Recovery and characterization of a replicase complex in rotavirus-infected cells by using a monoclonal antibody against NSP2

Rotavirus NSP2: A Master Orchestrator of Early Viral Particle Assembly

Rotaviruses (RVs) are 11-segmented, double-stranded (ds) RNA viruses and important causes of acute gastroenteritis in humans and other animal species. Early RV particle assembly is a multi-step process that includes the assortment, packaging and replication of the 11 genome segments in close connection with capsid morphogenesis. This process occurs inside virally induced, cytosolic, membrane-less organelles called viroplasms. While many viral and cellular proteins play roles during early RV assembly, the octameric nonstructural protein 2 (NSP2) has emerged as a master orchestrator of this key stage of the viral replication cycle. NSP2 is critical for viroplasm biogenesis as well as for the selective RNA-RNA interactions that underpin the assortment of 11 viral genome segments. Moreover, NSP2's associated enzymatic activities might serve to maintain nucleotide pools for use during viral genome replication, a process that is concurrent with early particle assembly. The goal of this review article is to summarize the available data about the structures, functions and interactions of RV NSP2 while also drawing attention to important unanswered questions in the field.

Cryo-EM structure of rotavirus B NSP2 reveals its unique tertiary architecture

Rotavirus (RV) NSP2 is a multifunctional RNA chaperone that exhibits numerous activities that are essential for replication and viral genome packaging. We performed an in silico analysis that highlighted a distant relationship of NSP2 from rotavirus B (RVB) to proteins from other human RVs. We solved a cryo-electron microscopy structure of RVB NSP2 that shows structural differences with corresponding proteins from other human RVs. Based on the structure, we identified amino acid residues that are involved in RNA interactions. Anisotropy titration experiments showed that these residues are important for nucleic acid binding. We also identified structural motifs that are conserved in all RV species. Collectively, our data complete the structural characterization of rotaviral NSP2 protein and demonstrate its structural diversity among RV species.IMPORTANCERotavirus B (RVB), also known as adult diarrhea rotavirus, has caused epidemics of severe diarrhea in China, India, and Bangladesh. Thousands of people are infected in a single RVB epidemic. However, information on this group of rotaviruses remains limited. As NSP2 is an essential protein in the viral life cycle, including its role in the formation of replication factories, it may be a target for future antiviral strategy against viruses with similar mechanisms.

Liquid-liquid phase separation underpins the formation of replication factories in rotaviruses

RNA viruses induce the formation of subcellular organelles that provide microenvironments conducive to their replication. Here we show that replication factories of rotaviruses represent protein-RNA condensates that are formed via liquid-liquid phase separation of the viroplasm-forming proteins NSP5 and rotavirus RNA chaperone NSP2. Upon mixing, these proteins readily form condensates at physiologically relevant low micromolar concentrations achieved in the cytoplasm of virus-infected cells. Early infection stage condensates could be reversibly dissolved by 1,6-hexanediol, as well as propylene glycol that released rotavirus transcripts from these condensates. During the early stages of infection, propylene glycol treatments reduced viral replication and phosphorylation of the condensate-forming protein NSP5. During late infection, these condensates exhibited altered material properties and became resistant to propylene glycol, coinciding with hyperphosphorylation of NSP5. Some aspects of the assembly of cytoplasmic rotavirus replication factories mirror the formation of other ribonucleoprotein granules. Such viral RNA-rich condensates that support replication of multi-segmented genomes represent an attractive target for developing novel therapeutic approaches.

