Proteolytic enhancement of rotavirus infectivity: molecular mechanisms

The polypeptide compositions of single-shelled and double-shelled simian rotavirus particles were modified by exposure to proteolytic enzymes. Specifically, a major outer capsid polypeptide (VP3) having a molecular weight of 88,000 in double-shelled particles was cleaved by trypsin to yield two polypeptides, VP5* and VP8* (molecular weights, 60,000 and 28,000, respectively). The cleavage of VP3 by enzymes that enhanced infectivity (trypsin, elastase, and pancreatin) yielded different products compared to those detected when VP3 was cleaved by chymotrypsin, which did not enhance infectivity. The appearance of VP5* was correlated with an enhancement of infectivity. Cleavages of the major internal capsid polypeptide VP2 were also observed. The VP2 cleavage products had molecular weights similar to those of known structural and nonstructural rotavirus polypeptides. We confirmed the precursor-product relationships by comparing the peptide maps of the polypeptides generated by digestions with V-8 protease and chymotrypsin. The remaining rotavirus structural polypeptides, including the outer capsid glycoproteins (VP7 and 7a), were not altered by exposure to pancreatic enzymes. Cleavage of VP3 was not required for virus assembly, and specific cleavage of the polypeptides occurred only on assembled particles. We also discuss the role of cleavage activation in other virus-specific biological functions (e.g., hemagglutination and virulence).     

Trypsin cleavage stabilizes the rotavirus VP4 spike

Trypsin enhances rotavirus infectivity by an unknown mechanism. To examine the structural basis of trypsin-enhanced infectivity in rotaviruses, SA11 4F triple-layered particles (TLPs) grown in the absence (nontrypsinized rotavirus [NTR]) or presence (trypsinized rotavirus [TR]) of trypsin were characterized to determine the structure, the protein composition, and the infectivity of the particles before and after trypsin treatment. As expected, VP4 was not cleaved in NTR particles and was cleaved into VP5(*) and VP8(*) in TR particles. However, surprisingly, while the VP4 spikes were clearly visible and well ordered in the electron cryomicroscopy reconstructions of TR TLPs, they were totally absent in the reconstructions of NTR TLPs. Biochemical analysis with radiolabeled particles indicated that the stoichiometry of the VP4 in NTR particles was the same as that in TR particles and that the VP8(*) portion of NTR, but not TR, particles is susceptible to further proteolysis by trypsin. Taken together, these structural and biochemical data show that the VP4 spikes in the NTR TLPs are icosahedrally disordered and that they are conformationally different. Structural studies on the NTR TLPs after trypsin treatment showed that spike structure could be partially recovered. Following additional trypsin treatment, infectivity was enhanced for both NTR and TR particles, but the infectivity of NTR remained 2 logs lower than that of TR particles. Increased infectivity in these particles corresponded to additional cleavages in VP5(*), at amino acids 259, 583, and putatively 467, which are conserved in all P serotypes of human and animal group A rotaviruses and also corresponded with a structural change in VP7. These biochemical and structural results show that trypsin cleavage imparts order to VP4 spikes on de novo synthesized virus particles, and these ordered spikes make virus entry into cells more efficient.     

Priming for rotavirus neutralizing antibodies by a VP4 protein-derived synthetic peptide.

In the rotavirus SA11 surface protein VP4, the trypsin cleavage sites associated with the enhancement of infectivity are flanked by two amino acid regions that are highly conserved among different rotaviruses. We have tested the ability of synthetic peptides that mimic these two regions to induce and prime for a rotavirus neutralizing antibody response in mice. After the peptide immunization schedule, both peptides induced peptide antibodies, but neither was able to induce virus antibodies, as measured by an enzyme-linked immunosorbent assay or a neutralization assay. However, when the peptide-inoculated mice were subsequently injected with intact SA11 virus, a rapid and high neutralizing antibody response was observed in mice that had previously received the peptide comprising amino acids 220 to 233 of the VP4 protein. This neutralizing activity was serotype specific; however, this peptide was also able to efficiently prime the immune system of mice for a neutralizing antibody response to the heterotypic rotavirus ST3 when the ST3 virus was used for the secondary inoculation.     

