INTRODUCTION

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Rotaviruses are nonenveloped, triple-layered icosahedral viruses with a genome of 11 segments of double-stranded RNA encoding six structural and five nonstructural polypeptides (11). These viruses have a unique mode of assembly involving specialized viroplasmic inclusions located adjacent to the ER. In the final stages of maturation, the immature inner capsid particle (ICP) is transferred across the membrane of the ER by a budding event initiated when the ICP interacts with a viral nonstructural glycoprotein, NSP4 (1, 23). During and after transfer to the ER lumen, the ICP is enveloped transiently in a membrane vesicle and the outer shell proteins VP7 and VP4 are assembled onto the surface of the particle. The precise details of how infectious virions are assembled within the ER lumen are poorly understood. Little is known about the requirement for the involvement of host proteins in this process, although several studies have demonstrated that a high concentration of Ca²⁺ within the ER lumen is necessary for productive assembly of virions (24, 25, 30).

Rotavirus exhibits a distinctive cytopathic effect characterized by a dramatic reduction in host cell gene expression (8, 10), elevated intracellular levels of calcium (24, 25), and, ultimately, lysis of the infected cell. The cytopathic effect is thought to derive from expression of a viral protein, and recent studies implicate NSP4 as the viral protein responsible. Expression of NSP4 demonstrated that this protein can raise intracellular levels of calcium in insect cells (37, 38) and leads to a loss of plasma membrane integrity and altered nuclear morphology in MA104 cells (28). An additional function recently attributed to NSP4 is that of a viral enterotoxin. Purified NSP4, or a synthetic peptide which represents residues 114 to 135, causes diarrhea in infant mice (2). It is not clear how NSP4 is released from the ER membranes of infected cells to fulfill this postulated role.

Few studies have addressed the issue of how host cells might respond to rotavirus in terms of altered gene expression or enzyme activity. A better knowledge of how the host cell responds to infection might shed light on the mechanism(s) of rotavirus assembly or cytopathogenesis. In this study, we have employed a proteome-mapping approach using two-dimensional (2D) gel electrophoresis to generate a difference map of proteins expressed in mock- and rotavirus-infected MA104 cells. Following infection, at least four host proteins are upregulated, and we have identified two of these as BiP (GRP78) and endoplasmin (GRP94). Furthermore, the transcriptional activity of the genes encoding these proteins has been analyzed, and a possible role for the upregulated proteins in rotavirus assembly has been suggested.

	MATERIALS AND METHODS

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Cells and viruses. The rhesus monkey kidney cell line MA104 was grown in 199 medium (Gibco) supplemented with 10% tryptose phosphate and 10% newborn calf serum. The SA11 strain of rotavirus was purified and the titer was determined as previously described (35). Reovirus type 3 (Dearing strain) was obtained from W. K. Joklik (Duke University, Durham, N.C.). Vaccinia virus VTF7.3, which expresses T7 RNA polymerase, was kindly provided by B. Moss (National Institutes of Health, Bethesda, Md.). Construction of the recombinant vaccinia virus vTMNSP4, which expresses NSP4 from the SA11 strain of rotavirus (5), has been described elsewhere (28). Prior to infection, rotavirus inocula were activated by addition of 10 µg of trypsin per ml and incubated at 37°C for 30 min. The virus was added to confluent MA104 cells at 10 PFU/cell in the presence of trypsin. After 3 h, the inoculum was removed and replaced by medium lacking trypsin.

