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

Rotavirus Particle Disassembly and Assembly In Vivo and In Vitro

Rotaviruses (RVs) are non-enveloped multilayered dsRNA viruses that are major etiologic agents of diarrheal disease in humans and in the young in a large number of animal species. The viral particle is composed of three different protein layers that enclose the segmented dsRNA genome and the transcriptional complexes. Each layer defines a unique subparticle that is associated with a different phase of the replication cycle. Thus, while single- and double-layered particles are associated with the intracellular processes of selective packaging, genome replication, and transcription, the viral machinery necessary for entry is located in the third layer. This modular nature of its particle allows rotaviruses to control its replication cycle by the disassembly and assembly of its structural proteins. In this review, we examine the significant advances in structural, molecular, and cellular RV biology that have contributed during the last few years to illuminating the intricate details of the RV particle disassembly and assembly processes.

Influence of Epicuticular Physicochemical Properties on Porcine Rotavirus Adsorption to 24 Leafy Green Vegetables and Tomatoes

Foodborne diseases are a persistent problem in the United States and worldwide. Fresh produce, especially those used as raw foods like salad vegetables, can be contaminated, causing illness. In this study, we determined the number of rotaviruses adsorbed on produce surfaces using group A porcine rotaviruses and 24 cultivars of leafy vegetables and tomato fruits. We also characterized the physicochemical properties of each produce's outermost surface layer, known as the epicuticle. The number of rotaviruses found on produce surfaces varied among cultivars. Three-dimensional crystalline wax structures on the epicuticular surfaces were found to significantly contribute to the inhibition of viral adsorption to the produce surfaces (p = 0.01). We found significant negative correlations between the number of rotaviruses adsorbed on the epicuticular surfaces and the concentrations of alkanes, fatty acids, and total waxes on the epicuticular surfaces. Partial least square model fitting results suggest that alkanes, ketones, fatty acids, alcohols, contact angle and surface roughness together can explain 60% of the variation in viral adsorption. The results suggest that various fresh produce surface properties need to be collectively considered for efficient sanitation treatments. Up to 10.8% of the originally applied rotaviruses were found on the produce surfaces after three washing treatments, suggesting a potential public health concern regarding rotavirus contamination.

Rotaviruses reach late endosomes and require the cation-dependent mannose-6-phosphate receptor and the activity of cathepsin proteases to enter the cell

Rotaviruses (RVs) enter cells through different endocytic pathways. Bovine rotavirus (BRV) UK uses clathrin-mediated endocytosis, while rhesus rotavirus (RRV) employs an endocytic process independent of clathrin and caveolin. Given the differences in the cell internalization pathway used by these viruses, we tested if the intracellular trafficking of BRV UK was the same as that of RRV, which is known to reach maturing endosomes (MEs) to infect the cell. We found that BRV UK also reaches MEs, since its infectivity depends on the function of Rab5, the endosomal sorting complex required for transport (ESCRT), and the formation of endosomal intraluminal vesicles (ILVs). However, unlike RRV, the infectivity of BRV UK was inhibited by knocking down the expression of Rab7, indicating that it has to traffic to late endosomes (LEs) to infect the cell. The requirement for Rab7 was also shared by other RV strains of human and porcine origin. Of interest, most RV strains that reach LEs were also found to depend on the activities of Rab9, the cation-dependent mannose-6-phosphate receptor (CD-M6PR), and cathepsins B, L, and S, suggesting that cellular factors from the trans-Golgi network (TGN) need to be transported by the CD-M6PR to LEs to facilitate RV cell infection. Furthermore, using a collection of UK × RRV reassortant viruses, we found that the dependence of BRV UK on Rab7, Rab9, and CD-M6PR is associated with the spike protein VP4. These findings illustrate the elaborate pathway of RV entry and reveal a new process (Rab9/CD-M6PR/cathepsins) that could be targeted for drug intervention.

IMPORTANCE

Rotavirus is an important etiological agent of severe gastroenteritis in children. In most instances, viruses enter cells through an endocytic pathway that delivers the viral particle to vesicular organelles known as early endosomes (EEs). Some viruses reach the cytoplasm from EEs, where they start to replicate their genome. However, other viruses go deeper into the cell, trafficking from EEs to late endosomes (LEs) to disassemble and reach the cytoplasm. In this work, we show that most RV strains have to traffic to LEs, and the transport of endolysosomal proteases from the Golgi complex to LEs, mediated by the mannose-6-phosphate receptor, is necessary for the virus to exit the vesicular compartment and efficiently start viral replication. We also show that this deep journey into the cell is associated with the virus spike protein VP4. These findings illustrate the elaborate pathway of RV entry that could be used for drug intervention.

Interactions between rotavirus and natural organic matter isolates with different physicochemical characteristics

Interaction forces between rotavirus and Suwanee River natural organic matter (SRNOM) or Colorado River NOM (CRNOM) were studied by atomic force microscopy (AFM) in NaCl solutions and at unadjusted pH (5.7-5.9). Compared to CRNOM, SRNOM has more aromatic carbon and phenolic/carboxylic functional groups. CRNOM is characterized with aliphatic structure and considerable presence of polysaccharide moieties rich in hydroxyl functional groups. Strong repulsive forces were observed between rotavirus and silica or mica or SRNOM. The interaction decay length derived from the approaching curves for these systems involving rotavirus in high ionic strength solution was significantly higher than the theoretical Debye length. While no adhesion was observed for rotavirus and SRNOM, attraction was observed between CRNOM and rotavirus during approach and adhesion during retraction. Moreover, these adhesion forces decreased with increasing ionic strength. Interactions due to ionic hydrogen bonding between deprotonated carboxyl groups on rotavirus and hydroxyl functional groups on CRNOM were suggested as the dominant interaction mechanisms between rotavirus and CRNOM.

