Reversion of Ebolavirus Disease from a Single Intramuscular Injection of a Pan-Ebolavirus Immunotherapeutic.

Intravenous (IV) administration of antiviral monoclonal antibodies (mAbs) can be challenging, particularly during an ongoing epidemic, due to the considerable resources required for performing infusions. An ebolavirus therapeutic administered via intramuscular (IM) injection would reduce the burdens associated with IV infusion and allow rapid treatment of exposed individuals during an outbreak. Here, we demonstrate how MBP134, a cocktail of two pan-ebolavirus mAbs, reverses the course of Sudan ebolavirus disease (Gulu variant) with a single IV or IM dose in non-human primates (NHPs) as late as five days post-exposure. We also investigate the utility of adding half-life extension mutations to the MBP134 mAbs, ultimately creating a half-life extended cocktail designated MBP431. When delivered as a post-exposure prophylactic or therapeutic, a single IM dose of MBP431 offered complete or significant protection in NHPs challenged with Zaire ebolavirus. In conjunction with previous studies, these results support the use of MBP431 as a rapidly deployable IM medical countermeasure against every known species of ebolavirus.

Genotype-specific features reduce the susceptibility of South American yellow fever virus strains to vaccine-induced antibodies.

The resurgence of yellow fever in South America has prompted vaccination against the etiologic agent, yellow fever virus (YFV). Current vaccines are based on a live-attenuated YF-17D virus derived from a virulent African isolate. The capacity of these vaccines to induce neutralizing antibodies against the vaccine strain is used as a surrogate for protection. However, the sensitivity of genetically distinct South American strains to vaccine-induced antibodies is unknown. We show that antiviral potency of the polyclonal antibody response in vaccinees is attenuated against an emergent Brazilian strain. This reduction was attributable to amino acid changes at two sites in central domain II of the glycoprotein E, including multiple changes at the domain I-domain II hinge, which are unique to and shared among most South American YFV strains. Our findings call for a reevaluation of current approaches to YFV immunological surveillance in South America and suggest approaches for updating vaccines.

Cross-Strain Neutralizing and Protective Monoclonal Antibodies against EEEV or WEEV.

The three encephalitic alphaviruses, namely, the Venezuelan, eastern, and western equine encephalitis viruses (VEEV, EEEV, and WEEV), are classified by the Centers for Disease Control and Prevention (CDC) as biothreat agents. Currently, no licensed medical countermeasures (MCMs) against these viruses are available for humans. Neutralizing antibodies (NAbs) are fast-acting and highly effective MCMs for use in both pre- and post-exposure settings against biothreat agents. While significant work has been done to identify anti-VEEV NAbs, less has been done to identify NAbs against EEEV and WEEV. In order to develop anti-EEEV or -WEEV NAbs, mice were immunized using complementary strategies with a variety of different EEEV or WEEV immunogens to maximize the generation of NAbs to each of these viruses. Of the hybridomas generated, three anti-EEEV and seven anti-WEEV monoclonal antibodies were identified with in vitro neutralization activity. The most potent neutralizers (two anti-EEEV NAbs and three anti-WEEV NAbs) were further evaluated for neutralization activity against additional strains of EEEV, a single strain of Madariaga virus (formerly South American EEEV), or WEEV. Of these, G1-2-H4 and G1-4-C3 neutralized all three EEEV strains and the Madariaga virus strain, whereas G8-2-H9 and 12 WA neutralized six out of eight WEEV strains. To determine the protective efficacy of these NAbs, the five most potent neutralizers were evaluated in respective mouse aerosol challenge models. All five NAbs demonstrated various levels of protection when administered at doses of 2.5 mg/kg or 10 mg/kg 24 h before the respective virus exposure via the aerosol route. Of these, anti-EEEV NAb G1-4-C3 and anti-WEEV NAb 8C2 provided 100% protection at both doses and all surviving mice were free of clinical signs throughout the study. Additionally, no virus was detected in the brain 14 days post virus exposure. Taken together, efficacious NAbs were developed that demonstrate the potential for the development of cross-strain antibody-based MCMs against EEEV and WEEV infections.