Radiolabeling and immunoprecipitation. Cells were washed twice with Dulbecco's minimal essential medium (DMEM) (Gibco) lacking methionine and cysteine and incubated for 20 min in this medium. The cells were then radiolabeled with 50 µCi of Trans-label (ICN) per ml in methionine and cysteine-free DMEM for 16 to 24 h in the case of prelabeled cells (i.e., labeled prior to virus infection) or for 7 h following virus infection. For pulse-labeling studies, cells were labeled at the indicated times for 10 min (unless otherwise stated), after which the medium was removed and replaced with DMEM containing a 20-fold molar excess of unlabeled cysteine and methionine. Cells were harvested with a rubber policeman and centrifuged at 1,000 × g for 5 min. Cell pellets were stored at 80°C until required. To immunoprecipitate specific proteins, cell pellets were solubilized in 1 ml of lysis buffer (2% CHAPS {3-[(3-cholamidopropyl)-dimethyl-ammonio]-1-propanesulfonate}, 50 mM HEPES [pH 7.5], 200 mM NaCl, 2 mM phenylmethylsulfonyl fluoride (PMSF), 10 U of apyrase per ml) for 30 min, and debris was removed by microcentrifugation. Immune complexes were formed by shaking the clarified cell lysate with specific antisera overnight at 4°C and recovered following addition of protein A-Sepharose for 30 min. Beads were washed three times in lysis buffer, and bound proteins were removed by boiling in sodium dodecyl sulfate (SDS) sample buffer prior to electrophoresis.

Preparation of cell lysates and analytical 2D gel electrophoresis. Frozen cell pellets were lysed by incubation in lysis buffer (8 M urea, 4% CHAPS {3-[(3-cholamidopropyl)-dimethyl-ammonio]-1-propanesulfonate}, 2% dithiothreitol [DTT], 2% Pharmalyte 3-10 [Pharmacia], 2 mM PMSF, 2 mM Pefabloc [Boehringer]) for 30 min at room temperature. Lysates were microcentrifuged at 12,000 rpm for 5 min to remove nucleic acid, and the protein concentration of the supernatant was determined in a dye-binding assay with Bradford reagent. Eighty-microgram aliquots of protein were focused on an Immobiline Drystrip 3-10 NL gel (Pharmacia) by using a Multiphor RII electrophoresis system (Pharmacia) with the following voltage program: 5 h to 500 V, 5 h to 3,500 V, and 10 h at 3,500 V at 1 mA. After separation in the first dimension, the Immobiline strips were equilibrated for 10 min in a buffer containing 50 mM Tris HCl (pH 6.8), 8 M urea, 30% glycerol, 1% SDS, and 1% DTT followed by a further 10 min in the same buffer containing 2.5% iodoacetamide. The strips were then laid atop precast 12 to 14% gradient gels for separation of polypeptides in the second dimension. Following electrophoresis, the gels were fixed in 10% glacial acetic acid-20% ethanol, and the proteins were visualized by silver staining and/or by autoradiography at 80°C. Gels were scanned with a Sharp JX325 scanner, and the protein spots were analyzed with Imagemaster software (Pharmacia).

Characterization of proteins by microsequencing and MALDI-TOF MS. Protein spots of interest were excised from Coomassie brilliant blue-stained preparative gels loaded with 800 µg of protein. Gel pieces were subjected to in-gel trypsin digestion as described by Rosenfeld et al. (34). The extracted peptide mixture was fractionated by reverse-phase high-performance liquid chromatography (HPLC) on a Brownlee RP300 C₁₈ column. Fractions were stored at 20°C prior to sequencing. For matrix-assisted laser desorption time-of-flight mass spectroscopy (MALDI-TOF MS), a modified protocol for in-gel trypsinization was employed. Briefly, the spots were excised from the gel, cut into <1-mm² pieces, and transferred to 0.5-ml microcentrifuge tubes. Gel pieces were washed twice in 50 mM ammonium bicarbonate in 50% acetonitrile at 30°C for 20 min with shaking. Proteins were reduced by incubation in 10 mM DTT-50 mM ammonium bicarbonate for 1 h, and cysteine residues were carboxymethylated with 25 mM iodoacetamide in 50 mM ammonium bicarbonate for a further hour at room temperature. The buffer was aspirated, and the gel pieces were washed as described above, dried, and digested with trypsin. The peptide mixture was dried and redissolved in 20 to 100 µl of 0.1% trifluoroacetic acid. Two-microliter aliquots were mixed with an equal amount of -cyano-4-hydroxycinnamic acid, and the samples were vacuum crystallized by using the sample preparation accessory (Hewlett-Packard). Crystals were carefully observed to ensure that crystal formation occurred evenly. The ion-accelerating potential was 30 kV. The MALDI mass spectrum of the peptide mixture was then obtained on a G2025A MALDI-TOF mass spectrometer (Hewlett-Packard).