Role of temperature and Suwannee River natural organic matter on inactivation kinetics of rotavirus and bacteriophage MS2 by solar irradiation

Although the sunlight-mediated inactivation of viruses has been recognized as an important process that controls surface water quality, the mechanisms of virus inactivation by sunlight are not yet clearly understood. We investigated the synergistic role of temperature and Suwannee River natural organic matter (SRNOM), an exogenous sensitizer, for sunlight-mediated inactivation of porcine rotavirus and MS2 bacteriophage. Upon irradiation by a full spectrum of simulated sunlight in the absence of SRNOM and in the temperature range of 14-42 °C, high inactivation rate constants, k(obs), of MS2 (k(obs) ≤ 3.8 h(-1) or 1-log(10) over 0.6 h) and rotavirus (k(obs) ≤ 11.8 h(-1) or ∼1-log(10) over 0.2 h) were measured. A weak temperature (14-42 °C) dependence of k(obs) values was observed for both viruses irradiated by the full sunlight spectrum. Under the same irradiation condition, the presence of SRNOM reduced the inactivation of both viruses due to attenuation of lower wavelengths of the simulated sunlight. For rotavirus and MS2 solutions irradiated by only UVA and visible light in the absence of SRNOM, inactivation kinetics were slow (k(obs) < 0.3 h(-1) or <1-log(10) unit reduction over 7 h) and temperature-independent for the range considered. Conversely, under UVA and visible light irradiation and in the presence of SRNOM, temperature-dependent inactivation of MS2 was observed. For rotavirus, the SRNOM-mediated exogenous inactivation was only important at temperatures >33 °C, with low rotavirus k(obs) values (k(obs) ≈ 0.2 h(-1); 1-log(10) unit reduction over 12 h) for the temperature range of 14-33 °C. These k(obs) values increased to 0.5 h(-1) at 43 °C and 1.5 h(-1) (1-log(10) reduction over 1.6 h) at 50 °C. While SRNOM-mediated exogenous inactivation of MS2 was triggered by singlet oxygen, the presence of hydrogen peroxide was important for rotavirus inactivation in the 40-50 °C range.

Iron oxide amended biosand filters for virus removal

Laboratory studies were performed to determine if the addition of iron oxides throughout biosand filter (BSF) media would increase virus removal due to adsorption. The proposed mechanism is electrostatic adsorption of negatively charged virion particles to positively charged iron oxides formed during the corrosion of zerovalent iron. Initial tests conducted using continuous flow, small-scale glass columns showed high MS2 bacteriophage removal in an iron-amended sand column (5log10) compared to a sand-only column (0.5log10) over 20 pore volumes. Additionally, two experiments with a column containing iron particles revealed 4log10 and 5log10 removal of rotavirus in the presence of 20 mg/L total organic carbon. Full-scale BSFs with iron particles removed>4log10 MS2 for the duration of the experiment (287 days), while BSF with steel wool removed>4log10 MS2 for the first 160 days. Plug flow for the BSF was shown to depend on uniformity between the iron oxide material and sand media grains. The results suggest that the duration of effective virus removal by iron-amended biosand filtration depends on source water conditions and the quantity and composition of iron material added. Overall, this study provides evidence that iron-amended BSFs may advance the field of point-of-use technologies and bring relief to millions of people suffering from waterborne diseases.

Cross-linking of rotavirus outer capsid protein VP7 by antibodies or disulfides inhibits viral entry

Antibodies that neutralize rotavirus infection target outer coat proteins VP4 and VP7 and inhibit viral entry. The structure of a VP7-Fab complex (S. T. Aoki, et al., Science 324:1444-1447, 2009) led us to reclassify epitopes into two binding regions at inter- and intrasubunit boundaries of the calcium-dependent trimer. It further led us to show that antibodies binding at the intersubunit boundary inhibit uncoating of the virion outer layer. We have now tested representative antibodies for each of the defined structural epitope regions and find that antibodies recognizing epitopes in either binding region neutralize by cross-linking VP7 trimers. Antibodies that bind at the intersubunit junction neutralize as monovalent Fabs, while those that bind at the intrasubunit region require divalency. The VP7 structure has also allowed us to design a disulfide cross-linked VP7 mutant which recoats double-layered particles (DLPs) as efficiently as does wild-type VP7 but which yields particles defective in cell entry as determined both by lack of infectivity and by loss of α-sarcin toxicity in the presence of recoated particles. We conclude that dissociation of the VP7 trimer is an essential step in viral penetration into cells.

Separation of rotavirus double-layered particles and triple-layered particles by capillary zone electrophoresis

Cell culture derived rotavirus preparations contain a mixture of double-layered particles (DLPs) and triple-layered particles (TLPs). Characterization of rotavirus vaccine products is important to demonstrate a consistent manufacturing process. A capillary zone electrophoresis (CZE) method was developed to separate and quantitate rotavirus DLPs and TLPs in cell lysate samples and CsCl-purified vaccine preparations of each of the five reassortant rotavirus vaccine strains (G1, G2, G3, G4 and P1) contained in the pentavalent rotavirus vaccine, RotaTeq. The CZE electropherograms showed that migration of DLPs and TLPs from both CsCl-purified and cell lysates resulted in a separation distance of approximately 3 min between the two rotavirus particle types. The identification of the peak(s) containing TLPs was confirmed for both CsCl-purified and cell lysate samples by treatment of the samples with 50mM EDTA, which converted TLPs to DLPs. The migration pattern of the DLPs was consistent (23-24 min) among all reassortant strains tested, whether the DLPs were CsCl-purified or from cell lysates. However, the migration pattern of the TLP electropherograms of the reassortant rotavirus strains in cell lysates differed from those of the CsCl-purified reassortant rotavirus strains. In the cell lysate samples, the TLPs of the G1 and G2 reassortant rotavirus strains migrated slower that the corresponding TLPs from the CsCl-purified samples, while the migration time of the TLPs of the G3, G4 and P1 reassortants strains from the cell lysate and CsCl-purified samples appeared similar. Also, the TLPs from the CsCl-purified samples appeared as a defined single peak, while most of the TLPs from the cell lysate samples appeared as a broad peak or as multiple peaks. All the migration patterns were reproducible and consistent. Taking into account reproducibility, objective quantitation, and minimal sample manipulation as well as volume, CZE allowed consistent and quantitative characterization of rotavirus vaccine preparations, which is required for evaluation of vaccine products, including process validation.