Protective neutralizing antibodies from human survivors of Crimean-Congo hemorrhagic fever.

Crimean-Congo hemorrhagic fever virus (CCHFV) is a World Health Organization priority pathogen. CCHFV infections cause a highly lethal hemorrhagic fever for which specific treatments and vaccines are urgently needed. Here, we characterize the human immune response to natural CCHFV infection to identify potent neutralizing monoclonal antibodies (nAbs) targeting the viral glycoprotein. Competition experiments showed that these nAbs bind six distinct antigenic sites in the Gc subunit. These sites were further delineated through mutagenesis and mapped onto a prefusion model of Gc. Pairwise screening identified combinations of non-competing nAbs that afford synergistic neutralization. Further enhancements in neutralization breadth and potency were attained by physically linking variable domains of synergistic nAb pairs through bispecific antibody (bsAb) engineering. Although multiple nAbs protected mice from lethal CCHFV challenge in pre- or post-exposure prophylactic settings, only a single bsAb, DVD-121-801, afforded therapeutic protection. DVD-121-801 is a promising candidate suitable for clinical development as a CCHFV therapeutic.

Therapy for Argentine hemorrhagic fever in nonhuman primates with a humanized monoclonal antibody.

The COVID-19 pandemic has reemphasized the need to identify safe and scalable therapeutics to slow or reverse symptoms of disease caused by newly emerging and reemerging viral pathogens. Recent clinical successes of monoclonal antibodies (mAbs) in therapy for viral infections demonstrate that mAbs offer a solution for these emerging biothreats. We have explored this with respect to Junin virus (JUNV), an arenavirus classified as a category A high-priority agent and the causative agent of Argentine hemorrhagic fever (AHF). There are currently no Food and Drug Administration-approved drugs available for preventing or treating AHF, although immune plasma from convalescent patients is used routinely to treat active infections. However, immune plasma is severely limited in quantity, highly variable in quality, and poses significant safety risks including the transmission of transfusion-borne diseases. mAbs offer a highly specific and consistently potent alternative to immune plasma that can be manufactured at large scale. We previously described a chimeric mAb, cJ199, that provided protection in a guinea pig model of AHF. To adapt this mAb to a format more suitable for clinical use, we humanized the mAb (hu199) and evaluated it in a cynomolgus monkey model of AHF with two JUNV isolates, Romero and Espindola. While untreated control animals experienced 100% lethality, all animals treated with hu199 at 6 d postinoculation (dpi) survived, and 50% of animals treated at 8 dpi survived. mAbs like hu199 may offer a safer, scalable, and more reproducible alternative to immune plasma for rare viral diseases that have epidemic potential.

Longitudinal dynamics of the human B cell response to the yellow fever 17D vaccine.

A comprehensive understanding of the development and evolution of human B cell responses induced by pathogen exposure will facilitate the design of next-generation vaccines. Here, we utilized a high-throughput single B cell cloning technology to longitudinally track the human B cell response to the yellow fever virus 17D (YFV-17D) vaccine. The early memory B cell (MBC) response was mediated by both classical immunoglobulin M (IgM) (IgM+CD27+) and switched immunoglobulin (swIg+) MBC populations; however, classical IgM MBCs waned rapidly, whereas swIg+ and atypical IgM+ and IgD+ MBCs were stable over time. Affinity maturation continued for 6 to 9 mo following vaccination, providing evidence for the persistence of germinal center activity long after the period of active viral replication in peripheral blood. Finally, a substantial fraction of the neutralizing antibody response was mediated by public clones that recognize a fusion loop-proximal antigenic site within domain II of the viral envelope glycoprotein. Overall, our findings provide a framework for understanding the dynamics and complexity of human B cell responses elicited by infection and vaccination.