A Genetically Engineered Rotavirus NSP2 Phosphorylation Mutant Impaired in Viroplasm Formation and Replication Shows an Early Interaction between vNSP2 and Cellular Lipid Droplets

Many RNA viruses replicate in cytoplasmic compartments (virus factories or viroplasms) composed of viral and cellular proteins, but the mechanisms required for their formation remain largely unknown. Rotavirus (RV) replication in viroplasms requires interactions between virus nonstructural proteins NSP2 and NSP5, which are associated with components of lipid droplets (LDs). We previously identified two forms of NSP2 in RV-infected cells, a cytoplasmically dispersed form (dNSP2) and a viroplasm-specific form (vNSP2), which interact with hypophosphorylated and hyperphosphorylated NSP5, respectively, indicating that a coordinated phosphorylation cascade controls viroplasm assembly. The cellular kinase CK1α phosphorylates NSP2 on serine 313, triggering the localization of vNSP2 to sites of viroplasm assembly and its association with hyperphosphorylated NSP5. Using reverse genetics, we generated a rotavirus with a phosphomimetic NSP2 (S313D) mutation to directly evaluate the role of CK1α NSP2 phosphorylation in viroplasm formation. Recombinant rotavirus NSP2 S313D (rRV NSP2 S313D) is significantly delayed in viroplasm formation and in virus replication and interferes with wild-type RV replication in coinfection. Taking advantage of the delay in viroplasm formation, the NSP2 phosphomimetic mutant was used as a tool to observe very early events in viroplasm assembly. We show that (i) viroplasm assembly correlates with NSP5 hyperphosphorylation and (ii) vNSP2 S313D colocalizes with RV-induced LDs without NSP5, suggesting that vNSP2 phospho-S313 is sufficient for interacting with LDs and may be the virus factor required for RV-induced LD formation. Further studies with the rRV NSP2 S313D virus are expected to reveal new aspects of viroplasm and LD initiation and assembly.IMPORTANCE Reverse genetics was used to generate a recombinant rotavirus with a single phosphomimetic mutation in nonstructural protein 2 (NSP2 S313D) that exhibits delayed viroplasm formation, delayed replication, and an interfering phenotype during coinfection with wild-type rotavirus, indicating the importance of this amino acid during virus replication. Exploiting the delay in viroplasm assembly, we found that viroplasm-associated NSP2 colocalizes with rotavirus-induced lipid droplets prior to the accumulation of other rotavirus proteins that are required for viroplasm formation and that NSP5 hyperphosphorylation is required for viroplasm assembly. These data suggest that NSP2 phospho-S313 is sufficient for interaction with lipid droplets and may be the virus factor that induces lipid droplet biogenesis in rotavirus-infected cells. Lipid droplets are cellular organelles critical for the replication of many viral and bacterial pathogens, and thus, understanding the mechanism of NSP2-mediated viroplasm/lipid droplet initiation and interaction will lead to new insights into this important host-pathogen interaction.

Understanding the penetrance of intrinsic protein disorder in rotavirus proteome

Rotavirus is a major cause of severe acute gastroenteritis in the infants and young children. The past decade has evidenced the role of intrinsically disordered proteins/regions (IDPs)/(IDPRs) in viral and other diseases. In general, (IDPs)/(IDPRs) are considered as dynamic conformational ensembles that devoid of a specific 3D structure, being associated with various important biological phenomena. Viruses utilize IDPs/IDPRs to survive in harsh environments, to evade the host immune system, and to highjack and manipulate host cellular proteins. The role of IDPs/IDPRs in Rotavirus biology and pathogenicity are not assessed so far, therefore, we have designed this study to deeply look at the penetrance of intrinsic disorder in rotavirus proteome consisting 12 proteins encoded by 11 segments of viral genome. Also, for all human rotaviral proteins, we have deciphered molecular recognition features (MoRFs), which are disorder based binding sites in proteins. Our study shows the wide spread of intrinsic disorder in several rotavirus proteins, primarily the nonstructural proteins NSP3, NSP4, and NSP5 that are involved in viral replication, translation, viroplasm formation and/or maturation. This study may serve as a primer for understanding the role of IDPs/MoRFs in rotavirus biology, design of alternative therapeutic strategies, and development of disorder-based drugs.