Antibodies to the trypsin cleavage peptide VP8 neutralize rotavirus by inhibiting binding of virions to target cells in culture.

Two distinct patterns of neutralization were identified by comparing the neutralization curves of monoclonal antibodies (MAbs) directed at the two surface proteins, VP4 and VP7, of rhesus rotavirus. VP7-specific MAbs were able to neutralize virus efficiently, and slight increases in antibody concentration resulted in a sharp decline in infectivity. On the other hand, MAbs to VP4 proved much less efficient at neutralizing rhesus rotavirus, and the fraction of infectious virus decreased gradually throughout a wide range of antibody concentrations. MAbs directed at VP8*, the smaller trypsin cleavage fragment of VP4, were shown to efficiently prevent binding of radiolabeled virions to MA104 cell monolayers, to an extent and at concentrations comparable to those required for neutralization of infectivity. Conversely, MAbs recognizing VP7 or the larger VP4 trypsin cleavage product, VP5*, showed little or no inhibitory effect on virus binding to cells. All MAbs studied were able to neutralize rotavirus that was already bound to the surface of cells. The MAbs directed at VP8*, but not those recognizing VP5* or VP7, were shown to mediate release of radiolabeled virus from the surface of the cells. With MAbs directed at VP7, papain digestion of virus-bound antibody molecules led to an almost complete recovery of infectivity. Neutralization could be fully restored by incubation of virus-Fab complexes with anti-mouse immunoglobulin G antiserum. Neutralization with MAbs directed at VP8* proved insensitive to digestion with papain as well as to the addition of anti-immunoglobulin antibodies.     

VP7 mediates the interaction of rotaviruses with integrin alphavbeta3 through a novel integrin-binding site

Rotavirus entry is a complex multistep process that depends on the trypsin cleavage of the virus spike protein VP4 into polypeptides VP5 and VP8 and on the interaction of these polypeptides and of VP7, the second viral surface protein, with several cell surface molecules, including integrin alphavbeta3. We characterized the effect of the trypsin cleavage of VP4 on the binding to MA104 cells of the sialic acid-dependent virus strain RRV and its sialic acid-independent variant, nar3. We found that, although the trypsin treatment did not affect the attachment of these viruses to the cell surface, their binding was qualitatively different. In contrast to the trypsin-treated viruses, which initially bound to the cell surface through VP4, the non-trypsin-treated variant nar3 bound to the cell through VP7. Amino acid sequence comparison of the surface proteins of rotavirus and hantavirus, both of which interact with integrin alphavbeta3 in an RGD-independent manner, identified a region shared by rotavirus VP7 and hantavirus G1G2 protein in which six of nine amino acids are identical. This region, which is highly conserved among the VP7 proteins of different rotavirus strains, mediates the binding of rotaviruses to integrin alphavbeta3 and probably represents a novel binding motif for this integrin.     

Binding to sialic acids is not an essential step for the entry of animal rotaviruses to epithelial cells in culture.

The infection of target cells by animal rotaviruses requires the presence of sialic acids on the cell surface. Treatment of the cells with neuraminidases or incubation of the viruses with some sialoglycoproteins, such as glycophorin A, greatly reduces virus binding, with the consequent reduction of viral infectivity. In this work, we report the isolation of animal rotavirus variants whose infectivity is no longer dependent on the presence of sialic acids on the cell surface. In addition, although these variants bind to glycophorin A as efficiently as the wild-type virus, this interaction no longer inhibit viral infectivity. These observations indicate that the initial interaction of the mutants with the cell occurs at a site different from the sialic acid-binding site located on VP8, the smaller trypsin cleavage product of VP4. Reassortant analysis showed that the mutant phenotype segregates with the VP4 gene. Neutralizing monoclonal antibodies directed to VP4 and VP7 were tested for their ability to neutralize the variants. Antibodies to VP7 and VP5, the larger trypsin cleavage product of VP4, neutralized the mutants as efficiently as the wild-type virus. In contrast, although antibodies to VP8 were able to bind to the mutants, they showed little or no neutralizing activity. The implications of these findings in rotavirus attachment to and penetration of epithelial cells in culture are discussed.     