Capsids and Portals Influence Each Other's Conformation During Assembly and Maturation.

The portal proteins of tailed bacteriophage and Herpesvirus capsids form dodecameric rings that occupy one capsid vertex and are incorporated during the assembly of capsid precursors called procapsids or proheads. Portals are essential and serve as the pore for DNA transit and the site of tail attachment; however, bacteriophage HK97 capsid proteins assemble efficiently without a portal when expressed from plasmids. Following portal co-expression, portals were incorporated into about half of the proheads that were made. In the absence of active capsid maturation protease, uncleaved proheads formed dimers, trimers, and tetramers of proheads during purification, but only if they had portals. These appeared bound to membrane-like fragments by their portals and could be disaggregated by detergents, supporting a role for membranes in their formation and in capsid assembly. The precursors to prohead oligomers were detected in cell extracts. These were able to bind to Octyl-Sepharose and could be released by detergent, while uncleaved proheads without portal or cleaved proheads with portal did not bind. Our results document a discrete change in the HK97 portal's hydrophobicity induced by cleavage of the procapsid shell in which it is embedded. Additionally, we detected an increase in the rate of expansion induced by the presence of a portal complex in cleaved HK97 proheads. These results suggest that portals and capsids influence each other's conformation during assembly. The formation of prohead oligomers also provides a rapid and sensitive assay for identification and analysis of portal incorporation mutants.

Structure and Characterization of Crimean-Congo Hemorrhagic Fever Virus GP38.

Crimean-Congo hemorrhagic fever virus (CCHFV) is the causative agent of the most widespread tick-borne viral infection in humans. CCHFV encodes a secreted glycoprotein (GP38) of unknown function that is the target of a protective antibody. Here, we present the crystal structure of GP38 at a resolution of 2.5 Å, which revealed a novel fold primarily consisting of a 3-helix bundle and a ß-sandwich. Sequence alignment and homology modeling showed distant homology between GP38 and the ectodomain of Gn (a structural glycoprotein in CCHFV), suggestive of a gene duplication event. Analysis of convalescent-phase sera showed high titers of GP38 antibodies indicating immunogenicity in humans during natural CCHFV infection. The only protective antibody for CCHFV in an adult mouse model reported to date, 13G8, bound GP38 with subnanomolar affinity and protected against heterologous CCHFV challenge in a STAT1-knockout mouse model. Our data strongly suggest that GP38 should be evaluated as a vaccine antigen and that its structure provides a foundation to investigate functions of this protein in the viral life cycle.IMPORTANCE Crimean-Congo hemorrhagic fever virus (CCHFV) is a priority pathogen that poses a high risk to public health. Due to the high morbidity and mortality rates associated with CCHFV infection, there is an urgent need to develop medical countermeasures for disease prevention and treatment. CCHFV GP38, a secreted glycoprotein of unknown function unique to the Nairoviridae family, was recently shown to be the target of a protective antibody against CCHFV. Here, we present the crystal structure of GP38, which revealed a novel fold with distant homology to another CCHFV glycoprotein that is suggestive of a gene duplication event. We also demonstrate that antibody 13G8 protects STAT1-knockout mice against heterologous CCHFV challenge using a clinical isolate from regions where CCHFV is endemic. Collectively, these data advance our understanding of GP38 structure and antigenicity and should facilitate future studies investigating its function.

Therapeutic monoclonal antibody treatment protects nonhuman primates from severe Venezuelan equine encephalitis virus disease after aerosol exposure.