Deposition and aggregation kinetics of rotavirus in divalent cation solutions

Aggregation kinetics of rotavirus in aqueous solutions and its deposition kinetics on silica surface in the presence of divalent (Ca(2+), Mg(2+)) cations were studied using complementary techniques of time-resolved dynamic light scattering (TR-DLS) and quartz crystal microbalance (QCM). Within a reasonable temporal window of 4 h, aggregation could be observed at levels as low as 10 mM of Ca(2+) and 20 mM of Mg(2+). Attachment efficiencies were always greater in Ca(2+) solutions of the same concentration, and the critical coagulation concentration (CCC) for rotavirus in Ca(2+) solutions was slightly smaller than that in Mg(2+) solutions. No aggregation was detected in Na(+) solution within the temporal window of 4 h. Deposition experiments showed higher attachment coefficients in solutions containing Ca(2+) compared to those obtained in Mg(2+) solution. The classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory failed to predict both the aggregation behavior of rotavirus and its deposition on silica surface. Besides electrostatic interactions, steric repulsions and specific interactions with divalent cations were important mechanisms in controlling rotavirus deposition and aggregation. Experimental results presented here suggest that rotavirus is not expected to aggregate in groundwater with typical hardness (up to 6 mM Ca(2+)) and rotavirus deposition on silica soil would be more favorable in the presence of Ca(2+) than Mg(2+).

Adsorption of rotavirus and bacteriophage MS2 using glass fiber coated with hematite nanoparticles

Batch and flow-through experiments were conducted to investigate the removal and inactivation of rotavirus (RV) and bacteriophage MS2 using glass fiber coated with hematite nanoparticles. Batch tests showed a high removal of MS2 (2.49x10(11) plaque forming unit/g) and RV (8.9x10(6) focal forming unit/g) at a low concentration of hematite nanoparticles in solution (0.043g/L and 0.26g/L, respectively). Virus adsorption was, however, decreased in the presence of bicarbonate ions and natural organic matter (NOM) in solution, suggesting a great affinity of iron oxide nanoparticles for these competitors. Adsorption on hematite nanoparticles by MS2 and RV was also tested with aquifer groundwater under saturated flow conditions to mimic environmental conditions with promising results (8x10(8) plaque forming unit/g and 3x10(4) focal forming unit/g, respectively). Desorption of up to 63% of infectious MS2 and only 2% of infectious RV from hematite nanoparticles were achieved when an eluant solution containing beef extract and glycine was used. Transmission electron microscopy (TEM) images showed evidence of electrostatic adsorption of apparently intact MS2 and structurally damaged RV particles to hematite nanoparticles. Results from this research suggest that a cartridge made of glass fiber coated with hematite nanoparticles could be used as a point-of-use device for virus removal for drinking water treatment.

Impact of physicochemical parameters on in vitro assembly and disassembly kinetics of recombinant triple-layered rotavirus-like particles

Virus-like particles constitute potentially relevant vaccine candidates. Nevertheless, their behavior in vitro and assembly process needs to be understood in order to improve their yield and quality. In this study we aimed at addressing these issues and for that purpose triple- and double-layered rotavirus-like particles (TLP 2/6/7 and DLP 2/6, respectively) size and zeta potential were measured using dynamic light scattering at different physicochemical conditions, namely pH, ionic strength, and temperature. Both TLP and DLP were stable within a pH range of 3-7 and at 5-25 degrees C. Aggregation occurred at 35-45 degrees C and their disassembly became evident at 65 degrees C. The isoelectric points of TLP and DLP were 3.0 and 3.8, respectively. In vitro kinetics of TLP disassembly was monitored. Ionic strength, temperature, and the chelating agent employed determined disassembly kinetics. Glycerol (10%) stabilized TLP by preventing its disassembly. Disassembled TLP was able to reassemble by dialysis at high calcium conditions. VP7 monomers were added to DLP in the presence of calcium to follow in vitro TLP assembly kinetics; its assembly rate being mostly affected by pH. Finally, DLP and TLP were found to coexist under certain conditions as determined from all reaction products analyzed by capillary electrophoresis. Overall, these results contribute to the design of new strategies for the improvement of TLP yield and quality by reducing the VP7 detachment from TLP.

Expression of nonstructural rotavirus protein NSP4 mimics Ca2+ homeostasis changes induced by rotavirus infection in cultured cells

Rotavirus infection modifies Ca(2+) homeostasis, provoking an increase in Ca(2+) permeation, the cytoplasmic Ca(2+) concentration ([Ca(2+)](cyto)), and total Ca(2+) pools and a decrease in Ca(2+) response to agonists. A glycosylated viral protein(s), NSP4 and/or VP7, may be responsible for these effects. HT29 or Cos-7 cells were infected by the SA11 clone 28 strain, in which VP7 is not glycosylated, or transiently transfected with plasmids coding for NSP4-enhanced green fluorescent protein (EGFP) or NSP4. The permeability of the plasma membrane to Ca(2+) and the amount of Ca(2+) sequestered in the endoplasmic reticulum released by carbachol or ATP were measured in fura-2-loaded cells at the single-cell level under a fluorescence microscope or in cell suspensions in a fluorimeter. Total cell Ca(2+) pools were evaluated as (45)Ca(2+) uptake. Infection with SA11 clone 28 induced an increase in Ca(2+) permeability and (45)Ca(2+) uptake similar to that found with the normally glycosylated SA11 strain. These effects were inhibited by tunicamycin, indicating that inhibition of glycosylation of a viral protein other than VP7 affects the changes of Ca(2+) homeostasis induced by infection. Expression of NSP4-EGFP or NSP4 in transfected cells induced the same changes observed with rotavirus infection, whereas the expression of EGFP or EGFP-VP4 showed the behavior of uninfected and untransfected cells. Increased (45)Ca(2+) uptake was also observed in cells expressing NSP4-EGFP or NSP4, as evidenced in rotavirus infection. These results indicate that glycosylated NSP4 is primarily responsible for altering the Ca(2+) homeostasis of infected cells through an initial increase of cell membrane permeability to Ca(2+).