Cleavage of Rhesus Rotavirus VP4 after Arginine 247 Is Essential for Rotavirus-Like Particle-Induced Fusion from Without

We recently described our finding that recombinant baculovirus-produced virus-like particles (VLPs) can induce cell-cell fusion similar to that induced by intact rotavirus in our assay for viral entry into tissue culture cells (J. M. Gilbert and H. B. Greenberg, J. Virol. 71:4555–4563, 1997). The conditions required for syncytium formation are similar to those for viral penetration of the plasma membrane during the course of viral infection. This VLP-mediated fusion activity was dependent on the presence of the outer-layer proteins, viral protein 4 (VP4) and VP7, and on the trypsinization of VP4. Fusion activity occurred only with cells that are permissive for rotavirus infection. Here we begin to dissect the role of VP4 in rotavirus entry by examining the importance of the precise trypsin cleavage of VP4 and the activation of VP4 function related to viral entry. We present evidence that the elimination of the three trypsin-susceptible arginine residues of VP4 by specific site-directed mutagenesis prevents syncytium formation. Two of the three arginine residues in VP4 are dispensable for syncytium formation, and only the arginine residue at site 247 appears to be required for activation of VP4 functions and cell-cell fusion. Using the recombinant VLPs in our syncytium assay will aid in understanding the conformational changes that occur in VP4 involved in rotavirus penetration into host cells.     

Trypsin enhancement of rotavirus infectivity: mechanism of enhancement.

The infectivity of most rotaviruses is enhanced by treatment with trypsin. We studied the mechanism of enhancement of examining the effect of trypsin on rotavirus infectivity, aggregation, early interactions with host cells, and structure. The results indicated that trypsin does not increase levels of infectious virus by dispersion of aggregates or affect the efficiency or rate of attachment of virus to cells. A fraction of virus that was not infections without trypsin treatment was found to attach to cells, but did not initiate antigen synthesis. When cells were infected with labeled, purified virus, increased levels of uncoated particles were found in cells infected with trypsin-treated virus. Infection of cells with trypsin-treated virus also led to greater levels of RNA synthesis early in the infection. The results suggest that trypsin converts a noninfectious fraction of virus into infectious virus by allowing this fraction to uncoat in the infected cell. Trypsin was found to cleave an 88,000-dalton structural polypeptide of bovine rotavirus generating 67,000- and 20,000-dalton cleavage products.     

Functional and structural analysis of the sialic acid-binding domain of rotaviruses.

The infectivity of most animal rotaviruses is dependent on the interaction of the virus spike protein VP4 with a sialic acid (SA)-containing cell receptor, and the SA-binding domain of this protein has been mapped between amino acids 93 and 208 of its trypsin cleavage fragment VP8. To identify which residues in this region are essential for the SA-binding activity, we performed alanine mutagenesis of the rotavirus RRV VP8 expressed in bacteria as a fusion polypeptide with glutathione S-transferase. Tyrosines were primarily targeted since tyrosine has been involved in the interaction of other viral hemagglutinins with SA. Of the 15 substitutions carried out, 10 abolished the SA-dependent hemagglutination activity of the protein, as well as its ability to bind to glycophorin A in a solid-phase assay. However, only alanine substitutions for tyrosines 155 and 188 and for serine 190 did not affect the overall conformation of the protein, as judged by their interaction with a panel of conformationally sensitive neutralizing VP8 monoclonal antibodies (MAbs). These findings suggest that these three amino acids play an essential role in the SA-binding activity of the protein, presumably by interacting directly with the SA molecule. The predicted secondary structure of VP8 suggests that it is organized as 11 beta-strands separated by loops; in this model, Tyr-155 maps to loop 7 while Tyr-188 and Ser-190 map to loop 9. The close proximity of these two loops is also supported by previous results from competition experiments with neutralizing MAbs directed at RRV VP8.     