Analysis of mRNA abundance and transcription activity. Cytoplasmic RNA was isolated from rotavirus-, reovirus-, vaccinia virus-, or mock-infected MA104 cells by using Trizol (Life Technology, Inc.). Total RNA (10 µg) was resolved by electrophoresis on a 1.2% agarose gel containing 2.2 M formaldehyde. The RNA was transferred to a nylon membrane overnight and baked at 80°C for 2 h. Plasmids p3C5 (15) and p4A3 (32), containing the mouse BiP and endoplasmin genes, respectively, were labeled with [-³²P]dCTP by a random primer method. The membranes were preincubated with hybridization buffer (1 mM EDTA, 40 mM Na₂HPO₄ [pH 7.2], 5% SDS) for 1 h at 65°C and subsequently incubated overnight in fresh buffer containing one of the labeled probes. Membranes were then washed twice in hybridization buffer, and the relative abundance of mRNA was determined with a Fujix BAS 1000 phosphoimager. Nuclear run-on transcription assays were performed essentially according to published protocols (21). Briefly, MA104 cells (10⁸ cells/reaction) were harvested with a rubber policeman at different times postinfection, pelleted by centrifugation at 1,000 × g for 5 min, and washed twice in ice-cold phosphate-buffered saline. Cells were resuspended in 4 ml of lysis buffer (10 mM Tris HCl [pH 7.4], 10 mM NaCl, 3 mM MgCl₂, 0.5% Nonidet P-40) by gentle vortexing, and the nuclei were pelleted (1,000 × g for 5 min). The nuclei were stored in 100 µl of storage buffer (50 mM Tris HCl [pH 8.3], 40% glycerol, 5 mM MgCl₂, 0.1 mM EDTA) and frozen in liquid N₂. Labeled transcripts were prepared by incubating isolated nuclei in reaction buffer (final concentration: 10 mM Tris HCl [pH 8.0], 5 mM MgCl₂, 0.3 M KCl, 1 mM ATP, 1 mM CTP, 1 mM GTP, 1 mM DTT, 40 U of RNasin per ml), with 400 µCi of [-³²P]UTP for 30 min at 37°C in a final volume of 200 µl. DNA was digested by the addition of RNase-free DNase (Boehringer Mannheim). Labeled RNA was extracted sequentially with guanadinium thiocyanate and phenol-chloroform and precipitated with isopropanol. RNA was resuspended in TES buffer [10 mM N-Tris (hydroxymethyl)methyl-2-aminoethanesulfonic acid (TES) (pH 7.4), 10 mM EDTA, 0.2% SDS], and aliquots of each sample were adjusted to 8 × 10⁸ cpm/ml by addition of TES buffer. One milliliter of RNA solution was mixed with 1 ml of TES-0.6 M NaCl, to which was added the appropriate cDNA immobilized to a nitrocellulose membrane (Schleicher & Schuell). Hybridization of the labeled RNA was performed over 36 h at 65°C, after which the nitrocellulose was washed twice in 25 ml of 2× SSC (1× SSC is 0.15 M NaCl plus 0.015 M sodium citrate) at 60°C for 30 min eachor under more stringent conditions if necessary. The strips were then dried and analyzed by phosphoimaging or exposed to Kodak Hyperfilm.