There are no FDA licensed vaccines or therapeutics for Venezuelan equine encephalitis virus (VEEV) which causes a debilitating acute febrile illness in humans that can progress to encephalitis. Previous studies demonstrated that murine and macaque monoclonal antibodies (mAbs) provide prophylactic and therapeutic efficacy against VEEV peripheral and aerosol challenge in mice. Additionally, humanized versions of two neutralizing mAbs specific for the E2 glycoprotein, 1A3B-7 and 1A4A-1, administered singly protected mice against aerosolized VEEV. However, no studies have demonstrated protection in nonhuman primate (NHP) models of VEEV infection. Here, we evaluated a chimeric antibody 1A3B-7 (c1A3B-7) containing mouse variable regions on a human IgG framework and a humanized antibody 1A4A-1 containing a serum half-life extension modification (Hu-1A4A-1-YTE) for their post-exposure efficacy in NHPs exposed to aerosolized VEEV. Approximately 24 hours after exposure, NHPs were administered a single bolus intravenous mAb. Control NHPs had typical biomarkers of VEEV infection including measurable viremia, fever, and lymphopenia. In contrast, c1A3B-7 treated NHPs had significant reductions in viremia and lymphopenia and on average approximately 50% reduction in fever. Although not statistically significant, Hu-1A4A-1-YTE administration did result in reductions in viremia and fever duration. Delay of treatment with c1A3B-7 to 48 hours post-exposure still provided NHPs protection from severe VEE disease through reductions in viremia and fever. These results demonstrate that post-exposure administration of c1A3B-7 protected macaques from development of severe VEE disease even when administered 48 hours following aerosol exposure and describe the first evaluations of VEEV-specific mAbs for post-exposure prophylactic use in NHPs. Viral mutations were identified in one NHP after c1A3B-7 treatment administered 24 hrs after virus exposure. This suggests that a cocktail-based therapy, or an alternative mAb against an epitope that cannot mutate without resulting in loss of viral fitness may be necessary for a highly effective therapeutic.

Cross-reactive neutralizing human survivor monoclonal antibody BDBV223 targets the ebolavirus stalk.

Three Ebolavirus genus viruses cause lethal disease and lack targeted therapeutics: Ebola virus, Sudan virus and Bundibugyo virus. Monoclonal antibody (mAb) cocktails against the surface glycoprotein (GP) present a potential therapeutic strategy. Here we report two crystal structures of the antibody BDBV223, alone and complexed with its GP2 stalk epitope, an interesting site for therapeutic/vaccine design due to its high sequence conservation among ebolaviruses. BDBV223, identified in a human survivor of Bundibugyo virus disease, neutralizes both Bundibugyo virus and Ebola virus, but not Sudan virus. Importantly, the structure suggests that BDBV223 binding interferes with both the trimeric bundle assembly of GP and the viral membrane by stabilizing a conformation in which the monomers are separated by GP lifting or bending. Targeted mutagenesis of BDBV223 to enhance SUDV GP recognition indicates that additional determinants of antibody binding likely lie outside the visualized interactions, and perhaps involve quaternary assembly or membrane-interacting regions.

Structural basis of broad ebolavirus neutralization by a human survivor antibody.

The structural features that govern broad-spectrum activity of broadly neutralizing anti-ebolavirus antibodies (Abs) outside of the internal fusion loop epitope are currently unknown. Here we describe the structure of a broadly neutralizing human monoclonal Ab (mAb), ADI-15946, which was identified in a human survivor of the 2013-2016 outbreak. The crystal structure of ADI-15946 in complex with cleaved Ebola virus glycoprotein (EBOV GPCL) reveals that binding of the mAb structurally mimics the conserved interaction between the EBOV GP core and its glycan cap ß17-ß18 loop to inhibit infection. Both endosomal proteolysis of EBOV GP and binding of mAb FVM09 displace this loop, thereby increasing exposure of ADI-15946's conserved epitope and enhancing neutralization. Our work also mapped the paratope of ADI-15946, thereby explaining reduced activity against Sudan virus, which enabled rational, structure-guided engineering to enhance binding and neutralization of Sudan virus while retaining the parental activity against EBOV and Bundibugyo virus.