Serotype classification and characterisation of the rotavirus SA11 VP6 protein using mass spectrometry and two-dimensional gel electrophoresis

VP6, which makes up the inner capsid of rotavirus, is the major structural protein of this virus. Whilst VP6 has been sequenced at the DNA level in several rotavirus strains, there has been less effort to characterise the protein at the amino acid level. This paper reports the use of peptide mass fingerprinting and post-source decay fragmentation studies using MALDI-TOF and electrospray ionisation mass spectrometry to identify and characterise, in detail, the VP6 protein. We show that mass spectrometric analysis of VP6 peptides successfully distinguished SA11 from other rotavirus serotypes, and identify unique peptides that can be used for serotypic differentiation. For VP6 characterisation, the ExPASy FindMod tool was used to predict post-translational modifications on the protein. Analysis of trypsin and AspN digests predicted that the N-terminal methionine of VP6 was acetylated and this was confirmed using post source decay and electrospray ionisation mass spectrometry–mass spectrometry. An asparagine residue (aa107), which is followed by a glycine residue, was shown to undergo partial deamidation to aspartic acid. VP6 has two additional asparagine-glycine sequences and, in this sequence context, asparagine is known to be particularly susceptible to deamidation. Two-dimensional gel electrophoresis revealed a complex series of VP6 isoforms with an apparent molecular mass of approximately 45,000 Da and a pI ranging from 5.25 to 5.8. This pattern could partly be explained by the potential for deamidation at several sites within the protein. Electronic Publication     

Polypeptide composition of rotavirus empty capsids and their possible use as a subunit vaccine.

Two types of empty capsid particles that differed with respect to the presence of the two outer shell proteins were isolated from MA-104 cells infected with bovine rotavirus V1005. Three previously uncharacterized polypeptides, I, II, and III, migrating between VP2 and VP6, were detected in empty capsids but not in single- and double-shelled rotavirus particles. Peptide mapping revealed that all three proteins were related to VP2. Polypeptides I, II, and III could be generated by in vitro trypsin digestion of empty capsids not exposed to trypsin in the infection medium. Labeled polypeptides appeared in empty capsids before they were detected in intracellular single- or double-shelled rotavirus particles. Empty capsids were also observed in MA-104 cells infected with bovine rotaviruses UK and NCDV, simian rotavirus SA11, and human rotavirus KU. VP7-containing empty capsid is the minimal subunit vaccine for cows; we failed to induce a substantial neutralizing antibody increase with VP7 purified under denaturating or nondenaturating conditions or with synthetic peptides corresponding to two regions of VP7.     

Comparative analysis of the VP3 gene of divergent strains of the rotaviruses simian SA11 and bovine Nebraska calf diarrhea virus.

The gene encoding outer capsid protein VP3 of subpopulations of two animal rotaviruses, simian SA11 and Nebraska calf diarrhea virus (NCDV), was analyzed. Two laboratory strains of simian SA11 rotavirus (SA11-SEM and SA11-FEM) differed with respect to VP3. This dimorphism was indicated by a difference in electrophoretic mobility and a difference in reactivity with anti-VP3 monoclonal antibodies. The overall VP3 amino acid homology between the two SA11 VP3 proteins was 82.7%, whereas the VP3 protein of SA11-FEM was 98.5% homologous in amino acid sequence to NCDV VP3, suggesting that SA11-FEM VP3 was derived by gene reassortment in the laboratory during contamination with a bovine rotavirus. A comparison of the deduced amino acid sequence of the VP3 of two virulent NCDV strains and an attenuated NCDV strain (RIT 4237), revealed only five amino acid differences which were scattered throughout the protein but did not involve the trypsin cleavage sites. Of interest, the VP3 of the standard strain of NCDV which is virulent for cows differed in only one amino acid (position 23, Gln to Lys) from the VP3 of an NCDV mutant which was attenuated both for cows and for children.     