	RESULTS

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Cellular protein expression is affected by rotavirus infection. Comparison of 2D protein maps prepared from ³⁵S-labeled MA104 cells reveals striking differences in the pattern of cellular protein expression following infection by rotavirus (Fig. 1A and B). The 2D protein map generated from infected cells is characterized by the presence of a large number of new proteins that are highly labeled (circled in Fig. 1B) and a general decrease in the labeling intensity of the majority of the other cellular proteins. This result was anticipated, because several previous studies have shown that rotavirus infection results in a dramatic decrease in the level of host cell translation and a high level of expression of viral polypeptides (6, 10). Forty-six new proteins were detected reproducibly in the infected-cell lysate when a matched set of scanned gel images was generated. The majority of these could be divided into three groups on the basis of molecular mass. For example, 11 new proteins that migrated with an apparent molecular mass of about 44 kDa were detected in virus-infected lysates, forming a string of protein spots with similar molecular masses but different pIs. All of these have been identified as VP6 by immunoblotting and/or mass spectrometry (data not shown). The presence of multiple VP6 isoforms in preparations of protein derived from purified virions has recently been demonstrated (9). A further 20 new proteins migrated between 25 and 30 kDa. Some of these were identified as NSP4 by comparison with immunoblotted 2D gels probed with anti-NSP4 antisera, and others were identified tentatively as NSP5 after labeling with ³²P (data not shown). A third group of new spots share a low molecular mass (<12 kDa) and are predominantly acidic. The identity of these is unknown, and it is possible that they represent degradation products of other viral proteins. The absence of new spots of higher molecular mass (e.g., VP1, VP2, etc.) probably reflects the failure of the viral inner core particles to denature and release their constituent polypeptides during the isoelectric focusing step (first dimension).

View larger version (99K): [in this window] [in a new window]   FIG. 1.   2D gel electrophoresis maps of protein from [35S]Met- and [35S]Cys-labeled MA104 cells either mock infected (A) or infected with 10 PFU of SA11 rotavirus per cell (B). The electrophoresis conditions were as described in Materials and Methods. Spots derived from virus-encoded proteins are circled. The large and small rectangles denote regions from each gel cell in which upregulated cellular proteins were detected. (C) Enlarged views of regions within the large and small rectangles from each gel showing the positions of a matched set of 10 polypeptides. (D) Increase or decrease in the amount of radioactivity associated with each of the marked spots from the virus-infected sample relative to those in the mock-infected control. MW, molecular mass.

Identification of rotavirus-induced proteins. To identify the upregulated polypeptides, preparative gels were run, and proteins were detected by Coomassie blue staining. Only proteins in spots 2 and 3 were sufficiently abundant and well resolved after this procedure to permit their characterization by N-terminal sequence analysis and peptide mass fingerprinting. These spots were excised from a total of eight preparative gels and subjected to in-gel trypsinization. The peptides were then eluted from the gel and purified by reverse-phase HPLC. From each digest, one well-resolved peptide was selected for N-terminal amino acid sequencing. The peptide from spot 2 yielded the N-terminal sequence VYEGERPL. Analysis of the Swissprot-PIR database (http://www.expasy.hcuge.ch) revealed 11 identical matches. Each match corresponded to residues 465 to 472 of the ER chaperone BiPs (GRP78) from various mammalian species. A similar analysis was performed for the sequence LGVIEDHS derived from spot 3. Twelve matches were found, each corresponding to endoplasmins (GRP94) from various mammalian species, all of which contain this sequence between residues 494 and 501. Like BiP, this protein is also an ER-resident chaperone. To confirm the identity of the protein in each spot, the peptide mass fingerprint of a tryptic digest of each protein (Fig. 2) was compared to the sequence in the database by using the PeptideSearch program (accessed via http://www.mann.embl-heidelberg.de/). Matches were restricted to proteins with a molecular mass of between 30 and 200 kDa. At least seven peptides were required to match each database entry, with a peptide mass accuracy of 1 Da. Again BiP (spot 2) and endoplasmin (spot 3) were selected as the top matched proteins from the database (Table 1). Therefore, we conclude that the proteins in spots 2 and 3 correspond to BiP and endoplasmin, respectively.

View larger version (30K): [in this window] [in a new window]   FIG. 2.   Peptide mass fingerprint of a tryptic digest of the protein from spots 2 and 3. Only peptides in the mass range 1,000 to 3,000 Da were analyzed and used for mass matching with the database. All peaks represent the M+H+ charge state.