Parenteral administration of RF 8-2/6/7 rotavirus-like particles in a one-dose regimen induce protective immunity in mice

Rotavirus virus-like particles (RV-VLPs) represent a novel strategy for development of a rotavirus subunit vaccine. In this study, RF 8-2/6/7-VLPs with rotavirus VP8 protein (amino acid 1-241 of VP4) fused to the amino terminal end of a truncated VP2, were evaluated for their immunogenic and protective properties. A single intramuscular dose of, either 2 or 20 microg, RF 8-2/6/7-VLPs alone or combined with a potent adjuvant poly[di(carboxylatophenoxy)]phosphazene] (PCPP) induced rotavirus-specific serum IgG and IgA, fecal IgG titers that were enhanced 5-90-fold by adjuvant. Passive protective immunity was achieved in offspring to dams vaccinated with 2 and 20 microg RV-VLPs in presence of adjuvant and 20 microg RV-VLP without adjuvant.

Endoplasmic reticulum chaperones are involved in the morphogenesis of rotavirus infectious particles

The final assembly of rotavirus particles takes place in the endoplasmic reticulum (ER). In this work, we evaluated by RNA interference the relevance to rotavirus assembly and infectivity of grp78, protein disulfide isomerase (PDI), grp94, calnexin, calreticulin, and ERp57, members of the two ER folding systems described herein. Silencing the expression of grp94 and Erp57 had no effect on rotavirus infectivity, while knocking down the expression of any of the other four chaperons caused a reduction in the yield of infectious virus of about 50%. In grp78-silenced cells, the maturation of the oligosaccharide chains of NSP4 was retarded. In cells with reduced levels of calnexin, the oxidative folding of VP7 was impaired and the trimming of NSP4 was accelerated, and in calreticulin-silenced cells, the formation of disulfide bonds of VP7 was also accelerated. The knockdown of PDI impaired the formation and/or rearrangement of the VP7 disulfide bonds. All these conditions also affected the correct assembly of virus particles, since compared with virions from control cells, they showed an altered susceptibility to EGTA and heat treatments, a decreased specific infectivity, and a diminished reactivity to VP7 with monoclonal antibody M60, which recognizes only this protein when its disulfide bonds have been correctly formed. In the case of grp78-silenced cells, the virus produced bound less efficiently to MA104 cells than virus obtained from control cells. All these results suggest that these chaperones are involved in the quality control of rotavirus morphogenesis. The complexity of the steps of rotavirus assembly that occur in the ER provide a useful model for studying the organization and operation of the complex network of chaperones involved in maintaining the quality control of this organelle.

Geometric mismatches within the concentric layers of rotavirus particles: a potential regulatory switch of viral particle transcription activity

Rotaviruses are prototypical double-stranded RNA viruses whose triple-layered icosahedral capsid constitutes transcriptional machinery activated by the release of the external layer. To understand the molecular basis of this activation, we studied the structural interplay between the three capsid layers by electron cryo-microscopy and digital image processing. Two viral particles and four virus-like particles containing various combinations of inner (VP2)-, middle (VP6)-, and outer (VP7)-layer proteins were studied. We observed that the absence of the VP2 layer increases the particle diameter and changes the type of quasi-equivalent icosahedral symmetry, as described by the shift in triangulation number (T) of the VP6 layer (from T = 13 to T = 19 or more). By fitting X-ray models of VP6 into each reconstruction, we determined the quasi-atomic structures of the middle layers. These models showed that the VP6 lattices, i.e., curvature and trimer contacts, are characteristic of the particle composition. The different functional states of VP6 thus appear as being characterized by trimers having similar conformations but establishing different intertrimeric contacts. Remarkably, the external protein VP7 reorients the VP6 trimers located around the fivefold axes of the icosahedral capsid, thereby shrinking the channel through which mRNA exits the transcribing rotavirus particle. We conclude that the constraints arising from the different geometries imposed by the external and internal layers of the rotavirus capsid constitute a potential switch regulating the transcription activity of the viral particles.

Intracellular disassembly of infectious rotavirus particles by depletion of Ca2+ sequestered in the endoplasmic reticulum at the end of virus cycle

Rotavirus infection is characterized by a number of Ca(2+) dependent virus-cell interactions. The structure of rotavirus triple-layered particles (TLP) is dependent on Ca(2+) concentration. Acquisition of the capsid outer layer requires a high Ca(2+) concentration inside the ER. Infection modifies Ca(2+) homeostasis of the cell, increasing ER Ca(2+) content, which may be advantageous to virus replication. We studied the role of sequestered Ca(2+) on the stabilization of already mature viral particles within the ER. Thapsigargin (TG), a SERCA pump inhibitor, added for 30min at the end of infection depleted ER Ca(2+) and reduced the titer of already mature TLP accumulated in the cell. Another inhibitor, cyclopiazonic acid, and two Ca(2+) ionophores (A23187 and ionomycin) in the presence of EGTA had similar effects. TG eliminated the peak of radiolabeled TLP, increasing that of DLP in CsCl gradients. Electron microscopy revealed accumulation of clustered particles in the ER, which had lost their integrity. The [Ca(2+)] in the ER of infected cells is important for virus maturation and for maintaining the integrity of mature TLP. Viral particles in this compartment may be potentially infectious, already containing VP7 and VP4.

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.

Adenovirus protein VI mediates membrane disruption following capsid disassembly

In contrast to enveloped viruses, the mechanisms involved in membrane penetration by nonenveloped viruses are not as well understood. In these studies, we determined the relationship between adenovirus (Ad) capsid disassembly and the development of membrane lytic activity. Exposure to low pH or heating induced conformational changes in wild-type Ad but not in temperature-sensitive Ad (ts1) particles that fail to escape the early endosome. Wild-type Ad but not ts1 particles permeabilized model membranes (liposomes) and facilitated the cytosolic delivery of a ribotoxin. Alterations in wild-type Ad capsids were associated with the exposure of a pH-independent membrane lytic factor. Unexpectedly, this factor was identified as protein VI, a 22-kDa cement protein located beneath the peripentonal hexons in the viral capsid. Recombinant protein VI and preprotein VI, but not a deletion mutant lacking an N-terminal amphipathic alpha-helix, possessed membrane lytic activity similar to partially disassembled virions. A new model of Ad entry is proposed based on our present observations of capsid disassembly and membrane penetration.