The VP5 domain of VP4 can mediate attachment of rotaviruses to cells

Some animal rotaviruses require the presence of sialic acid (SA) on the cell surface to infect the cell. We have isolated variants of rhesus rotavirus (RRV) whose infectivity no longer depends on SA. Both the SA-dependent and -independent interactions of these viruses with the cell are mediated by the virus spike protein VP4, which is cleaved by trypsin into two domains, VP5 and VP8. In this work we have compared the binding characteristics of wild-type RRV and its variant nar3 to MA104 cells. In a direct nonradioactive binding assay, both viruses bound to the cells in a saturable and specific manner. When neutralizing monoclonal antibodies directed to both the VP8 and VP5 domains of VP4 were used to block virus binding, antibodies to VP8 blocked the cell attachment of wild-type RRV but not that of the variant nar3. Conversely, an antibody to VP5 inhibited the binding of nar3 but not that of RRV. These results suggest that while RRV binds to the cell through VP8, the variant does so through the VP5 domain of VP4. This observation was further sustained by the fact that recombinant VP8 and VP5 proteins, produced in bacteria as fusion products with glutathione S-transferase, were found to bind to MA104 cells in a specific and saturable manner and, when preincubated with the cell, were capable of inhibiting the binding of wild-type and variant viruses, respectively. In addition, the VP5 and VP8 recombinant proteins inhibited the infectivity of nar3 and RRV, respectively, confirming the results obtained in the binding assays. Interestingly, when the infectivity assay was performed on neuraminidase-treated cells, the VP5 fusion protein was also found to inhibit the infectivity of RRV, suggesting that RRV could bind to the cell through two sequential steps mediated by the interaction of VP8 and VP5 with SA-containing and SA-independent cell surface receptors, respectively.     

Localization of a neutralization site of Theiler''s murine encephalomyelitis viruses.

Theiler's murine encephalomyelitis viruses (TMEV) are picornaviruses that produce enteric and neurological diseases in mice. Subgroup TO strains of TMEV cause persistent infections with demyelination, while subgroup GDVII strains neither persist nor demyelinate. We produced neutralizing monoclonal antibodies (mAbs) to clarify the mechanisms of persistence and demyelination. Some of the neutralizing mAbs reacted with isolated VP1 on Western blots, while others were conformation specific. The neutralization site for the former TMEV mAbs was on the VP1 trypsin cleavage site of the intact virion. The neutralization site for the conformation-specific mAbs was distinct and was not affected by trypsin. Trypsin treatment of subgroup TO strains increased their infectivity for L cells, whereas the infectivity of subgroup GDVII strains was decreased by trypsin treatment. Subpopulations of virus in subgroup TO-infected tissue culture cells and in infected mouse brain homogenates contained VP1-cleaved virus; this VP1-cleaved virus gave rise to a large persistent fraction in neutralization tests when it was reacted with VP1-specific mAbs. These findings have implications regarding the pathogenesis of subgroup TO demyelinating disease. TMEV VP1 cleavage may be important for virus persistence because of disruption of a major neutralization epitope. The change in virus surface structure caused by VP1 cleavage may affect cell binding and lead to altered cytotropism. Immunocytes, which have been implicated in subgroup TO demyelination, may provide a source for proteases for VP1 cleavage.     

