Rotavirus follows an atypical pathway to the apical membrane of intestinal cells that bypasses the Golgi. The involvement of rafts in this process was explored here. VP4 is the most peripheral protein of the triple-layered structure of this nonenveloped virus. High proportions of VP4 associated with rafts within the cell as early as 3 h postinfection. In the meantime a significant part of VP4 was targeted to the Triton X-100-resistant microdomains of the apical membrane, suggesting that this protein possesses an autonomous signal for its targeting. At a later stage the other structural rotavirus proteins were also found in rafts within the cells together with NSP4, a nonstructural protein required for the final stage of virus assembly. Rafts purified from infected cells were shown to contain infectious particles. Finally purified VP4 and mature virus were shown to interact with cholesterol-and sphingolipid-enriched model lipid membranes that changed their phase preference from inverted hexagonal to lamellar structures. Together these results indicate that a direct interaction of VP4 with rafts promotes assembly and atypical targeting of rotavirus in intestinal cells.Lipids membrane microdomains are dynamic entities involved in the control of the lipid-lipid and lipid-protein interactions that play a key role in numerous cellular functions such as signal transduction and membrane transport and trafficking (27). Membrane microdomains enriched in cholesterol and sphingolipids, also termed rafts, are thought to act as transitory platforms on which lipids and proteins may interact dynamically to exert a function that may be interrupted as the microdomain dissociates. It is thought that rafts emerge from the Golgi apparatus and reach the plasma membrane through a still-discussed intracellular pathway. Among the numerous functions of microdomains so far explored, various steps of virus interactions with their host cells have been proposed (8,32,44,47,53,66,71). These findings mainly concerned enveloped viruses, whose lipid membranes are expected to interact with the host cell membranes. By contrast, nonenveloped viruses that replicate and assemble in the cytoplasm of host cells have been scarcely explored for their putative interactions with membrane microdomains (37,48).Rotavirus, a triple-layered nonenveloped virus (70), is a worldwide cause of infantile gastroenteritis, accounting for an estimated 600,000 deaths annually (2). Knowledge of the detailed process of virus assembly is required to provide a molecular basis for the design of drugs or strategies able to interfere with virus entry, assembly, and/or replication. The interest in this approach has been enhanced since the withdrawal of the tetravalent vaccine because of side effects (7). In vivo rotavirus specifically targets highly polarized intestinal cells (59). This prompted us to develop studies on rotavirus infection of Caco-2 cells (17), which originate from human colon and which display a well-polarized and differentiated enterocytic phenotype when grown...

VP4 is an unglycosylated protein of the outer layer of the capsid of rotavirus. It forms spikes that project from the outer layer of mature virions, which is mainly constituted by glycoprotein VP7. VP4 has been implicated in several important functions, such as cell attachment, penetration, hemagglutination, neutralization, virulence, and host range. Previous studies indicated that VP4 is located in the space between the periphery of the viroplasm and the outside of the endoplasmic reticulum in rotavirus-infected cells. Confocal microscopy of infected MA104 monolayers, immunostained with specific monoclonal antibodies, revealed that a significant fraction of VP4 was present at the plasma membrane early after infection. Another fraction of VP4 is cytoplasmic and colocalizes with ␤-tubulin. Flow cytometry analysis confirmed that at the early stage of viral infection, VP4 was present on the plasma membrane and that its N-terminal region, the VP8* subunit, was accessible to antibodies. Biotin labeling of the infected cell surface monolayer with a cell-impermeable reagent allowed the identification of the noncleaved form of VP4 that was associated with the glycoprotein VP7. The localization of VP4 was not modified in cells transfected with a plasmid allowing the expression of a fusion protein consisting of VP4 and the green fluorescent protein. The present data suggest that VP4 reaches the plasma membrane through the microtubule network and that other viral proteins are dispensable for its targeting and transport.Rotaviruses are the most important etiologic agents of severe dehydrating infantile gastroenteritis in developed and developing countries (17). They are responsible for more than 850,000 deaths per year (14). As a member of the Reoviridae family, rotavirus has a segmented double-stranded RNA genome, enclosed in a viral capsid constituted of three concentric protein layers (37). Electron microscopy studies show that viral morphogenesis begins in cytoplasmic inclusions, termed viroplasms, where the central core and single-shelled particles are assembled (3, 10). VP4 is an unglycosylated protein and forms spikes that project from the outer layer of mature virions, which is mainly constituted by the glycoprotein VP7 (1, 34). VP4 has been implicated in several important functions, such as cell attachment, penetration, hemagglutination, neutralization, virulence, and host range (5,12,18,23). It has been shown that the infectivity of rotaviruses is increased and is probably dependent on trypsin treatment of the virus (11). This proteolytic treatment results in the specific cleavage of VP4 to polypeptides VP8* and VP5*, which represent, respectively, the amino-and carboxyl-terminal regions of the protein (22). VP4 possesses a conserved hydrophobic region located between amino acids 384 and 401 that shares some homology with the internal fusion sites of Semliki Forest virus and Sindbis virus E1 spike proteins (25). Recently, it has been shown that VP5*, which includes this hydrophobic domain, is a specific membrane-p...

