Conformational changes of murine polyomavirus capsid proteins induced by sialic acid binding

Murine polyomavirus (Py) infection initiates by the recognition of cell membrane molecules containing terminal sialic acid (SA) residues through specific binding pockets formed at the major capsid protein VP1 surface. VP1 Pockets 1, 2, and 3 bind terminal SA, Gal, and second branched SA, respectively. The consequence of recognition on viral cell entry remains elusive. In this work, we show that preincubation of Py with soluble compounds within Pocket 1 (N-acetyl or N-glycolyl neuraminic acids) increases Py cell binding and infectivity in murine 3T6 fibroblasts. In contrast, Gal does not significantly alter Py binding nor infectivity, whereas sialyllactose, in Pockets 1 and 2, decreases cell binding and infectivity. Binding experiments with Py virus-like particles confirmed the direct involvement of VP1 in this effect. To determine whether such results could reflect VP1 conformational changes induced by SA binding, protease digestion assays were performed after pretreatment of Py or virus-like particles with soluble receptor fragments. Binding of SA with the VP1 Pocket 1, but not of compounds interacting with Pocket 2, was associated with a transition of this protein from a protease-sensitive to a protease-resistant state. This effect was transmitted to the minor capsid proteins VP2 and VP3 in virus particles. Attachment of Py to cell monolayers similarly led to a VP1 trypsin-resistant pattern. Taken together, these data present evidence that initial binding of Py to terminal SA induces conformational changes in the viral capsid, which may influence subsequent virus cell entry steps.     

Development of a enterovirus diagnostic assay system for diagnosis of viral myocarditis in humans

The coxsackieviruses type B3 (CVB3) are members of the genus Enterovirus of the family Picornaviridae. They are the commonest cause of chronic myocarditis and dilated cardiomyopathy. However, there is still no effective method for diagnosing CVB3 infection in humans. Here, a fast and accurate system that uses a capsid‐protein‐specific peptide sequence to detect CVB3 in the sera of patients with viral myocarditis was established. The peptide sequence was selected from the whole CVB3 capsid protein sequence by computationally predicting fragments with high antigenicity and low hydrophobicity. Two of eight possible peptide sequences were selected and commercially synthesized. The synthesized peptides encoded either the VP2 or VP1 capsid protein and induced immunoglobulin G antibody expression in immunized rabbits. Anti‐VP2 and anti‐VP1 sera detected the viral proteins extracted from CVB3‐infected HeLa cells. The newly synthesized peptides successfully induced antibody production. These peptides, applied in an ELISA system, detected anti‐CVB3 antibodies in virus‐infected mouse serum. Moreover, an ELISA system based on the VP2 peptide detected CVB3 infection in patients with positively identified CVB3‐induced fulminant myocarditis. These results indicate that these new peptides specifically interact with anti‐CVB3 IgG antibodies in mouse and human sera. This ELISA system should be useful for the clinical diagnosis of enterovirus‐induced myocarditis.     

Expression of Rotavirus Capsid Protein VP6 in Transgenic Potato and Its Oral Immunogenicity in Mice

Murine rotavirus gene six encoding the 41 kDa group specific capsid structural protein VP6 was stably inserted into the Solanum tuberosum genome by Agrobacterium tumefaciens mediated transformation. The molecular mass of plant synthesized VP6 capsid protein determined by immunoblot was similar to the size of both purified virus VP6 monomeric peptides and partially assembled virus-like particles. The amount of VP6 protein synthesized in transgenic potato leaf and tuber was determined by enzyme-linked immunosorbent assay to be approximately 0.01% of total soluble protein. Oral immunization of CD-1 mice with transformed potato tuber tissues containing VP6 capsid protein generated measurable titers of both anti-VP6 serum IgG and intestinal IgA antibodies. The presence of detectable humoral and intestinal antibody responses against the rotavirus capsid protein following mucosal immunization provides an optimistic basis for the development of edible plant vaccines against enteric viral pathogens.     