Initial interaction of rotavirus strains with N-acetylneuraminic (sialic) acid residues on the cell surface correlates with VP4 genotype, not species of origin

We examined 41 human and animal rotavirus strains representative of all known P genotypes for their dependency on cellular N-acetylneuraminic (sialic) acid (SA) residues for infectivity. Our results showed that all rotaviruses studied, whether of animal or human origin, belonging to P genotypes [1], [2], [3], and [7] depended on SA residues on the cell surface for efficient infectivity but that all human and animal rotavirus strains representative of the remaining known P genotypes were SA independent. The SA residue requirement for efficient infectivity did not change for reassortant rotavirus strains with altered VP4-VP7 combinations. The initial interaction of rotavirus strains with SA residues on the cell surface correlated with VP4 genotype specificity, not with species of origin or VP7 G serotype specificity (P = 0.001; r2 = 1.00, Pearson's correlation coefficient). In addition to being a requirement for infectivity, the presence of SA residues on the cell surface is a requirement for efficient growth in cell culture; recognition of the association of specific P genotypes with the binding of rotavirus to SA residues will facilitate our understanding of the molecular basis of the early events of rotavirus-cell interactions in cell culture models and of pathogenicity in vivo.     

Interaction of rotaviruses with Hsc70 during cell entry is mediated by VP5

Rotavirus infection seems to be a multistep process in which the viruses are required to interact with several cell surface molecules to enter the cell. The virus spike protein VP4, which is cleaved by trypsin into two subunits, VP5 and VP8, is involved in some of these interactions. We have previously shown that the neuraminidase-sensitive rotavirus strain RRV initially attaches to a sialic acid-containing cell molecule through the VP8 subunit of VP4 and subsequently interacts with integrin alpha2beta1 through VP5. After these initial contacts, the virus interacts with at least two additional proteins located at the cell surface, the integrin alphavbeta3 and the heat shock cognate protein Hsc70. In this work, we have shown that rotavirus RRV and its neuraminidase-resistant variant nar3 interact with Hsc70 through a VP5 domain located between amino acids 642 and 658 of the protein. This conclusion is based on the observation that a recombinant protein comprising the 300 carboxy-terminal amino acids of VP5 binds specifically to Hsc70 and a synthetic peptide containing amino acids 642 to 658 competes with the binding of the RRV and nar3 viruses to the heat shock protein. The VP5 peptide also competed with the binding to Hsc70 of the recombinant VP5 protein, and an antibody to Hsc70 reduced the binding of the recombinant protein to the surface of MA104 cells. The fact that the synthetic peptide blocks the infectivity of rotaviruses RRV and nar3 but not their binding to cells indicates that the interaction of VP5 with Hsc70 most probably occurs at a postattachment step during the virus entry process.     

Functional and topographical analyses of epitopes on the hemagglutinin (VP4) of the simian rotavirus SA11.

An immunochemical analysis of the hemagglutinin (VP4) of the simian rotavirus SA11 was performed to better understand the structure and function of this molecule. Following immunization of mice with double-shelled virus particles and VP4-enriched fractions from CsCl gradients, a battery of anti-SA11 hybridomas was generated. A total of 13 clones secreting high levels of anti-VP4 monoclonal antibody (MAb) was characterized and compared with two cross-reactive anti-VP4 MAbs generated against heterologous rhesus (RRV) and porcine (OSU) rotavirus strains. These cross-reactive MAbs effectively neutralized SA11 infectivity in vitro. The epitopes recognized by these 15 MAbs were grouped into six antigenic sites on the SA11 hemagglutinin. These sites were identified following analysis of the MAbs by using a simple competitive binding enzyme-linked immunosorbent assay (ELISA) and biological assays. Three of the antigenic sites were involved in neutralization of virus infectivity in vitro. All the MAbs with neutralization activity and two nonneutralizing MAbs were able to inhibit viral hemagglutination of human erythrocytes. Competitive binding ELISA data showed a positive cooperative binding effect with some pairs of the anti-VP4 MAbs, apparently due to a conformational change induced by the binding of the first MAb. Some of the MAbs also bound better to trypsin-treated virus than to non-trypsin-treated virus. A topographic map for VP4 is proposed on the basis of the observed properties of each antigenic site.     