Mercuric chloride induces an autoimmune glomerulonephritis in Brown-Norway (BN) but not in Lewis (LEW) rats. Injection of HGCl2 into BN rats regularly produced a transient appearance of plaque-forming cells (PFC) of anti-2,4,6-trinitrophenyl and anti-sheep red blood cell specificity and circulating anti-single-stranded DNA antibodies. Addition of HgCl2 to spleen cell cultures from BN rats induced an increase in anti-trinitrophenyl PFC and reverse PFC. This effect was no longer observed when nylon wool column-depleted or anti-Thy-1 antiserum-treated spleen cells were cultured in the presence of HgCl1. These data suggest that HgCl2 acts as a polyclonal activator on spleen cells in BN rats, but not on isolated B lymphocytes. In contrast, no effect of HgCl2 on immunoglobulin production was observed in LEW rats. Since polyclonal activation and immune-type nephritis are both seen in BN but not in LEW rats, polyclonal activation may participate in the pathogenesis of the HgCl2-induced autoimmune disease of BN rats.

Rotavirus infection is the most common cause of severe infantile gastroenteritis worldwide. These viruses infect mature enterocytes of the small intestine and cause structural and functional damage, including a reduction in disaccharidase activity. It was previously hypothesized that reduced disaccharidase activity resulted from the destruction of rotavirus-infected enterocytes at the villus tips. However, this pathophysiological model cannot explain situations in which low disaccharidase activity is observed when rotavirus-infected intestine exhibits few, if any, histopathologic changes. In a previous study, we demonstrated that the simian rotavirus strain RRV replicated in and was released from human enterocyte-like Caco-2 cells without cell destruction (N. Jourdan, M. Maurice, D. Delautier, A. M. Quero, A. L. Servin, and G. Trugnan, J. Virol. 71:8268–8278, 1997). In the present study, to reinvestigate disaccharidase expression during rotavirus infection, we studied sucrase-isomaltase (SI) in RRV-infected Caco-2 cells. We showed that SI activity and apical expression were specifically and selectively decreased by RRV infection without apparent cell destruction. Using pulse-chase experiments and cell surface biotinylation, we demonstrated that RRV infection did not affect SI biosynthesis, maturation, or stability but induced the blockade of SI transport to the brush border. Using confocal laser scanning microscopy, we showed that RRV infection induces important alterations of the cytoskeleton that correlate with decreased SI apical surface expression. These results lead us to propose an alternate model to explain the pathophysiology associated with rotavirus infection.

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Rotavirus is a nonenveloped virus with a three-layered capsid. The inner layer, made of VP2, encloses the genomic RNA and two minor proteins, VP1 and VP3, with which it forms the viral core. Core assembly is coupled with RNA viral replication and takes place in definite cellular structures termed viroplasms. Replication and encapsidation mechanisms are still not fully understood, and little information is available about the intermolecular interactions that may exist among the viroplasmic proteins. NSP2 and NSP5 are two nonstructural viroplasmic proteins that have been shown to interact with each other. They have also been found to be associated with precore replication intermediates that are precursors of the viral core. In this study, we show that NSP5 interacts with VP2 in infected cells. This interaction was demonstrated with recombinant proteins expressed from baculovirus recombinants or in bacterial systems. NSP5-VP2 interaction also affects the stability of VP6 bound to VP2 assemblies. The data presented showed evidence, for the first time, of an interaction between VP2 and a nonstructural rotavirus protein. Published data and the interaction demonstrated here suggest a possible role for NSP5 as an adapter between NSP2 and the replication complex VP2-VP1-VP3 in core assembly and RNA encapsidation, modulating the role of NSP2 as a molecular motor involved in the packaging of viral mRNA.Rotaviruses, members of the Reoviridae family, are the major cause of severe gastroenteritis in infants and young children (18). The rotavirus genome consists of 11 segments of doublestranded RNA (dsRNA) and is surrounded by three concentric layers of protein (28). The outer layer is made up of 60 spikes formed by dimers of VP4 and of 260 trimers of the glycoprotein VP7. The middle layer consists of 260 trimers of VP6. The inner layer has a T ϭ 1 symmetry and is made of 60 dimers of the capsid protein VP2, which shows nonspecific singlestranded RNA and dsRNA binding activities (21). The amino terminus of VP2 is essential for the incorporation of the RNAdependent RNA polymerase VP1 and guanylyltransferase methylase VP3 into the core of the virion (23). The RNAdependent RNA polymerase (VP1) has both transcriptase and replicase activities, which catalyze the synthesis of viral mRNA and dsRNA genome, respectively. Synthesis of dsRNA occurs in association with subviral particles, since free dsRNA cannot be detected in infected cells. Furthermore, the packaging and replication of the viral genome must be a highly coordinated process, given that the 11 dsRNA segments are present in equimolar concentrations in virions and that the ratio of number of virus particles to infectious units is low (16,25). Although several reports have described the characterization of rotavirus replication intermediates (RI), molecular details of the replication mechanisms remain unclear (12). Structural proteins VP1 and VP2 are essential components of the in vitro replicase activity (33). Two nonstructural proteins, NSP2 and NSP5, are associated with th...

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