Effect of the DnaK chaperone on the conformational quality of JCV VP1 virus‐like particles produced in Escherichia coli

Protein nanoparticles such as virus‐like particles (VLPs) can be obtained by recombinant protein production of viral capsid proteins and spontaneous self‐assembling in cell factories. Contrarily to infective viral particles, VLPs lack infective viral genome while retaining important viral properties like cellular tropism and intracellular delivery of internalized molecules. These properties make VLPs promising and fully biocompatible nanovehicles for drug delivery. VLPs of human JC virus (hJCV) VP1 capsid protein produced in Escherichia coli elicit variable hemagglutination properties when incubated at different NaCl concentrations and pH conditions, being optimal at 200 mM NaCl and at pH range between 5.8 and 7.5. In addition, the presence or absence of chaperone DnaK in E. coli cells influence the solubility of recombinant VP1 and the conformational quality of this protein in the VLPs. The hemagglutination ability of hJCV VP1 VLPs contained in E. coli cell extracts can be modulated by buffer composition in the hemagglutination assay. It has been also determined that the production of recombinant hJCV VP1 in E. coli is favored by the absence of chaperone DnaK as observed by Western Blot analysis in different E. coli genetic backgrounds, indicating a proteolysis targeting role for DnaK. However, solubility is highly compromised in a DnaK^? E. coli strain suggesting an important role of this chaperone in reduction of protein aggregates. Finally, hemagglutination efficiency of recombinant VP1 is directly related to the presence of DnaK in the producing cells. © 2014 American Institute of Chemical Engineers Biotechnol. Prog., 30:744–748, 2014     

Adeno-associated virus type 2 VP2 capsid protein is nonessential and can tolerate large peptide insertions at its N terminus

Direct insertion of amino acid sequences into the adeno-associated virus type 2 (AAV) capsid open reading frame (cap ORF) is one strategy currently being developed for retargeting this prototypical gene therapy vector. While this approach has successfully resulted in the formation of AAV particles that have expanded or retargeted viral tropism, the inserted sequences have been relatively short, linear receptor binding ligands. Since many receptor-ligand interactions involve nonlinear, conformation-dependent binding domains, we investigated the insertion of full-length peptides into the AAV cap ORF. To minimize disruption of critical VP3 structural domains, we confined the insertions to residue 138 within the VP1-VP2 overlap, which has been shown to be on the surface of the particle following insertion of smaller epitopes. The insertion of coding sequences for the 8-kDa chemokine binding domain of rat fractalkine (CX3CL1), the 18-kDa human hormone leptin, and the 30-kDa green fluorescent protein (GFP) after residue 138 failed to lead to formation of particles due to the loss of VP3 expression. To test the ability to complement these insertions with the missing capsid proteins in trans, we designed a system for producing AAV vectors in which expression of one capsid protein is isolated and combined with the remaining two capsid proteins expressed separately. Such an approach allows for genetic modification of a specific capsid protein across its entire coding sequence leaving the remaining capsid proteins unaffected. An examination of particle formation from the individual components of the system revealed that genome-containing particles formed as long as the VP3 capsid protein was present and demonstrated that the VP2 capsid protein is nonessential for viral infectivity. Viable particles composed of all three capsid proteins were obtained from the capsid complementation groups regardless of which capsid proteins were supplied separately in trans. Significant overexpression of VP2 resulted in the formation of particles with altered capsid protein stoichiometry. The key finding was that by using this system we successfully obtained nearly wild-type levels of recombinant AAV-like particles with large ligands inserted after residue 138 in VP1 and VP2 or in VP2 exclusively. While insertions at residue 138 in VP1 significantly decreased infectivity, insertions at residue 138 that were exclusively in VP2 had a minimal effect on viral assembly or infectivity. Finally, insertion of GFP into VP1 and VP2 resulted in a particle whose trafficking could be temporally monitored by using confocal microscopy. Thus, we have demonstrated a method that can be used to insert large (up to 30-kDa) peptide ligands into the AAV particle. This system allows greater flexibility than current approaches in genetically manipulating the composition of the AAV particle and, in particular, may allow vector retargeting to alternative receptors requiring interaction with full-length conformation-dependent peptide ligands.     

Capsid intermediates assembled in a foot-and-mouth disease virus genome RNA-programmed cell-free translation system and in infected cells.