Nanoscale organization of rotavirus replication machineries

Rotavirus genome replication and assembly take place in cytoplasmic electron dense inclusions termed viroplasms (VPs). Previous conventional optical microscopy studies observing the intracellular distribution of rotavirus proteins and their organization in VPs have lacked molecular-scale spatial resolution, due to inherent spatial resolution constraints. In this work we employed super-resolution microscopy to reveal the nanometric-scale organization of VPs formed during rotavirus infection, and quantitatively describe the structural organization of seven viral proteins within and around the VPs. The observed viral components are spatially organized as five concentric layers, in which NSP5 localizes at the center of the VPs, surrounded by a layer of NSP2 and NSP4 proteins, followed by an intermediate zone comprised of the VP1, VP2, VP6. In the outermost zone, we observed a ring of VP4 and finally a layer of VP7. These findings show that rotavirus VPs are highly organized organelles.

Probing the sites of interactions of rotaviral proteins involved in replication

Replication and packaging of the rotavirus genome occur in cytoplasmic compartments called viroplasms, which form during virus infection. These processes are orchestrated by yet-to-be-understood complex networks of interactions involving nonstructural proteins (NSPs) 2, 5, and 6 and structural proteins (VPs) 1, 2, 3, and 6. The multifunctional enzyme NSP2, an octamer with RNA binding activity, is critical for viroplasm formation with its binding partner, NSP5, and for genome replication/packaging through its interactions with replicating RNA, the viral polymerase VP1, and the inner core protein VP2. Using isothermal calorimetry, biolayer interferometry, and peptide array screening, we examined the interactions between NSP2, VP1, VP2, NSP5, and NSP6. These studies provide the first evidence that NSP2 can directly bind to VP1, VP2, and NSP6, in addition to the previously known binding to NSP5. The interacting sites identified from reciprocal peptide arrays were found to be in close proximity to the RNA template entry and double-stranded RNA (dsRNA) exit tunnels of VP1 and near the catalytic cleft and RNA-binding grooves of NSP2; these sites are consistent with the proposed role of NSP2 in facilitating dsRNA synthesis by VP1. Peptide screening of VP2 identified NSP2-binding sites in the regions close to the intersubunit junctions, suggesting that NSP2 binding could be a regulatory mechanism for preventing the premature self-assembly of VP2. The binding sites on NSP2 for NSP6 were found to overlap that of VP1, and the NSP5-binding sites overlap those of VP2 and VP1, suggesting that interaction of these proteins with NSP2 is likely spatially and/or temporally regulated.

IMPORTANCE

Replication and packaging of the rotavirus genome occur in cytoplasmic compartments called viroplasms that form during virus infection and are orchestrated by complex networks of interactions involving nonstructural proteins (NSPs) and structural proteins (VPs). A multifunctional RNA-binding NSP2 octamer with nucleotidyl phosphatase activity is central to viroplasm formation and RNA replication. Here we provide the first evidence that NSP2 can directly bind to VP1, VP2, and NSP6, in addition to the previously known binding to NSP5. The interacting sites identified from peptide arrays are consistent with the proposed role of NSP2 in facilitating dsRNA synthesis by VP1 and also point to NSP2's possible role in preventing the premature self-assembly of VP2 cores. Our findings lead us to propose that the NSP2 octamer with multiple enzymatic activities is a principal regulator of viroplasm formation, recruitment of viral proteins into the viroplasms, and possibly genome replication.

A novel form of rotavirus NSP2 and phosphorylation-dependent NSP2-NSP5 interactions are associated with viroplasm assembly