Rotavirus infection increases mRNA steady-state abundance and transcriptional activity of the genes encoding BiP and endoplasmin. To determine whether the increase in BiP and endoplasmin was due to transcriptional or posttranscriptional control, we investigated the kinetics of mRNA accumulation. RNA was prepared from mock-infected or SA11 rotavirus-infected MA104 cells at various times postinfection, and the levels of BiP and endoplasmin mRNA were determined by Northern blotting (Fig. 3). mRNA extracted from reovirus-infected MA104 cells was also included as a further control. Levels of both BiP and endoplasmin mRNA were increased markedly in rotavirus-infected cells from 7.5 h postinfection (hpi). In contrast, levels of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) mRNA were unchanged during the first 10 h of infection. No change in the steady-state abundance of BiP or endoplasmin mRNA was observed in either mock-infected cells or reovirus-infected cells.

View larger version (62K): [in this window] [in a new window]   FIG. 3.   Rotavirus infection increases BiP and endoplasmin mRNA abundance in MA104 cells. mRNA levels were analyzed at 2, 5, 7.5, and 10 hpi. At each time point, RNA was extracted from mock-infected cells (mock), reovirus-infected cells (reo), and rotavirus-infected cells (rota) and resolved by electrophoresis. The membranes were then probed for the abundance of mRNA encoding BiP, endoplasmin, and GAPDH. For further details, see Materials and Methods.

View larger version (71K): [in this window] [in a new window]   FIG. 4.   Transcriptional activation of genes encoding BiP and endoplasmin following rotavirus infection. Nuclei were isolated from rotavirus-infected MA104 cells at the indicated times and used for run-on transcription assays in the presence of [-32P]UTP as described in Materials and Methods. Labeled RNA was purified and hybridized to nitrocellulose strips containing probes for BiP, endoplasmin, hsp70, and GAPDH. Bound RNA was quantified by analysis in a Fuji phosphoimager. The increase in each mRNA species relative to the initial amount is illustrated in the attached table. Note that negative values indicate a decrease.

NSP4 expression induces transcription of the BiP and endoplasmin genes. We next considered whether induction of BiP and endoplasmin required the assembly of virus in the infected cell, or whether one or more rotavirus gene products could specifically trigger the induction process. Both BiP and endoplasmin are thought to have a molecular chaperone function within the ER and to act by preventing the misfolding and aggregation of proteins following a variety of conditions that result in ER stress (18). Any viral polypeptide that imparts stress to the ER might therefore cause induction of BiP and/or endoplasmin. The nonstructural glycoprotein NSP4 is a likely candidate for such a role given (i) the localization of this protein to the ER membrane and (ii) the ability of this protein to increase intracellular calcium and perturb the ER membrane (36).

View larger version (61K): [in this window] [in a new window]   FIG. 5.   Expression of NSP4 enhances the transcription of genes encoding BiP and endoplasmin. Nuclei were isolated from MA104 cells infected with either 20 PFU of vTF7.3 per cell or 10 PFU of vTF7.3 plus 10 PFU of vTMNSP4 per cell and used in run-on transcription assays as described in Materials and Methods. The presence of mRNA species encoding BiP, endoplasmin, hsp70, and GAPDH was determined with the corresponding probes at 4, 8, and 12 hpi.

Association of rotavirus protein with BiP and endoplasmin. Both BiP and endoplasmin have been shown to associate with certain newly synthesized, unfolded, or misfolded polypeptides (20, 27). Moreover, several studies have shown that association of polypeptides with BiP or endoplasmin is accompanied by the transcriptional upregulation of these genes (14, 31, 39). Therefore, we investigated whether BiP or endoplasmin was associated with any rotavirus proteins during infection. Given the ability of NSP4 to induce the transcription of these genes, we tested whether this might reflect the association of NSP4 with either BiP or endoplasmin. MA104 cells were prelabeled for 24 h prior to infection with either vTF7-3 or a combination of vTF7-3 and vTMNSP4. At 7 hpi, the cells were given a 10-min pulse with ³⁵S and either lysed immediately or chased in unlabeled medium for different intervals and then lysed. Cells were lysed in nondenaturing buffer to limit disruption of weakly interacting protein complexes. Lysates were then immunoprecipitated, either with a monoclonal antibody against NSP4, or with a polyclonal serum against the carboxy-terminal portion of endoplasminwhich cross-reacts with BiP. Immediately after labeling, NSP4 in various states of glycosylation was precipitated by the anti-NSP4 monoclonal antibody (Fig. 6, lane 1). After a short chase period, only a single 28-kDa species, representing the diglycosylated form of NSP4, was evident. Antibodies against BiP and endoplasmin precipitated both proteins as well as a further 36-kDa band that was common to both sera and thus deemed to be nonspecific (Fig. 6, lanes 7 and 8). No evidence was found for association between NSP4 and either BiP or endoplasmin.