Emerging themes in rotavirus cell entry, genome organization, transcription and replication

Rotaviruses, causative agents of gastroenteritis in young animals and humans, are large icosahedral viruses with a complex architecture. The double-stranded RNA (dsRNA) genome composed of 11 segments, which codes for 6 structural and 6 non-structural proteins, is enclosed within three concentric capsid layers. In addition to facilitating host-specific interactions, the design of the capsid architecture in rotaviruses as in other dsRNA viruses should also be conducive to the requirement of transcribing the enclosed genome segments repeatedly and simultaneously within the capsid interior. Several non-structural proteins facilitate the subsequent processes of genome replication and packaging. Electron cryomicroscopy studies of intact virions, recombinant virus-like particles, functional complexes, together with recent X-ray crystallographic studies on rotavirus proteins have provided structural insights into the capsid architecture, genome organization, antibody interaction, cell entry, trypsin-enhanced infectivity, endogenous transcription and replication. These studies underscore contrasting features and unifying themes between rotavirus and other dsRNA viruses.

Discrete domains within the rotavirus VP5* direct peripheral membrane association and membrane permeability

Cleavage of the rotavirus spike protein, VP4, is required for rotavirus-induced membrane permeability and viral entry into cells. The VP5* cleavage product selectively permeabilizes membranes and liposomes and contains an internal hydrophobic domain that is required for membrane permeability. Here we investigate VP5* domains (residues 248 to 474) that direct membrane binding. We determined that expressed VP5 fragments containing residues 248 to 474 or 265 to 474, including the internal hydrophobic domain, bind to cellular membranes but are not present in Triton X-100-resistant membrane rafts. Expressed VP5 partitions into aqueous but not detergent phases of Triton X-114, suggesting that VP5 is not integrally inserted into membranes. Since high-salt or alkaline conditions eluted VP5 from membranes, our findings demonstrate that VP5 is peripherally associated with membranes. Interestingly, mutagenesis of residue 394 (W-->R) within the VP5 hydrophobic domain, which abolishes VP5-directed permeability, had no effect on VP5's peripheral membrane association. In contrast, deletion of N-terminal VP5 residues (residues 265 to 279) abolished VP5 binding to membranes. Alanine mutagenesis of two positively charged residues within this domain (residues 274R and 276K) dramatically reduced (>95%) binding of VP5 to membranes and suggested their potential interaction with polar head groups of the lipid bilayer. Mutations in either the VP5 hydrophobic or basic domain blocked VP5-directed permeability of cells. These findings indicate that there are at least two discrete domains within VP5* required for pore formation: an N-terminal basic domain that permits VP5* to peripherally associate with membranes and an internal hydrophobic domain that is essential for altering membrane permeability. These results provide a fundamental understanding of interactions between VP5* and the membrane, which are required for rotavirus entry.

Requirement for vacuolar H+ -ATPase activity and Ca2+ gradient during entry of rotavirus into MA104 cells

The mechanism by which rotavirus and other nonenveloped viruses enter the cell is still not clear. We have proposed an endocytosis model where the critical step for virus uncoating and membrane permeabilization is the decrease in Ca(2+) concentration in the endosome. In this paper, we monitored rotavirus entry by measuring alpha-sarcin-rotavirus coentry and infectivity in MA104 cells. The participation of endocytosis, acidification, and endosomal Ca(2+) concentration on virus entry was studied by inhibiting the endosomal H(+)-ATPase with bafilomycin A1 and/or increasing the extracellular calcium reservoir by addition of 10 mM CaEGTA. Rotavirus-alpha-sarcin coentry was inhibited by bafilomycin A1 and by addition of 10 mM CaEGTA. These effects were additive. These substances induced a significant inhibition of infectivity without affecting virus binding and postentry steps. These results are compatible with the interpretation that bafilomycin A1 and CaEGTA block rotavirus penetration from the endosome into the cytoplasm and support our hypothesis of a Ca(2+)-dependent endocytosis model.

Identification of rotavirus VP6 residues located at the interface with VP2 that are essential for capsid assembly and transcriptase activity

Rotavirus has a complex triple-layered icosahedral capsid. The external layer consists of VP7 and VP4, the intermediate layer consists of VP6 trimers, and the internal layer consists of VP2. Double-layered particles (DLP) derived from the virus by solubilization of VP4 and VP7 are transcriptionally competent and extrude capped mRNA from their vertices. Analysis of the pseudoatomic model of the VP6 layer, obtained by placing the atomic structure of VP6 into electron microscopy reconstructions of the DLP, has identified the regions of the protein involved in interactions with the internal layer. To study the role of VP6 both in the assembly of DLP and in transcription, 13 site-specific substitution mutations of VP6, targeting the contacts between the two inner layers, were constructed and expressed in the baculovirus system. The effects of these mutations on VP6 expression, trimerization, and formation of macromolecular assemblies were investigated. Using either in vitro reconstituted DLP derived from purified viral cores and recombinant VP6 or in vivo self-assembled virus-like particles resulting from the coexpression of VP2 and VP6 in the baculovirus-Sf9 system (VLP2/6), we have identified the amino acids essential for recovery of transcription or assembly. All VP6 mutants formed stable trimers which, like wild-type VP6, assembled into tubular structures. The ability of VP6 to interact with VP2 was examined by several assays, including electron microscopy, coimmunoprecipitation, purification of VLP2/6, and monitoring of the transcriptase activity of reconstituted DLP. Of the 13 VP6 mutants examined, 3 were unable to assemble with VP2 and 3 others partially assembled. These mutants either did not rescue the transcriptase activity of core particles or did so only marginally. Four mutants as well as the wild-type VP6 assembled and transcribed very well. Three mutants assembled well on cores but, surprisingly, did not rescue the transcriptase activity of reconstituted DLP. Our results indicate that hydrophobic interactions between VP6 and VP2 residues are responsible for the stability of the DLP. They also show that subtle electrostatic interactions between VP6 and the underlying transcriptase machinery can be essential for mRNA synthesis.