Purification and characterization of adult diarrhea rotavirus: identification of viral structural proteins.

Adult diarrhea rotavirus (ADRV) is a newly identified strain of noncultivable human group B rotavirus that has been epidemic in the People's Republic of China since 1982. We have used sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western (immuno-) blot analysis to examine the viral proteins present in the outer and inner capsids of ADRV and compared these with the proteins of a group A rotavirus, SA11. EDTA treatment of double-shelled virions removed the outer capsid and resulted in the loss of three polypeptides of 64, 61, and 41, kilodaltons (kDa). Endo-beta-N-acetylglucosaminidase H digestion of double-shelled virions identified the 41-kDa polypeptide as a glycoprotein. CaCl2 treatment of single-shelled particles removed the inner capsid and resulted in the loss of one polypeptide with a molecular mass of 47 kDa. The remaining core particle had two major structural proteins of 136 and 113 kDa. All of the proteins visualized on sodium dodecyl sulfate-polyacrylamide gel electrophoresis were antigenic by Western blot analysis when probed with convalescent-phase human and animal antisera. A 47-kDa polypeptide was most abundant and was strongly immunoreactive with human sera, animal sera raised against ADRV and against other group B animal rotaviruses (infectious diarrhea of infant rat virus, bovine and porcine group B rotavirus, and bovine enteric syncytial virus) and a monoclonal antibody prepared against infectious diarrhea of infant rat virus. This 47-kDa inner capsid polypeptide contains a common group B antigen and is similar to the VP6 of the group A rotaviruses. Human convalescent-phase sera also responded to a 41-kDa polypeptide of the outer capsid that seems similar to the VP7 of group A rotavirus. Other polypeptides have been given tentative designations on the basis of similarities to the control preparation of SA11, including a 136-kDa polypeptide designated VP1, a 113-kDa polypeptide designated VP2, 64- and 61-kDa polypeptides designated VP5 and VP5a, and several proteins in the 110- to 72-kDa range that may be VP3, VP4, or related proteins. The lack of cross-reactivity on Western blots between antisera to group A versus group B rotaviruses confirmed that these viruses are antigenically quite distinct.     

The concentration of Ca2+ that solubilizes outer capsid proteins from rotavirus particles is dependent on the strain.

It has been previously shown that rotavirus maturation and stability of the outer capsid are calcium-dependent processes. More recently, it has been hypothesized that penetration of the cell membrane is also affected by conformational changes of the capsid induced by Ca2+. In this study, we determined quantitatively the critical concentration of calcium ion that leads to solubilization of the outer capsid proteins VP4 and VP7. Since this critical concentration is below or close to trace levels of Ca2+, we have used buffered solutions based on ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) and Ca-EGTA. This method allowed us to show a very high variability of the free [Ca2+] needed to stabilize, at room temperature, the outer capsid of several rotavirus strains. This concentration is about 600 nM for the two bovine strains tested (RF and UK), 100 nM for the porcine strain OSU, and only 10 to 20 nM for the simian strain SA11. Titration of viral infectivity after incubation in buffer of defined [Ca2+] confirmed that the loss of infectivity occurs at different [Ca2+] for these three strains. For the bovine strain, the cleavage of VP4 by trypsin has no significant effect on the [Ca2+] that solubilizes outer shell proteins. The outer layer (VP7) of virus-like particles (VLP) made of recombinant proteins VP2, VP6, and VP7 (VLP2/6/7) was also solubilized by lowering the [Ca2+]. The critical concentration of Ca2+ needed to solubilize VP7 from VLP2/6/7 made of protein from the bovine strain is close to the concentration needed for the corresponding virus. Genetic analysis of this phenotype in a set of reassortant viruses from two parental strains having the phenotypes of strains OSU (porcine) and UK (bovine) confirmed that this property of viral particles is probably associated with the gene coding for VP7. The analysis of VLP by reverse genetics might allow the identification of the region(s) essential for calcium binding.