View larger version (63K): [in this window] [in a new window]   FIG. 6.   NSP4 does not associate with BiP or endoplasmin. MA104 cells were labeled for 24 h with 35S-Trans-label prior to infection with 10 PFU of vTF7.3 plus 10 PFU of vTMNSP4 per cell and thereafter grown in unlabeled medium. At 7 hpi, cells were pulse-labeled for 10 min and then chased for the indicated times, after which the cells were lysed and proteins were immunoprecipitated with either a monoclonal antiserum against NSP4 (lanes 1 to 5) or a polyclonal antiserum against endoplasmin that cross-reacts with BiP (lanes 7 and 8). The asterisk denotes the position of an unidentified cellular protein that coprecipitated with NSP4. MW, molecular mass.

View larger version (49K): [in this window] [in a new window]   FIG. 7.   BiP and endoplasmin bind to rotavirus (RV) structural polypeptides during virion maturation in the ER. Rotavirus-infected MA104 cells were pulse-labeled at 7 hpi and chased for the indicated times. The cells were lysed and the proteins were immunoprecipitated with either anti-SA11 rotavirus, antiendoplasmin antiserum, or nonimmune serum (lanes 1 to 8). Alternatively, cells were labeled for 16 h with 35S-Trans-label, chased for 1 h before infection, and immunoprecipitated 7 hpi (lanes 9 to 11).

	DISCUSSION

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Refinement of methods for the resolution of protein mixtures by 2D gel electrophoresis has enabled the analysis of protein expression in a variety of cells and tissues. The ability to map the total complement of proteins expressed in a given cell (referred to as the "proteome") enables the establishment of a reference against which alterations in the pattern of protein expression following virus infection may be determined. When applied to the analysis of MA104 cells infected with rotavirus, the most striking feature of the proteome maps is the appearance of many new virus-encoded proteins (compare Fig. 1A and B). Surprisingly, although rotavirus encodes only 11 proteins, a total of 46 highly labeled new spots were evident in the map derived from virus-infected cells. Our data suggest that many rotavirus proteins exist in multiple isoforms, possibly reflecting differences in the extent of posttranslational modification. In support of this notion, a previous analysis of NSP5 by 2D polyacrylamide gel electrophoresis revealed the existence of several isoforms that differed in molecular mass and pI and which were also labeled with ³²P, suggesting that this protein is phosphorylated at several sites (4). In this study, immunoblotting of the gels with antisera against VP6 and NSP4 demonstrated that these proteins also exist in multiple states (data not shown). 2D gel electrophoresis and mass spectrometry analysis of VP6 from purified virions have also revealed a level of heterogeneity similar to that observed in this study (9).

The majority of host cell proteins were expressed at a level approximately two- to threefold less than that in mock-infected cells. However, careful analysis of the gels revealed four proteins that increased in abundance following virus infection. Limitations to the resolution of the total proteome in a single 2D gel mean that is possible that the number of upregulated host cell proteins is greater. We are currently refining the conditions of electrophoresis to expand the resolution in selective regions of the gels and analyzing subcellular fractions of infected MA104 cells. Together these approaches should yield more informative data about the effect of virus infection on host cell protein expression.