Antibodies to rotavirus outer capsid glycoprotein VP7 neutralize infectivity by inhibiting virion decapsidation

The rotavirus capsid is composed of three concentric protein layers. Proteins VP4 and VP7 comprise the outer layer. VP4 forms spikes, is the viral attachment protein, and is cleaved by trypsin into VP8* and VP5*. VP7 is a glycoprotein and the major constituent of the outer protein layer. Both VP4 and VP7 induce neutralizing and protective antibodies. To gain insight into the virus neutralization mechanisms, the effects of neutralizing monoclonal antibodies (MAbs) directed against VP8*, VP5*, and VP7 on the decapsidation process of purified OSU and RRV virions were studied. Changes in virion size were followed in real time by 90 degrees light scattering. The transition from triple-layered particles to double-layered particles induced by controlled low calcium concentrations was completely inhibited by anti-VP7 MAbs but not by anti-VP8* or anti-VP5* MAbs. The inhibitory effect of the MAb directed against VP7 was concentration dependent and was abolished by papain digestion of virus-bound antibody under conditions that generated Fab fragments but not under conditions that generated F(ab')(2) fragments. Electron microscopy showed that RRV virions reacted with an anti-VP7 MAb stayed as triple-layered particles in the presence of excess EDTA. Furthermore, the infectivity of rotavirus neutralized via VP8*, but not that of rotavirus neutralized via VP7, could be recovered by lipofection of neutralized particles into MA-104 cells. These data are consistent with the notion that antibodies directed at VP8* neutralize by inhibiting binding of virus to the cell. They also indicate that antibodies directed at VP7 neutralize by inhibiting virus decapsidation, in a manner that is dependent on the bivalent binding of the antibody.

Ionic strength- and temperature-induced K(Ca) shifts in the uncoating reaction of rotavirus strains RF and SA11: correlation with membrane permeabilization

The hydrodynamic diameters of native rotavirus particles, bovine RF and simian SA11 strains, were determined by quasielastic light scattering. By using this method and agarose gel electrophoresis, the Ca(2+) dissociation constant, K(Ca), governing the transition from triple-layer particles (TLPs) to double-layer particles (DLPs), was shown to increase, at constant pH, as the temperature and/or the ionic strength of the incubation medium increased. We report the novel observation that, under physiological conditions, K(Ca) values for both RF and SA11 rotaviruses were well above the intracytoplasmic Ca(2+) concentrations of various cells, which may explain why TLP uncoating takes place within vesicles (possibly endosomes) during the entry process. A correlation between TLP uncoating and cell membrane permeabilization was found, as shown by the release of carboxyfluorescein (CF) from CF-loaded intestinal brush-border membrane vesicles. Conditions stabilizing the virion in the TLP form inhibited CF release, whereas conditions favoring the TLP-to-DLP transformation activated this process. We conclude that membrane permeabilization must be preceded by the loss of the outer-capsid proteins from trypsinized TLP and that physiological ionic strength is required for permeabilization to take place. Finally, the paper develops an alternative explanation for the mechanism of rotavirus entry, compatible with the Ca(2+)-dependent endocytic pathway. We propose that there must be an iterative process involving tight coupling in time between the lowering of endosomal Ca(2+) concentration, virion decapsidation, and membrane permeabilization, which would cause the transcriptionally active DLPs to enter the cytoplasm of cells.

VP4 differentially regulates TRAF2 signaling, disengaging JNK activation while directing NF-kappa B to effect rotavirus-specific cellular responses

Rotaviruses rapidly activate NF-kappaB and induce the secretion of selected chemokines after infection. The ability of rotavirus particles lacking genomic RNA to activate NF-kappaB suggested that rotavirus proteins direct cell signaling responses. We identified conserved TNFR-associated factor (TRAF) binding motifs within the rotavirus capsid protein VP4 and its N-terminal VP8* cleavage product. TRAFs (-1, -2, and -3) are bound by the rhesus rotavirus VP8* protein through three discrete TRAF binding domains. Expression of VP4 or VP8* from rhesus or human rotaviruses induced a 5-7-fold increase in NF-kappaB activity and synergistically enhanced TRAF2-mediated NF-kappaB activation. Mutagenesis of VP8* TRAF binding motifs abolished VP8* binding to TRAFs and the ability of the protein to activate NF-kappaB. Expression of pathway-specific dominant negative (DN) inhibitors DN-TRAF2 or DN-NF-kappaB-inducing kinase also abolished VP8*-, VP4-, or rotavirus-mediated NF-kappaB activation. These findings demonstrate that rotavirus primarily activates NF-kappaB through a TRAF2-NF-kappaB-inducing kinase signaling pathway and that VP4 and VP8* proteins direct pathway activation through interactions with cellular TRAFs. In contrast, transcriptional responses from AP-1 reporters were inhibited 5-fold by VP8* and were not activated by rotavirus infection, suggesting the differential regulation of TRAF2 signaling responses by VP8*. VP8* blocked JNK activation directed by TRAF2 or TRAF5 but had no effect on JNK activation directed by TRAF6 or MEKK1. This establishes that fully cytoplasmic rotaviruses selectively engage signaling pathways, which regulate cellular transcriptional responses. These findings also demonstrate that TRAF2 interactions can disengage JNK signaling from NF-kappaB activation and thereby provide a new means for TRAF2 interactions to determine pathway-specific responses.

Individual rotavirus-like particles containing 120 molecules of fluorescent protein are visible in living cells

Rotaviruses are large, complex icosahedral particles consisting of three concentric capsid layers. When the innermost capsid protein VP2 is expressed in the baculovirus-insect cell system it assembles as core-like particles. The amino terminus region of VP2 is dispensable for assembly of virus-like particles (VLP). Coexpression of VP2 and VP6 produces double layered VLP. We hypothesized that the amino end of VP2 could be extended without altering the auto assembly properties of VP2. Using the green fluorescent protein (GFP) or the DsRed protein as model inserts we have shown that the chimeric protein GFP (or DsRed)-VP2 auto assembles perfectly well and forms fluorescent VLP (GFP-VLP2/6 or DsRed-VLP2/6) when coexpressed with VP6. The presence of GFP inside the core does not prevent the assembly of the outer capsid layer proteins VP7 and VP4 to give VLP2/6/7/4. Cryo-electron microscopy of purified GFP-VLP2/6 showed that GFP molecules are located at the 5-fold vertices of the core. It is possible to visualize a single fluorescent VLP in living cells by confocal fluorescent microscopy. In vitro VLP2/6 did not enter into permissive cells or in dendritic cells. In contrast, fluorescent VLP2/6/7/4 entered the cells and then the fluorescence signal disappear rapidly. Presented data indicate that fluorescent VLP are interesting tools to follow in real time the entry process of rotavirus and that chimeric VLP could be envisaged as "nanoboxes" carrying macromolecules to living cells.