Rotavirus (RV) replication occurs in cytoplasmic inclusions called viroplasms whose formation requires the interactions of RV proteins NSP2 and NSP5; however, the specific role(s) of NSP2 in viroplasm assembly remains largely unknown. To study viroplasm formation in the context of infection, we characterized two new monoclonal antibodies (MAbs) specific for NSP2. These MAbs show high-affinity binding to NSP2 and differentially recognize distinct pools of NSP2 in RV-infected cells; a previously unrecognized cytoplasmically dispersed NSP2 (dNSP2) is detected by an N-terminal binding MAb, and previously known viroplasmic NSP2 (vNSP2) is detected by a C-terminal binding MAb. Kinetic experiments in RV-infected cells demonstrate that dNSP2 is associated with NSP5 in nascent viroplasms that lack vNSP2. As viroplasms mature, dNSP2 remains in viroplasms, and the amount of diffuse cytoplasmic dNSP2 increases. vNSP2 is detected in increasing amounts later in infection in the maturing viroplasm, suggesting a conversion of dNSP2 into vNSP2. Immunoprecipitation experiments and reciprocal Western blot analysis confirm that there are two different forms of NSP2 that assemble in complexes with NSP5, VP1, VP2, and tubulin. dNSP2 associates with hypophosphorylated NSP5 and acetylated tubulin, which is correlated with stabilized microtubules, while vNSP2 associates with hyperphosphorylated NSP5. Mass spectroscopy analysis of NSP2 complexes immunoprecipitated from RV-infected cell lysates show both forms of NSP2 are phosphorylated, with a greater proportion of vNSP2 being phosphorylated compared to dNSP2. Together, these data suggest that dNSP2 interacts with viral proteins, including hypophosphorylated NSP5, to initiate viroplasm formation, while viroplasm maturation includes phosphorylation of NSP5 and vNSP2.

Crystallographic Analysis of Rotavirus NSP2-RNA Complex Reveals Specific Recognition of 5' GG Sequence for RTPase Activity

Rotavirus nonstructural protein NSP2, a functional octamer, is critical for the formation of viroplasms, which are exclusive sites for replication and packaging of the segmented double-stranded RNA (dsRNA) rotavirus genome. As a component of replication intermediates, NSP2 is also implicated in various replication-related activities. In addition to sequence-independent single-stranded RNA-binding and helix-destabilizing activities, NSP2 exhibits monomer-associated nucleoside and 5' RNA triphosphatase (NTPase/RTPase) activities that are mediated by a conserved H225 residue within a narrow enzymatic cleft. Lack of a 5' γ-phosphate is a common feature of the negative-strand RNA [(-)RNA] of the packaged dsRNA segments in rotavirus. Strikingly, all (-)RNAs (of group A rotaviruses) have a 5' GG dinucleotide sequence. As the only rotavirus protein with 5' RTPase activity, NSP2 is implicated in the removal of the γ-phosphate from the rotavirus (-)RNA. To understand how NSP2, despite its sequence-independent RNA-binding property, recognizes (-)RNA to hydrolyze the γ-phosphate within the catalytic cleft, we determined a crystal structure of NSP2 in complex with the 5' consensus sequence of minus-strand rotavirus RNA. Our studies show that the 5' GG of the bound oligoribonucleotide interacts extensively with highly conserved residues in the NSP2 enzymatic cleft. Although these residues provide GG-specific interactions, surface plasmon resonance studies suggest that the C-terminal helix and other basic residues outside the enzymatic cleft account for sequence-independent RNA binding of NSP2. A novel observation from our studies, which may have implications in viroplasm formation, is that the C-terminal helix of NSP2 exhibits two distinct conformations and engages in domain-swapping interactions, which result in the formation of NSP2 octamer chains.

Phylogenetic analysis of rotavirus A NSP2 gene sequences and evidence of intragenic recombination