Structural protein complexes sedimenting at 140S, 70S (empty capsids), and 14S were isolated from foot-and-mouth disease virus-infected cells. The empty capsids were stable, while 14S complexes were relatively short-lived. Radioimmune binding assays involving the use of neutralizing monoclonal antibodies to six distinct epitopes on type A12 virus and polyclonal antisera to A12 structural proteins demonstrated that native empty capsids were indistinguishable from virus. Infected cell 14S particles possessed all the neutralizing epitopes and reacted with VP2 antiserum. Cell-free structural protein complexes sedimenting at 110S, 60S, and 14S containing capsid proteins VP0, VP3, and VP1 are assembled in a rabbit reticulocyte lysate programmed with foot-and-mouth viral RNA. These structures also contain the six epitopes, and cell-free 14S structures like their in vivo counterparts reacted with VP2 antiserum. Capsid structures from infected cells and the cell-free complexes adsorbed to susceptible cells, and this binding was inhibited, to various degrees, by saturating levels of unlabeled virus. These assays and other biochemical evidence indicate that capsid assembly in the cell-free system resembles viral morphogenesis in infected cells. In addition, epitopes on the virus surface possibly involved in interaction with cellular receptor sites are found early in virion morphogenesis.     

Structural peptides of a nonenveloped virus are involved in assembly and membrane translocation

The capsid of infectious bursal disease virus (IBDV), a nonenveloped virus of the family Birnaviridae, has a T=13l icosahedral shell constituted by a single protein, VP2, and several disordered peptides, all derived from the precursor pVP2. In this study, we show that two of the peptides, pep11 and pep46, control virus assembly and cell entry. Deletion of pep11 or even simple substitution of most of its residues blocks the capsid morphogenesis. Removal of pep46 also prevents capsid assembly but leads to the formation of subviral particles formed by unprocessed VP2 species. Fitting with the VP2 atomic model into three-dimensional reconstructions of these particles demonstrates that the presence of uncleaved pep46 causes a steric hindrance at the vertices, blocking fivefold axis formation. Mutagenesis of the pVP2 maturation sites confirms that C terminus processing is necessary for VP2 to acquire the correct icosahedral architecture. All peptides present on virions are accessible to proteases or biochemical labeling. One of them, pep46, is shown to induce large structural rearrangements in liposomes and to destabilize target membranes, demonstrating its implication in cell entry.     

Identification of the region including the epitope for a monoclonal antibody which can neutralize human parvovirus B19.

In this study, we identified a region in the human parvovirus structural protein which involves the neutralization of the virus by a monoclonal antibody and site-specific synthetic peptides. A newly established monoclonal antibody reacted with both viral capsid proteins VP1 and VP2. The epitope was found in six strains of independently isolated human parvovirus B19. The monoclonal antibody could protect colony-forming unit erythroid in human bone marrow cell culture from injury by the virus. The monoclonal antibody reacted with only 1 of 12 peptides that were synthesized according to a predicted amino acid sequence based on nucleotide sequences of the coding region for the structural protein of B19 virus. The sequence recognized by the antibody was a site corresponding to amino acids 328 to 344 from the amino-terminal portion of VP2. This evidence suggests that the epitope of the viral capsid protein is located on the surface of the virus and may be recognized by virus-neutralizing antibodies.     

Chromosome-protein interactions in polyomavirus virions

In this work, we sought to determine whether the components of the murine polyomavirus capsid establish specific interactions with the minichromosome encapsidated into the mature viral particles by using the cis-diamminedichloroplatinum(II) cross-linking reagent. Our data indicated that VP1, but not minor capsid proteins, interacts with the viral genome in vivo. In addition, semiquantitative PCR assays performed on cross-linked DNA complexes revealed that VP1 binds to all regions of the viral genome but significantly more to the regulatory region. The implications of such an interaction for viral infectivity are discussed.     

Review: Production, characterization, and testing of banked mammalian cell substrates used to produce biological products

Summary A critical component in controlling the production of biological products derived from human and animal cell lines in the characterization and testing of banked cell substrates. The objective is to confirm the identity, purity, and suitability of these cells for manufacturing use. Quality concerns for biological products derived from cell lines originate from the presence of cellular and adventitious contaminants. Well-characterized cell banks not only permit a consistent source of production cells throughout the life of a product but also decrease the likelihood of contamination by other cell lines and adventitious agents. An important part of qualifying a cell line is choosing the appropriate testing for the presence of adventitious contaminants. The qualification of cell banks includes tests for cell identity and endogenous and adventitious microbial contaminants (bacteria, fungi, mycoplasmas, and viruses). For cells producing recombinant deoxyribonucleic acid-derived products, analysis of the expression construct at the nucleic acid level (genetic stability) is also a primary concern. The strategy for designing a safety-testing program for banked cells should be based on sound scientific principles and current regulatory guidance.     