Purified recombinant rotavirus VP7 forms soluble, calcium-dependent trimers

Rotavirus is a major cause of severe, dehydrating childhood diarrhea. VP7, the rotavirus outer capsid glycoprotein, is a target of protective antibodies and is responsible for the calcium-dependent uncoating of the virus during cell entry. We have purified, characterized, and crystallized recombinant rhesus rotavirus VP7, expressed in insect cells. A critical aspect of the purification is the elution of VP7 from a neutralizing monoclonal antibody column by EDTA. Gel filtration chromatography and equilibrium analytical ultracentrifugation demonstrate that, in the presence of calcium, purified VP7 trimerizes. Trimeric VP7 crystallizes into hexagonal plates. Preliminary X-ray analysis suggests that the crystal packing reproduces the hexagonal component of the icosahedral lattice of VP7 on triple-layered rotavirus particles. These data indicate that the rotavirus outer capsid assembles from calcium-dependent VP7 trimers and that dissociation of these trimers is the biochemical basis for EDTA-induced rotavirus uncoating and loss of VP7 neutralizing epitopes.

Role of Ca2+in the replication and pathogenesis of rotavirus and other viral infections

Ca2+ plays a key role in many pathological processes, including viral infections. Rotavirus, the major etiological agent of viral gastroenteritis in children and young animals, provides a useful model to study a number of Ca2+ dependent virus-cell interactions. Rotavirus entry, activation of transcription, morphogenesis, cell lysis, particle release, and the distant action of viral proteins are Ca2+ dependent processes. In the extracellular medium, Ca2+ stabilizes the structure of the viral capsid. During entry into the cell the low cytoplasmic Ca2+ concentration induced the solubilization of the outer protein layer of the capsid and transcriptase activation. Viral protein synthesis modifies Ca2+ homeostasis which, in turn, favours viral morphogenesis and induces cell death. The generation of diarrhea is a multifactorial process involving Ca2+ dependent secretory processes of mediators and water and electrolytes, as well as the induction of cell death in the different cell types that compose the intestinal epithelium. The discovery of the non-structural viral protein NSP4 as a viral enterotoxin and the possible participation of the enteric nervous system in the pathogenesis of diarrhea represent significant advances in its understanding. Ca2+ also plays a role in the replication cycles and pathogenesis of other viral diseases such as poliovirus, Coxsackie virus, cytomegalovirus, vaccinia and measles virus and HIV.

Selective membrane permeabilization by the rotavirus VP5* protein is abrogated by mutations in an internal hydrophobic domain

Rotavirus infectivity is dependent on the proteolytic cleavage of the VP4 spike protein into VP8* and VP5* proteins. Proteolytically activated virus, as well as expressed VP5*, permeabilizes membranes, suggesting that cleavage exposes a membrane-interactive domain of VP5* which effects rapid viral entry. The VP5* protein contains a single long hydrophobic domain (VP5*-HD, residues 385 to 404) at an internal site. In order to address the role of the VP5*-HD in permeabilizing cellular membranes, we analyzed the entry of o-nitrophenyl-beta-D-galactopyranoside (ONPG) into cells induced to express VP5* or mutated VP5* polypeptides. Following IPTG (isopropyl-beta-D-thiogalactopyranoside) induction, VP5* and VP5* truncations containing the VP5*-HD permeabilized cells to the entry and cleavage of ONPG, while VP8* and control proteins had no effect on cellular permeability. Expression of VP5* deletions containing residues 265 to 474 or 265 to 404 permeabilized cells; however, C-terminal truncations which remove the conserved GGA (residues 399 to 401) within the HD abolished membrane permeability. Site-directed mutagenesis of the VP5-HD further demonstrated a requirement for residues within the HD for VP5*-induced membrane permeability. Functional analysis of mutant VP5*s indicate that conserved glycines within the HD are required and suggest that a random coiled structure rather than the strictly hydrophobic character of the domain is required for permeability. Expressed VP5* did not alter bacterial growth kinetics or lyse bacteria following induction. Instead, VP5*-mediated size-selective membrane permeability, releasing 376-Da carboxyfluorescein but not 4-kDa fluorescein isothiocyanate-dextran from preloaded liposomes. These findings suggest that the fundamental role for VP5* in the rotavirus entry process may be to expose triple-layered particles to low [Ca](i), which uncoats the virus, rather than to effect the detergent-like lysis of early endosomal membranes.

Rotavirus capsid protein VP5* permeabilizes membranes

Proteolytic cleavage of the VP4 outer capsid spike protein into VP8* and VP5* proteins is required for rotavirus infectivity and for rotavirus-induced membrane permeability. In this study we addressed the function of the VP5* cleavage fragment in permeabilizing membranes. Expressed VP5* and truncated VP5* proteins were purified by nickel affinity chromatography and assayed for their ability to permeabilize large unilamellar vesicles (LUVs) preloaded with carboxyfluorescein (CF). VP5* and VP5* truncations, but not VP4 or VP8*, permeabilized LUVs as measured by fluorescence dequenching of released CF. Similar to virus-induced CF release, VP5*-induced CF release was concentration and temperature dependent, with a pH optimum of 7.35 at 37 degrees C, but independent of the presence of divalent cations or cholesterol. VP5*-induced permeability was completely inhibited by VP5*-specific neutralizing monoclonal antibodies (2G4, M2, or M7) which recognize conformational epitopes on VP5* but was not inhibited by VP8*-specific neutralizing antibodies. In addition, N-terminal and C-terminal VP5* truncations including residues 265 to 474 are capable of permeabilizing LUVs. These findings demonstrate that VP5* permeabilizes membranes in the absence of other rotavirus proteins and that membrane-permeabilizing VP5* truncations contain the putative fusion region within predicted virion surface domains. The ability of recombinant expressed VP5* to permeabilize membranes should permit us to functionally define requirements for VP5*-membrane interactions. These findings indicate that VP5* is a specific membrane-permeabilizing capsid protein which is likely to play a role in the cellular entry of rotaviruses.