Development of a Human Antibody Cocktail that Deploys Multiple Functions to Confer Pan-Ebolavirus Protection.

Passive administration of monoclonal antibodies (mAbs) is a promising therapeutic approach for Ebola virus disease (EVD). However, all mAbs and mAb cocktails that have entered clinical development are specific for a single member of the Ebolavirus genus, Ebola virus (EBOV), and ineffective against outbreak-causing Bundibugyo virus (BDBV) and Sudan virus (SUDV). Here, we advance MBP134, a cocktail of two broadly neutralizing human mAbs, ADI-15878 from an EVD survivor and ADI-23774 from the same survivor but specificity-matured for SUDV GP binding affinity, as a candidate pan-ebolavirus therapeutic. MBP134 potently neutralized all ebolaviruses and demonstrated greater protective efficacy than ADI-15878 alone in EBOV-challenged guinea pigs. A second-generation cocktail, MBP134AF, engineered to effectively harness natural killer (NK) cells afforded additional improvement relative to its precursor in protective efficacy against EBOV and SUDV in guinea pigs. MBP134AF is an optimized mAb cocktail suitable for evaluation as a pan-ebolavirus therapeutic in nonhuman primates.

Structural Basis of Pan-Ebolavirus Neutralization by a Human Antibody against a Conserved, yet Cryptic Epitope.

Only one naturally occurring human antibody has been described thus far that is capable of potently neutralizing all five ebolaviruses. Here we present two crystal structures of this rare, pan-ebolavirus neutralizing human antibody in complex with Ebola virus and Bundibugyo virus glycoproteins (GPs), respectively. The structures delineate the key protein and glycan contacts for binding that are conserved across the ebolaviruses, explain the antibody's unique broad specificity and neutralization activity, and reveal the likely mechanism behind a known escape mutation in the fusion loop region of GP2. We found that the epitope of this antibody, ADI-15878, extends along the hydrophobic paddle of the fusion loop and then dips down into a highly conserved pocket beneath the N-terminal tail of GP2, a mode of recognition unlike any other antibody elicited against Ebola virus, and likely critical for its broad activity. The fold of Bundibugyo virus glycoprotein, not previously visualized, is similar to the fold of Ebola virus GP, and ADI-15878 binds to each virus's GP with a similar strategy and angle of attack. These findings will be useful in deployment of this antibody as a broad-spectrum therapeutic and in the design of immunogens that elicit the desired broadly neutralizing immune response against all members of the ebolavirus genus and filovirus family.IMPORTANCE There are five different members of the Ebolavirus genus. Provision of vaccines and treatments able to protect against any of the five ebolaviruses is an important goal of public health. Antibodies are a desired result of vaccines and can be delivered directly as therapeutics. Most antibodies, however, are effective against only one or two, not all, of these pathogens. Only one human antibody has been thus far described to neutralize all five ebolaviruses, antibody ADI-15878. Here we describe the molecular structure of ADI-15878 bound to the relevant target proteins of Ebola virus and Bundibugyo virus. We explain how it achieves its rare breadth of activity and propose strategies to design improved vaccines capable of eliciting more antibodies like ADI-15878.

The Ebola Virus VP30-NP Interaction Is a Regulator of Viral RNA Synthesis.

Filoviruses are capable of causing deadly hemorrhagic fevers. All nonsegmented negative-sense RNA-virus nucleocapsids are composed of a nucleoprotein (NP), a phosphoprotein (VP35) and a polymerase (L). However, the VP30 RNA-synthesis co-factor is unique to the filoviruses. The assembly, structure, and function of the filovirus RNA replication complex remain unclear. Here, we have characterized the interactions of Ebola, Sudan and Marburg virus VP30 with NP using in vitro biochemistry, structural biology and cell-based mini-replicon assays. We have found that the VP30 C-terminal domain interacts with a short peptide in the C-terminal region of NP. Further, we have solved crystal structures of the VP30-NP complex for both Ebola and Marburg viruses. These structures reveal that a conserved, proline-rich NP peptide binds a shallow hydrophobic cleft on the VP30 C-terminal domain. Structure-guided Ebola virus VP30 mutants have altered affinities for the NP peptide. Correlation of these VP30-NP affinities with the activity for each of these mutants in a cell-based mini-replicon assay suggests that the VP30-NP interaction plays both essential and inhibitory roles in Ebola virus RNA synthesis.