Pressure-induced formation of inactive triple-shelled rotavirus particles is associated with changes in the spike protein Vp4

Rotaviruses are non-enveloped, triple-shelled particles that cause enteritis in animals and humans. The interactions among the different viral proteins located in the three concentric layers make the rotavirus particle an excellent model for physico-chemical and biological studies of viral assemblage. SA11-4S rotaviruses subjected to high pressure were inactivated by more than five log units. After pressure treatment, the particles were recovered with slight structural changes when compared to the control. Electron microscopy suggested subtle changes in the viral outer layer in some pressurised particles. Fluorescence spectroscopy showed that much more dramatic changes were produced by urea denaturation than by pressure. Based on the fluorescence spectrum, the genome resistance to ribonuclease, and the absence of changes in hydrodynamic properties, there was little or no disruption of the capsid under pressure. On the other hand, hemagglutination assays indicated that the main component affected by pressure was the spike protein VP4, thus accounting for changes in interaction with host cells and greatly reduced infectivity. The changes leading to inactivation did not cause removal of VP4 from the outer capsid, as verified by size-exclusion chromatography. Antibodies raised against pressurised material were as effective as antibodies raised against the intact virus, based on their neutralisation titre in plaque reduction assays, enzyme-linked immunosorbent assays and direct interaction with the particle, as measured by gel-filtration chromatography. Therefore, the new conformation of the pressurised particle did not result in loss of immunogenicity. We propose that pressure alters the receptor-binding protein VP4 by triggering changes similar to those produced when the virus interacts with target cells. As the changes in VP4 conformation caused by pressure occur prior to virus exposure to target cells, it leads to non-infectious particles and may lead to the exposure of previously occult epitopes, important for vaccine development.     

Synthetic antibodies and peptides recognizing progressive multifocal leukoencephalopathy-specific point mutations in polyomavirus JC capsid viral protein 1

Polyomavirus JC (JCV) is the causative agent of progressive multifocal leukoencephalopathy (PML), a rare and frequently fatal brain disease that afflicts a small fraction of the immune-compromised population, including those affected by AIDS and transplantation recipients on immunosuppressive drug therapy. Currently there is no specific therapy for PML. The major capsid viral protein 1 (VP1) involved in binding to sialic acid cell receptors is believed to be a key player in pathogenesis. PML-specific mutations in JCV VP1 sequences present at the binding pocket of sialic acid cell receptors, such as L55F and S269F, abolish sialic acid recognition and might favor PML onset. Early diagnosis of these PML-specific mutations may help identify patients at high risk of PML, thus reducing the risks associated with immunosuppressive therapy. As a first step in the development of such early diagnostic tools, we report identification and characterization of affinity reagents that specifically recognize PML-specific mutations in VP1 variants using phage display technology. We first identified 2 peptides targeting wild type VP1 with moderate specificity. Fine-tuning via selection of biased libraries designed based on 2 parental peptides yielded peptides with different, yet still moderate, bindinspecificities. In contrast, we had great success in identifying synthetic antibodies that recognize one of the PML-specific mutations (L55F) with high specificity from the phage-displayed libraries. These peptides and synthetic antibodies represent potential candidates for developing tailored immune-based assays for PML risk stratification in addition to complementing affinity reagents currently available for the study of PML and JCV.     

Characterization of virus-like particles produced by the expression of rotavirus capsid proteins in insect cells.

Rotaviruses are triple-layered particles that contain four major capsid proteins, VP2, VP4, VP6, and VP7, and two minor proteins, VP1 and VP3. We have cloned each of the rotavirus genes coding for a major capsid protein into the baculovirus expression system and expressed each protein in insect cells. Coexpression of different combinations of the rotavirus major structural proteins resulted in the formation of stable virus-like particles (VLPs). The coexpression of VP2 and VP6 alone or with VP4 resulted in the production of VP2/6 or VP2/4/6 VLPs, which were similar to double-layered rotavirus particles. Coexpression of VP2, VP6, and VP7, with or without VP4, produced triple-layered VP2/6/7 or VP2/4/6/7 VLPs, which were similar to native infectious rotavirus particles. The VLPs maintained the structural and functional characteristics of native particles, as determined by electron microscopic examination of the particles, the presence of nonneutralizing and neutralizing epitopes on VP4 and VP7, and hemagglutination activity of the VP2/4/6/7 VLPs. The production of VP2/4/6 particles indicated that VP4 interacts with VP6. Cell binding assays performed with each of the VLPs indicated that VP4 is the viral attachment protein. Chimeric particles containing VP7 from two different G serotypes also were obtained. The ability to express individual proteins or to coexpress different subsets of proteins provides a system with which to examine the interactions of the rotavirus structural proteins, the role of individual proteins in virus morphogenesis, and the feasibility of a subunit vaccine.     