Characterization of a membrane calcium pathway induced by rotavirus infection in cultured cells

Some viruses induce changes in membrane permeability during infection. We have shown previously that the porcine strain of rotavirus, OSU, induced an increase in the permeability to Na+, K+, and Ca2+ during replication in MA104 cells. In this work, we have characterized the divalent cation entry pathway by measuring intracellular Ca2+ in fura-2-loaded MA104 and HT29 cells in suspension. The permeability to Ca2+ and other cations was evaluated by the change of the intracellular concentration following an extracellular cation pulse. Rotavirus infection induced an increase in permeability to Ca2+, Ba2+, Sr2+, Mn2+, and Co2+. The rate of cation entry decreased over time as the intracellular concentration increased during the first 20 s. This indicates that regulatory mechanisms, including channel inactivation, are triggered. La3+ did not enter the cell and blocked the entry of the divalent cations in a dose-dependent manner. Metoxyverapamil (D600), a blocker of L-type voltage-gated channels, partially inhibited the entry of Ca2+ in virus-infected MA104 and HT29 cells. The results suggest that rotavirus infection of cultured cells activates a cation channel rather than nonspecific permeation through the plasma membrane. This activation involves the synthesis of viral proteins through mechanisms yet unknown. The increase in intracellular Ca2+ induced by the activation of this channel may be related to the increase in cytoplasmic and endoplasmic reticulum Ca2+ pools required for virus maturation and cell death.

The molecular chaperone calnexin interacts with the NSP4 enterotoxin of rotavirus in vivo and in vitro

Calnexin is an endoplasmic reticulum (ER)-associated molecular chaperone proposed to promote folding and assembly of glycoproteins that traverse the secretory pathway in eukaryotic cells. In this study we examined if calnexin interacts with the ER-associated luminal (VP7) and transmembrane (NSP4) proteins of rotavirus. Only glycosylated NSP4 interacted with calnexin and did so in a time-dependent manner (half-life, 20 min). In vitro translation experiments programmed with gene 10 of rhesus rotavirus confirmed that calnexin recognizes only glycosylated NSP4. Castanospermine (a glucosidase I and II inhibitor) experiments established that calnexin associates only with partly deglucosylated (di- or monoglucosylated) NSP4. Furthermore, enzymatic removal of the remaining glucose residues on the N-linked glycan units was essential to disengage the NSP4-calnexin complex. Novel experiments with castanospermine revealed that glucose trimming and the calnexin-NSP4 interaction were not critical for the assembly of infectious virus.

Rotaviruses induce an early membrane permeabilization of MA104 cells and do not require a low intracellular Ca2+ concentration to initiate their replication cycle

In this work, we found that rotavirus infection induces an early membrane permeabilization of MA104 cells and promotes the coentry of toxins, such as alpha-sarcin, into the cell. This cell permeability was shown to depend on infectious virus and was also shown to be virus dose dependent, with 10 infectious particles per cell being sufficient to achieve maximum permeability; transient, lasting no more than 15 min after virus entry and probably occurring concomitantly with virus penetration; and specific, since cells that are poorly permissive for rotavirus were not permeabilized. The rotavirus-mediated coentry of toxins was not blocked by the endocytosis inhibitors dansylcadaverine and cytochalasin D or by the vacuolar proton-ATPase inhibitor bafilomycin A1, suggesting that neither endocytocis nor an intraendosomal acidic pH or a proton gradient is required for permeabilization of the cells. Compounds that raise the intracellular concentration of calcium ([Ca2+]i) by different mechanisms, such as the calcium ionophores A23187 and ionomycin and the endoplasmic reticulum calcium-ATPase inhibitor thapsigargin, did not block the coentry of alpha-sarcin or affect the onset of viral protein synthesis, suggesting that a low [Ca2+]i is not essential for the initial steps of the virus life cycle. Since the entry of alpha-sarcin correlates with virus penetration in all parameters tested, the assay for permeabilization to toxins might be a useful tool for studying and characterizing the route of entry and the mechanism used by rotaviruses to traverse the cell membrane and initiate a productive replication cycle.

Poliovirus protein 2BC increases cytosolic free calcium concentrations

Poliovirus-infected cells undergo an increase in cytoplasmic calcium concentrations from the 4th h postinfection. The protein responsible for this effect was identified by the expression of different poliovirus nonstructural proteins in HeLa cells by using a recombinant vaccinia virus system. Synthesis of protein 2BC enhances cytoplasmic calcium concentrations in a manner similar to that observed in poliovirus-infected cells. To identify the regions in 2BC involved in modifying cytoplasmic calcium levels, several 2BC variants were generated. Regions present in both 2B and 2C are necessary to augment cellular free calcium levels. Therefore, in addition to inducing proliferation of membranous vesicles, poliovirus protein 2BC also alters cellular calcium homeostasis.

Two proline residues are essential in the calcium-binding activity of rotavirus VP7 outer capsid protein

Rotavirus maturation and stability of the outer capsid are calcium-dependent processes. It has been shown previously that the concentration of Ca2+-solubilizing outer capsid proteins from rotavirus particles is dependent on the virus strain. This property of viral particles has been associated with the gene coding for VP7 (gene 9). In this study the correlation between VP7 and resistance to low [Ca2+] was confirmed by analyzing the origin of gene 9 from reassortant viruses prepared under the selective pressure of low [Ca2+]. After chemical mutagenesis, we selected mutant viruses of the bovine strain RF that are more resistant to low [Ca2+]. The genes coding for the VP7 proteins of these independent mutants have been sequenced. Sequence analysis confirmed that these mutants are independent and revealed that all mutant VP7 proteins have proline 75 changed to leucine and have an outer capsid that solubilized at low [Ca2+]. The mutation of proline 279 to serine is found in all but two mutants. The phenotype of mutants having a single proline change can be distinguished from the phenotype of mutants having two proline changes. Sequence analysis showed that position 75 is in a region (amino acids 65 to 78) of great variability and that proline 75 is present in most of the bovine strains. In contrast, proline 279 is in a conserved region and is conserved in all the VP7 sequences in data banks. This region is rich in oxygenated residues that are correctly allocated in the metal-coordinating positions of the Ca2+-binding EF-hand structure pattern, suggesting that this region is important in the Ca2+ binding of VP7.