The rotavirus non-structural protein NSP2 is one of the earliest and most abundant viral proteins produced during infection. This protein has multiple essential roles in the replication cycle involving RNA binding, viroplasm formation, helicase and can hydrolyse the γ-phosphate of RNA and NTPs acting as an RTPase and NTPase. In studying sequences from rotavirus strains isolated in Australia between 1984 and 2009, the NSP2 gene was seen to be highly conserved and clustered with defined NSP2 genotypes N1 and N2 according to the full genome based rotavirus classification system. Phylogenetic analysis indicated that NSP2 gene sequences isolated from Australian rotavirus strains formed four distinct lineages. Temporal variation was observed in several clusters during the 26 year period, with lineage D identified throughout the entire study period and lineage A only detected since 1999. Phylogenetic analysis and dendrograms identified NSP2 genes that exhibited reassortment between different virus VP7 genotypes, as well as a sequence from a human strain that grouped closely with the NSP2 genes of bovine rotavirus strains. This study also identified a sequence that fell between lineages and exhibited evidence of recombination, the first time that intergenic recombination has been detected in a NSP2 gene sequence. This study increases the understanding of the evolution mechanisms of NSP2 in view of improved vaccine design.

Assortment and packaging of the segmented rotavirus genome

The rotavirus (RV) genome comprises 11 segments of double-stranded RNA (dsRNA) and is contained within a non-enveloped, icosahedral particle. During assembly, a highly coordinated selective packaging mechanism ensures that progeny RV virions contain one of each genome segment. Cis-acting signals thought to mediate assortment and packaging are associated with putative panhandle structures formed by base-pairing of the ends of RV plus-strand RNAs (+RNAs). Viral polymerases within assembling core particles convert the 11 distinct +RNAs to dsRNA genome segments. It remains unclear whether RV +RNAs are assorted before or during encapsidation, and the functions of viral proteins during these processes are not resolved. However, as reviewed here, recent insights gained from the study of RV and two other segmented RNA viruses, influenza A virus and bacteriophage Φ6, reveal potential mechanisms of RV assortment and packaging.

Functional and biochemical properties of Mal de Río Cuarto virus (Fijivirus, Reoviridae) P9-1 viroplasm protein show further similarities to animal reovirus counterparts

Mal de Río Cuarto virus (MRCV) is a plant virus of the genus Fijivirus within the family Reoviridae that infects several monocotyledonous species and is transmitted by planthoppers in a persistent and propagative manner. Other members of the family replicate in viral inclusion bodies (VIBs) termed viroplasms that are formed in the cytoplasm of infected plant and insect cells. In this study, the protein coded by the first ORF of MRCV segment S9 (P9-1) was shown to establish cytoplasmic inclusion bodies resembling viroplasms after transfection of Spodoptera frugiperda insect cells. In accordance, MRCV P9-1 self-associates giving rise to high molecular weight complexes when expressed in bacteria. Strong self-interaction was also evidenced by yeast two-hybrid assays. Furthermore, biochemical characterization showed that MRCV P9-1 bound single stranded RNA and had ATPase activity. Finally, the MRCV P9-1 region required for the formation of VIB-like structures was mapped to the protein carboxy-terminal half. This extensive functional and biochemical characterization of MRCV P9-1 revealed further similarities between plant and animal reovirus viroplasm proteins.

Nuclear localization of cytoplasmic poly(A)-binding protein upon rotavirus infection involves the interaction of NSP3 with eIF4G and RoXaN

Rotavirus nonstructural protein NSP3 interacts specifically with the 3' end of viral mRNAs, with the eukaryotic translation initiation factor eIF4G, and with RoXaN, a cellular protein of yet-unknown function. By evicting cytoplasmic poly(A) binding protein (PABP-C1) from translation initiation complexes, NSP3 shuts off the translation of cellular polyadenylated mRNAs. We show here that PABP-C1 evicted from eIF4G by NSP3 accumulates in the nucleus of rotavirus-infected cells. Through modeling of the NSP3-RoXaN complex, we have identified mutations in NSP3 predicted to interrupt its interaction with RoXaN without disturbing the NSP3 interaction with eIF4G. Using these NSP3 mutants and a deletion mutant unable to associate with eIF4G, we show that the nuclear localization of PABP-C1 not only is dependent on the capacity of NSP3 to interact with eIF4G but also requires the interaction of NSP3 with a specific region in RoXaN, the leucine- and aspartic acid-rich (LD) domain. Furthermore, we show that the RoXaN LD domain functions as a nuclear export signal and that RoXaN tethers PABP-C1 with RNA. This work identifies RoXaN as a cellular partner of NSP3 involved in the nucleocytoplasmic localization of PABP-C1.