Poliovirus-associated protein kinase: destabilization of the virus capsid and stimulation of the phosphorylation reaction by Zn2+.

The previously described poliovirus-associated protein kinase activity phosphorylates viral proteins VP0 and VP2 as well as exogenous proteins in the presence of Mg2+. In this paper, the effect of Zn2+ on the phosphorylation reaction and the stability of the poliovirus capsid has been studied in detail and compared to that of Mg2+. Phosphorylation patterns of viral and other proteins depend on the divalent cation present. In the presence of Zn2+, phosphorylation of capsid proteins VP2 and VP4 is significantly higher while phosphorylation of VP0 and exogenous phosphate acceptor proteins is not detected. Our results indicate the activation of more than one virus-associated protein kinase by Zn2+. The ion-dependent behavior of the enzyme activities is observed independently of whether the virus was obtained from HeLa or green monkey kidney cells. The poliovirus capsid is destabilized by Zn2+. The destabilization leads to a substantially increased permeability of virus particles to ethidium bromide and RNase, concomitant with decreased infectivity of the sample. This alteration of the poliovirus capsid structure is a prerequisite for effective phosphorylation of viral capsid proteins. The increased level of phosphorylation of viral capsid proteins results in further destabilization of the viral capsid. As a result of the conformational changes, poliovirus-associated protein kinase activities dissociate from the virus particle. High-performance liquid chromatography-purified viral protein VP2 is phosphorylated by the released enzymes on serine, threonine, and tyrosine in the presence of Zn2+. We suggest that the destabilizing effect of phosphorylation on the viral capsid plays a role in uncoating of poliovirus.     

Low pH-dependent endosomal processing of the incoming parvovirus minute virus of mice virion leads to externalization of the VP1 N-terminal sequence (N-VP1), N-VP2 cleavage, and uncoating of the full-length genome

Minute virus of mice (MVM) enters the host cell via receptor-mediated endocytosis. Although endosomal processing is required, its role remains uncertain. In particular, the effect of low endosomal pH on capsid configuration and nuclear delivery of the viral genome is unclear. We have followed the progression and structural transitions of DNA full-virus capsids (FC) and empty capsids (EC) containing the VP1 and VP2 structural proteins and of VP2-only virus-like particles (VLP) during the endosomal trafficking. Three capsid rearrangements were detected in FC: externalization of the VP1 N-terminal sequence (N-VP1), cleavage of the exposed VP2 N-terminal sequence (N-VP2), and uncoating of the full-length genome. All three capsid modifications occurred simultaneously, starting as early as 30 min after internalization, and all of them were blocked by raising the endosomal pH. In particles lacking viral single-stranded DNA (EC and VLP), the N-VP2 was not exposed and thus it was not cleaved. However, the EC did externalize N-VP1 with kinetics similar to those of FC. The bulk of all the incoming particles (FC, EC, and VLP) accumulated in lysosomes without signs of lysosomal membrane destabilization. Inside lysosomes, capsid degradation was not detected, although the uncoated DNA of FC was slowly degraded. Interestingly, at any time postinfection, the amount of structural proteins of the incoming virions accumulating in the nuclear fraction was negligible. These results indicate that during the early endosomal trafficking, the MVM particles are structurally modified by low-pH-dependent mechanisms. Regardless of the structural transitions and protein composition, the majority of the entering viral particles and genomes end in lysosomes, limiting the efficiency of MVM nuclear translocation.     

