity of some antibodies and the activities of assembly effectors. Solution studies suggest that the assembly-inactive state is more accurately an ensemble of conformations. Simulations show that allostery supports controlled assembly and results in capsids that are resistant to dissociation. We propose that allostery, as demonstrated in HBV, is common to most self-assembling viruses.

Simplified Reverse Genetics Method to Recover Recombinant Rotaviruses Expressing Reporter Proteins.

Rotaviruses are a large and evolving population of segmented double-stranded RNA viruses that cause severe gastroenteritis in the young of many mammalian and avian host species, including humans. With the recent advent of rotavirus reverse genetics systems, it has become possible to use directed mutagenesis to explore rotavirus biology, modify and optimize existing rotavirus vaccines, and develop rotavirus multitarget vaccine vectors. In this report, we describe a simplified reverse genetics system that allows the efficient and reliable recovery of recombinant rotaviruses. The system is based on co-transfection of T7 transcription vectors expressing full-length rotavirus (+)RNAs and a CMV vector encoding an RNA capping enzyme into BHK cells constitutively producing T7 RNA polymerase (BHK-T7). Recombinant rotaviruses are amplified by overseeding the transfected BHK-T7 cells with MA104 cells, a monkey kidney cell line that is highly permissive for virus growth. In this report, we also describe an approach for generating recombinant rotaviruses that express a separate fluorescent reporter protein through the introduction of a 2A translational stop-restart element into genome segment 7 (NSP3). This approach avoids deleting or modifying any of the viral open reading frames, thus allowing the production of recombinant rotaviruses that retain fully functional viral proteins while expressing a fluorescent protein.

A mutant hepatitis B virus core protein mimics inhibitors of icosahedral capsid self-assembly.

Understanding self-assembly of icosahedral virus capsids is critical to developing assembly directed antiviral approaches and will also contribute to the development of self-assembling nanostructures. One approach to controlling assembly would be through the use of assembly inhibitors. Here we use Cp149, the assembly domain of the hepatitis B virus capsid protein, together with an assembly defective mutant, Cp149-Y132A, to examine the limits of the efficacy of assembly inhibitors. By itself, Cp149-Y132A will not form capsids. However, Cp-Y132A will coassemble with the wild-type protein on the basis of light scattering and size exclusion chromatography. The resulting capsids appear to be indistinguishable from normal capsids. However, coassembled capsids are more fragile, with disassembly observed by chromatography under mildly destabilizing conditions. The relative persistence of capsids assembled under conditions where association energy is weak compared to the fragility of those where association is strong suggests a mechanism of "thermodynamic editing" that allows replacement of defective proteins in a weakly associated complex. There is fine line between weak assembly, where assembly defective protein is edited from a growing capsid, and relatively strong assembly, where assembly defective subunits may dramatically compromise virus stability. Thus, attempts to control virus self-assembly (with small molecules or defective proteins) must take into account the competing process of thermodynamic editing.

Evolution of Intermediates during Capsid Assembly of Hepatitis B Virus with Phenylpropenamide-Based Antivirals.

Self-assembly of virus capsids is a potential target for antivirals due to its importance in the virus lifecycle. Here, we investigate the effect of phenylpropenamide derivatives B-21 and AT-130 on the assembly of hepatitis B virus (HBV) core protein. Phenylpropenamides are widely believed to yield assembly of spherical particles resembling native, empty HBV capsids. Because the details of assembly can be overlooked with ensemble measurements, we performed resistive-pulse sensing on nanofluidic devices with four pores in series to characterize the size distributions of the products in real time. With its single particle sensitivity and compatibility with typical assembly buffers, resistive-pulse sensing is well-suited for analyzing virus assembly in vitro. We observed that assembly with B-21 and AT-130 produced a large fraction of partially complete virus particles that may be on-path, off-path, or trapped. For both B-21 and AT-130, capsid assembly was more sensitive to disruption under conditions where the interprotein association energy was low at lower salt concentrations. Dilution of the reaction solutions led to the rearrangement of the incomplete particles and demonstrated that these large intermediates may be on-path, but are labile, and exist in a frustrated dynamic equilibrium. During capsid assembly, phenylpropenamide molecules modestly increase the association energy of dimers, prevent intermediates from dissociating, and lead to kinetic trapping where the formation of too many capsids has been initiated, which results in both empty and incomplete particles.

Collection of Recombinant Rotaviruses Expressing Fluorescent Reporter Proteins.

A collection of recombinant rotaviruses that express the fluorescent markers UnaG, mKate, mRuby, TagBFP, CFP, or YFP as separate proteins was generated. Genes for the fluorescent proteins were inserted into genome segment 7 without compromising expression of the protein NSP3. These recombinant rotaviruses are valuable for analyzing rotavirus biology by fluorescence-based live-cell imaging.

Repurposing staples for viruses: applying peptide design to RSV prophylaxis.

Respiratory syncytial virus (RSV) is responsible for lower respiratory tract infections and annually results in 200,000 deaths worldwide. Despite the burden of RSV-associated disease, treatments and preventative measures are limited. In this issue of JCI, Bird and colleagues describe their work using a peptide stapling technique that allowed synthesis of a stable peptide mimic of a portion of the RSV fusion protein. Pretreatment of cells with the stable peptide effectively blocked virus entry. When introduced into mice prior to RSV exposure, the peptide produced a substantial prophylactic effect. This work provides a new way forward in RSV prevention.

Assembly-directed antivirals differentially bind quasiequivalent pockets to modify hepatitis B virus capsid tertiary and quaternary structure.

Hepatitis B virus (HBV) is a major cause of liver disease. Assembly of the HBV capsid is a critical step in virus production and an attractive target for new antiviral therapies. We determined the structure of HBV capsid in complex with AT-130, a member of the phenylpropenamide family of assembly effectors. AT-130 causes tertiary and quaternary structural changes but does not disrupt capsid structure. AT-130 binds a hydrophobic pocket that also accommodates the previously characterized heteroaryldihydropyrimidine compounds but favors a unique quasiequivalent location on the capsid surface. Thus, this pocket is a promiscuous drug-binding site and a likely target for different assembly effectors with a broad range of mechanisms of activity. That AT-130 successfully decreases virus production by increasing capsid assembly rate without disrupting capsid structure delineates a paradigm in antiviral design, that disrupting reaction timing is a viable strategy for assembly effectors of HBV and other viruses.

Trapping of hepatitis B virus capsid assembly intermediates by phenylpropenamide assembly accelerators.

Understanding the biological self-assembly process of virus capsids is key to understanding the viral life cycle, as well as serving as a platform for the design of assembly-based antiviral drugs. Here we identify and characterize the phenylpropenamide family of small molecules, known to have antiviral activity in vivo, as assembly effectors of the hepatitis B virus (HBV) capsid. We have found two representative phenylpropenamides to be assembly accelerators, increasing the rate of assembly with only modest increases in the stability of the HBV capsids; these data provide a physical-chemical basis for their antiviral activity. Unlike previously described HBV assembly effectors, the phenylpropenamides do not misdirect assembly; rather, the accelerated reactions proceed on-path to produce morphologically normal capsids. However, capsid assembly in the presence of phenylpropenamides is characterized by kinetic trapping of assembly intermediates. These traps resolve under conditions close to physiological, but we found that trapped intermediates persist under conditions that favor phenylpropenamide binding and strong core protein-protein interactions. The phenylpropenamides serve as chemical probes of the HBV capsid assembly pathway by trapping on-path assembly intermediates, illustrating the governing influence of reaction kinetics on capsid assembly.