A guide to viral inclusions, membrane rearrangements, factories, and viroplasm produced during virus replication

Virus replication can cause extensive rearrangement of host cell cytoskeletal and membrane compartments leading to the “cytopathic effect” that has been the hallmark of virus infection in tissue culture for many years. Recent studies are beginning to redefine these signs of viral infection in terms of specific effects of viruses on cellular processes. In this chapter, these concepts have been illustrated by describing the replication sites produced by many different viruses. In many cases, the cellular rearrangements caused during virus infection lead to the construction of sophisticated platforms in the cell that concentrate replicase proteins, virus genomes, and host proteins required for replication, and thereby increase the efficiency of replication. Interestingly, these same structures, called virus factories, virus inclusions, or virosomes, can recruit host components that are associated with cellular defences against infection and cell stress. It is possible that cellular defence pathways can be subverted by viruses to generate sites of replication. The recruitment of cellular membranes and cytoskeleton to generate virus replication sites can also benefit viruses in other ways. Disruption of cellular membranes can, for example, slow the transport of immunomodulatory proteins to the surface of infected cells and protect against innate and acquired immune responses, and rearrangements to cytoskeleton can facilitate virus release.

Crystallographic and biochemical analysis of rotavirus NSP2 with nucleotides reveals a nucleoside diphosphate kinase-like activity

Rotavirus, the major pathogen of infantile gastroenteritis, carries a nonstructural protein, NSP2, essential for viroplasm formation and genome replication/packaging. In addition to RNA-binding and helix-destabilizing properties, NSP2 exhibits nucleoside triphosphatase activity. A conserved histidine (H225) functions as the catalytic residue for this enzymatic activity, and mutation of this residue abrogates genomic double-stranded RNA synthesis without affecting viroplasm formation. To understand the structural basis of the phosphatase activity of NSP2, we performed crystallographic analyses of native NSP2 and a functionally defective H225A mutant in the presence of nucleotides. These studies showed that nucleotides bind inside a cleft between the two domains of NSP2 in a region that exhibits structural similarity to ubiquitous cellular HIT (histidine triad) proteins. Only minor conformational alterations were observed in the cleft upon nucleotide binding and hydrolysis. This hydrolysis involved the formation of a stable phosphohistidine intermediate. These observations, reminiscent of cellular nucleoside diphosphate (NDP) kinases, prompted us to investigate whether NSP2 exhibits phosphoryl-transfer activity. Bioluminometric assay showed that NSP2 exhibits an NDP kinase-like activity that transfers the bound phosphate to NDPs. However, NSP2 is distinct from the highly conserved cellular NDP kinases in both its structure and catalytic mechanism, thus making NSP2 a potential target for antiviral drug design. With structural similarities to HIT proteins, which are not known to exhibit NDP kinase activity, NSP2 represents a unique example among structure-activity relationships. The newly observed phosphoryl-transfer activity of NSP2 may be utilized for homeostasis of nucleotide pools in viroplasms during genome replication.

Association of rotavirus viroplasms with microtubules through NSP2 and NSP5

Rotavirus replication and virus assembly take place in electrodense spherical structures known as viroplasms whose main components are the viral proteins NSP2 and NSP5. The viroplasms are produced since early times after infection and seem to grow by stepwise addition of viral proteins and by fusion, however, the mechanism of viropIasms formation is unknown. In this study we found that the viroplasms surface colocalized with microtubules, and seem to be caged by a microtubule network. Moreover inhibition of microtubule assembly with nocodazole interfered with viroplasms growth in rotavirus infected cells. We searched for a physical link between viroplasms and microtubules by co-immunoprecipitation assays, and we found that the proteins NSP2 and NSP5 were co-immunoprecipitated with anti-tubulin in rotavirus infected cells and also when they were transiently co-expressed or individually expressed. These results indicate that a functional microtubule network is needed for viroplasm growth presumably due to the association of viroplasms with microtubules via NSP2 and NSP5.