A Single Maturation Cleavage Site in Adenovirus Impacts Cell Entry and Capsid Assembly.

UNLABELLED: Proteolytic maturation drives the conversion of stable, immature virus particles to a mature, metastable state primed for cell infection. In the case of human adenovirus, this proteolytic cleavage is mediated by the virally encoded protease AVP. Protein VI, an internal capsid cement protein and substrate for AVP, is cleaved at two sites, one of which is near the N terminus of the protein. In mature capsids, the 33 residues at the N terminus of protein VI (pVIn) are sequestered inside the cavity formed by peripentonal hexon trimers at the 5-fold vertex. Here, we describe a glycine-to-alanine mutation in the N-terminal cleavage site of protein VI that profoundly impacts proteolytic processing, the generation of infectious particles, and cell entry. The phenotypic effects associated with this mutant provide a mechanistic framework for understanding the multifunctional nature of protein VI. Based on our findings, we propose that the primary function of the pVIn peptide is to mediate interactions between protein VI and hexon during virus replication, driving hexon nuclear accumulation and particle assembly. Once particles are assembled, AVP-mediated cleavage facilitates the release of the membrane lytic region at the amino terminus of mature VI, allowing it to lyse the endosome during cell infection. These findings highlight the importance of a single maturation cleavage site for both infectious particle production and cell entry and emphasize the exquisite spatiotemporal regulation governing adenovirus assembly and disassembly. IMPORTANCE: Postassembly virus maturation is a cornerstone principle in virology. However, a mechanistic understanding of how icosahedral viruses utilize this process to transform immature capsids into infection-competent particles is largely lacking. Adenovirus maturation involves proteolytic processing of seven precursor proteins. There is currently no information for the role of each independent cleavage event in the generation of infectious virions. To address this, we investigated the proteolytic maturation of one adenovirus precursor molecule, protein VI. Structurally, protein VI cements the outer capsid shell and links it to the viral core. Functionally, protein VI is involved in endosome disruption, subcellular trafficking, transcription activation, and virus assembly. Our studies demonstrate that the multifunctional nature of protein VI is largely linked to its maturation. Through mutational analysis, we show that disrupting the N-terminal cleavage of preprotein VI has major deleterious effects on the assembly of infectious virions and their subsequent ability to infect host cells.

The delta domain of the HK97 major capsid protein is essential for assembly.

The 102 residue N-terminal extension of the HK97 major capsid protein, the delta domain, is normally present during the assembly of immature HK97 procapsids, but it is removed during maturation like well-known internal scaffolding proteins of other tailed phages and herpesviruses. The delta domain also shares other unusual properties usually found in other viral and phage scaffolding proteins, including its location on the inside of the capsid, a high predicted and measured α-helical content, and an additional prediction for the ability to form parallel coiled-coils. Viral scaffolding proteins are essential for capsid assembly and phage viability, so we tested whether the HK97 delta domain was essential for capsid assembly. We studied the effects of deleting all or parts of the delta domain on capsid assembly and on complementation of capsid-protein-defective phage, and our results demonstrate that the delta domain is required for HK97 capsid assembly.

Inhibition of adenovirus replication by a trisubstituted piperazin-2-one derivative.