The Herpesvirus capsid surface protein, VP26, and the majority of the tegument proteins are dispensable for capsid transport toward the nucleus

Upon entering a cell, alphaherpesvirus capsids are transported toward the minus ends of microtubules and ultimately deposit virus DNA within the host nucleus. The virus proteins that mediate this centripetal transport are unknown but are expected to be either viral tegument proteins, which are a group of capsid-associated proteins, or a surface component of the capsid itself. Starting with derivatives of pseudorabies virus that encode a fluorescent protein fused to a structural component of the virus, we have made a collection of 12 mutant viruses that lack either the VP26 capsid protein or an individual tegument protein. Using live-cell fluorescence microscopy, we tracked individual virus particles in axons following infection of primary sensory neurons. Quantitative analysis of the VP26-null virus indicates that this protein plays no observable role in capsid transport. Furthermore, viruses lacking tegument proteins that are nonessential for virus propagation in cell culture were also competent for axonal transport. These results indicate that a protein essential for viral propagation mediates transport of the capsid to the nucleus.     

In silico prediction and in vitro identification of bluetongue virus 4 VP5 protein B-cell epitopes

VP5, the outer capsid protein of bluetongue virus (BTV), plays an important role in viral penetration and antibody-mediated viral neutralization. Therefore, VP5 represents an important target for development of vaccines and diagnostic tests. In this study, we use bioinformatic tools to predict nine antigenic B cell epitopes in the VP5 protein of a BTV serotype 4 (BTV4) isolate from China. Further, we generate five BTV4 VP5-specific monoclonal antibodies (MAbs) and define their corresponding epitopes using a set of VP5-derived peptides expressed as maltose-binding protein (MBP) fusion proteins. The five identified epitopes map to amino acids 119–134, 257–272, 286–301, 322–337, and 481–496 of the VP5 protein. Importantly, the epitopes identified using VP5-derived peptides do not correlate with our bioinformatic prediction of antibody epitopes. Identification and characterization of BTV4 VP5 protein epitopes may aid the development of diagnostic tools and provide information with which to study the structure of the BTV VP5 protein.     

Susceptibility of mouse minute virus to inactivation by heat in two cell culture media types

Viral contaminations of biopharmaceutical manufacturing cell culture facilities are a significant threat and one for which having a risk mitigation strategy is highly desirable. High temperature, short time (HTST) mammalian cell media treatment may potentially safeguard manufacturing facilities from such contaminations. HTST is thought to inactivate virions by denaturing proteins of the viral capsid, and there is evidence that HTST provides ample virucidal efficacy against nonenveloped or naked viruses such as mouse minute virus (MMV), a parvovirus. The aim of the studies presented herein was to further delineate the susceptibility of MMV, known to have contaminated mammalian cell manufacturing facilities, to heat by exposing virus‐spiked cell culture media to a broad range of temperatures and for various times of exposure. The results of these studies show that HTST is capable of inactivating MMV by three orders of magnitude or more. Thus, we believe that HTST is a useful technology for the purposes of providing a barrier to adventitious contamination of mammalian cell culture processes in the biopharmaceutical industry. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009     

Assembly of highly infectious rotavirus particles recoated with recombinant outer capsid proteins

Assembly of the rotavirus outer capsid is the final step of a complex pathway. In vivo, the later steps include a maturational membrane penetration that is dependent on the scaffolding activity of a viral nonstructural protein. In vitro, simply adding the recombinant outer capsid proteins VP4 and VP7 to authentic double-layered rotavirus subviral particles (DLPs) in the presence of calcium and acidic pH increases infectivity by a factor of up to 10(7), yielding particles as infectious as authentic purified virions. VP4 must be added before VP7 for high-level infectivity. Steep dependence of infectious recoating on VP4 concentration suggests that VP4-VP4 interactions, probably oligomerization, precede VP4 binding to particles. Trypsin sensitivity analysis identifies two populations of VP4 associated with recoated particles: properly mounted VP4 that can be specifically primed by trypsin, and nonspecifically associated VP4 that is degraded by trypsin. A full complement of properly assembled VP4 is not required for efficient infectivity. Minimal dependence of recoating on VP7 concentration suggests that VP7 binds DLPs with high affinity. The parameters for efficient recoating and the characterization of recoated particles suggest a model in which, after a relatively weak interaction between oligomeric VP4 and DLPs, VP7 binds the particles and locks VP4 in place. Recoating will allow the use of infectious modified rotavirus particles to explore rotavirus assembly and cell entry and could lead to practical applications in novel immunization strategies.