Two upregulated proteins were identified as BiP and endoplasmin (also known as glucose-regulated proteins GRP78 and GRP94, respectively). Northern analysis and nuclear run-on transcription assays clearly demonstrate that a marked increase in transcription accounts for the upregulation of BiP and endoplasmin in rotavirus-infected cells. These proteins function as ER-resident molecular chaperones that assist in the folding and oligomerization of proteins within the ER lumen. The induction of BiP and endoplasmin is believed to protect cells from a variety of external and internal stimuli that exhibit, as a common feature, the potential to induce stress within the ER lumen and therefore lead to the accumulation of misfolded proteins (reviewed in reference 18). A variety of pharmacological agents, such as the Ca²⁺ ionophore A23187 and thapsigargin (an inhibitor of the ER Ca²⁺ ATPase), lead to a reduction in ER Ca²⁺ levels (16, 32). It has been suggested that this reduction in ER Ca²⁺ impairs normal protein folding in the ER, and thus increased BiP is required to prevent protein aggregation (19). Similarly, internal stimuli such as the expression of folding-incompetent mutant polypeptides or high-level expression of complex oligomeric proteins, require an increase in BiP levels to prevent the aggregation of protein within the organelle (27). Virus infection can also lead to the induction of such a stress within the ER. For example, infection with the paramyxovirus simian virus 5 (SV5) causes an increase in synthesis of BiP (29). The expression of the SV5 hemagglutinin-neuraminidase (HN) protein alone was sufficient to promote induction of BiP, suggesting that proteins whose folding and oligomerization proceed more slowly, as evidenced by a prolonged association with BiP, may require increased levels of this chaperone (39).

NSP4 is localized to the ER membrane and has been reported to possess membrane-destabilizing activity (36). Therefore, we reasoned that its expression may be sufficient to cause the induction of BiP and endoplasmin. Delivery of NSP4 via a recombinant vaccinia virus vector resulted in a three- to fourfold increase in BiP and endoplasmin transcription over vaccinia virus-infected control cells. We could not detect any association between NSP4 and either BiP or endoplasmin in coimmunoprecipitation experiments (Fig. 5). This result was not unexpected, because only a small portion of the N terminus of NSP4 is believed to project into the ER lumen (3, 7). We conclude that the ability of NSP4 to induce transcription of the BiP and endoplasmin genes derives not from its own folding requirements, but rather from its effects on the integrity of the ER, possibly by contributing to the reduction in the lumenal Ca²⁺ concentration as a result of the disruption of the ER membrane. Michelangeli et al. have suggested that a high ER Ca²⁺ concentration is required for the productive maturation of rotavirus in this organelle (24). It is interesting to note that BiP has recently been assigned a Ca²⁺ storage role and contributes ~25% of the exchangeable Ca²⁺ pool within the ER (17). We hypothesize that a depletion of the ER Ca²⁺ pool in response to NSP4 expression may partially be offset by upregulation of BiP expression and its recruitment to the ER lumenwhere it may function to prevent protein aggregation and restore Ca²⁺ homeostasis within this compartment.

BiP and endoplasmin may also participate in the folding of the rotavirus outer capsid proteins during assembly of virions within the ER. Antibodies against endoplasmin which cross-react with BiP efficiently precipitated the rotavirus structural proteins from lysates of infected cells (Fig. 6). The proportion of VP4 and VP7 that was bound to BiP or endoplasmin increased immediately following their synthesis, reaching a maximum after 30 min. A proportion of the rotavirus structural proteins could still be coimmunoprecipitated with antiendoplasmin antibodies 2 h after their synthesis (Fig. 6, lane 7). Recently VP7 has been shown to associate with another ER-localized chaperone, protein disulfide isomerase (PDI) (26). The kinetics of the VP7-PDI interaction showed a maximal association after 120 min. As rotavirus particles assemble in the ER, they might interact sequentially with several different chaperone proteins until the mature virions are completely assembled and can be released. The fact that an antirotavirus antibody precipitated both BiP and endoplasmin from prelabeled infected cells suggests that both chaperones are involved in the folding and assembly process and thus may function in concert.

In summary, by applying a proteome-mapping approach to the analysis of rotavirus-infected cells, we have identified BiP and endoplasmin as two proteins whose synthesis is upregulated postinfection. A further two proteins remain to be identified from our initial 2D gel analysis. The roles of these and other host proteins in the assembly and pathology of rotavirus infection remain to be established.