The number of disseminated adenovirus (Ad) infections continues to increase mostly due to the growing use of immunosuppressive treatments. Recipients of solid organ or hematopoietic stem cell transplants, mainly in pediatric units, exhibit a high morbidity and mortality due to these infections. Unfortunately, there are no Ad-specific antiviral drugs currently approved for medical use. To address this situation, we used high-throughput screening (HTS) of synthetic small molecule libraries to identify compounds that restrict Ad infection. Among the more than 25,000 compounds screened, we identified a hit compound that significantly inhibited Ad infection. The compound (15D8) is a trisubstituted piperazin-2-one derivative that showed substantial antiviral activity with little or no cytotoxicity at low micromolar concentrations. Compound 15D8 selectively inhibits Ad DNA replication in the nucleus, providing a potential candidate for the development of a new class of antiviral compounds to treat Ad infections.

Adenovirus composition, proteolysis, and disassembly studied by in-depth qualitative and quantitative proteomics.

Using high-resolution MS-based proteomics in combination with multiple protease digestion, we profiled, with on average 90% sequence coverage, all 13 viral proteins present in an human adenovirus (HAdV) vector. This in-depth profile provided multiple peptide-based evidence on intrinsic protease activity affecting several HAdV proteins. Next, the generated peptide library was used to develop a targeted proteomics method using selected reaction monitoring (SRM) aimed at quantitative profiling of the stoichiometry of all 13 proteins present in the HAdV. We also used this method to probe the release of specific virus proteins initiated by thermal stimulation, mimicking the early stage of HAdV disassembly during entry into host cells. We confirmed the copy numbers of the most well characterized viral capsid components and established the copy numbers for proteins whose stoichiometry has so far not been accurately defined. We also found that heating HAdV induces the complete release of the penton base and fiber proteins as well as a substantial release of protein VIII and VI. For these latter proteins, maturational proteolysis by the adenoviral protease leads to the differential release of fragments with certain peptides being fully released and others largely retained in the AdV particles. This information is likely to be beneficial for the ongoing interpretation of high resolution cryoEM and x-ray electron density maps.

The cleaved N-terminus of pVI binds peripentonal hexons in mature adenovirus.

Mature human adenovirus particles contain four minor capsid proteins, in addition to the three major capsid proteins (penton base, hexon and fiber) and several proteins associated with the genomic core of the virion. Of the minor capsid proteins, VI plays several crucial roles in the infection cycle of the virus, including hexon nuclear targeting during assembly, activation of the adenovirus proteinase (AVP) during maturation and endosome escape following cell entry. VI is translated as a precursor (pVI) that is cleaved at both N- and C-termini by AVP. Whereas the role of the C-terminal fragment of pVI, pVIc, is well established as an important co-factor of AVP, the role of the N-terminal fragment, pVIn, is currently elusive. In fact, the fate of pVIn following proteolytic cleavage is completely unknown. Here, we use a combination of proteomics-based peptide identification, native mass spectrometry and hydrogen-deuterium exchange mass spectrometry to show that pVIn is associated with mature human adenovirus, where it binds at the base of peripentonal hexons in a pH-dependent manner. Our findings suggest a possible role for pVIn in targeting pVI to hexons for proper assembly of the virion and timely release of the membrane lytic mature VI molecule.

Disulfide-bond formation by a single cysteine mutation in adenovirus protein VI impairs capsid release and membrane lysis.

The internal capsid protein VI mediates adenovirus (AdV) endosome penetration during cell entry. Essential to this process is the release of protein VI from the AdV capsid and subsequent membrane targeting and insertion by the liberated VI molecules within the endocytic vesicle. In this study, we describe a human AdV (HAdV) substitution mutant (AdV VI-G48C) within the critical N-terminal amphipathic α-helical domain of protein VI. The VI-G48C virus displays altered capsid stability that impacts protein VI release, membrane disruption and virus infectivity. This is due in part to aberrant disulfide-bonding of protein VI molecules within the AdV particle. Our results provide insight into the structural organization of protein VI in the virus particle, as well as highlight the role of protein VI in cell entry.