Interaction of rotavirus polymerase VP1 with nonstructural protein NSP5 is stronger than that with NSP2

Rotavirus morphogenesis starts in intracellular inclusion bodies called viroplasms. RNA replication and packaging are mediated by several viral proteins, of which VP1, the RNA-dependent RNA polymerase, and VP2, the core scaffolding protein, were shown to be sufficient to provide replicase activity in vitro. In vivo, however, viral replication complexes also contain the nonstructural proteins NSP2 and NSP5, which were shown to be essential for replication, to interact with each other, and to form viroplasm-like structures (VLS) when coexpressed in uninfected cells. In order to gain a better understanding of the intermediates formed during viral replication, this work focused on the interactions of NSP5 with VP1, VP2, and NSP2. We demonstrated a strong interaction of VP1 with NSP5 but only a weak one with NSP2 in cotransfected cells in the absence of other viral proteins or viral RNA. By contrast, we failed to coimmunoprecipitate VP2 with anti-NSP5 antibodies or NSP5 with anti-VP2 antibodies. We constructed a tagged form of VP1, which was found to colocalize in viroplasms and in VLS formed by NSP5 and NSP2. The tagged VP1 was able to replace VP1 structurally by being incorporated into progeny viral particles. When applying anti-tag-VP1 or anti-NSP5 antibodies, coimmunoprecipitation of tagged VP1 with NSP5 was found. Using deletion mutants of NSP5 or different fragments of NSP5 fused to enhanced green fluorescent protein, we identified the 48 C-terminal amino acids as the region essential for interaction with VP1.

Rotavirus proteins: structure and assembly

Rotavirus is a major pathogen of infantile gastroenteritis. It is a large and complex virus with a multilayered capsid organization that integrates the determinants of host specificity, cell entry, and the enzymatic functions necessary for endogenous transcription of the genome that consists of 11 dsRNA segments. These segments encode six structural and six nonstructural proteins. In the last few years, there has been substantial progress in our understanding of both the structural and functional aspects of a variety of molecular processes involved in the replication of this virus. Studies leading to this progress using of a variety of structural and biochemical techniques including the recent application of RNA interference technology have uncovered several unique and intriguing features related to viral morphogenesis. This review focuses on our current understanding of the structural basis of the molecular processes that govern the replication of rotavirus.

Rotavirus genome replication and morphogenesis: role of the viroplasm

The rotaviruses, members of the family Reoviridae, are icosahedral triple-layered viruses with genomes consisting of 11 segments of double-stranded (ds)RNA. A characteristic feature of rotavirus-infected cells is the formation of large cytoplasmic inclusion bodies, termed viroplasms. These dynamic and highly organized structures serve as viral factories that direct the packaging and replication of the viral genome into early capsid assembly intermediates. Migration of the intermediates to the endoplasmic reticulum (ER) initiates a budding process that culminates in final capsid assembly. Recent information on the development and organization of viroplasms, the structure and function of its components, and interactive pathways linking RNA synthesis and capsid assembly provide new insight into how these microenvironments serve to interface the replication and morphogenetic processes of the virus.

Histidine triad-like motif of the rotavirus NSP2 octamer mediates both RTPase and NTPase activities

Rotavirus NSP2 is an abundant non-structural RNA-binding protein essential for forming the viral factories that support replication of the double-stranded RNA genome. NSP2 exists as stable doughnut-shaped octamers within the infected cell, representing the tail-to-tail interaction of two tetramers. Extending diagonally across the surface of each octamer are four highly basic grooves that function as binding sites for single-stranded RNA. Between the N and C-terminal domains of each monomer is a deep electropositive cleft containing a catalytic site that hydrolyzes the γ-β phosphoanhydride bond of any NTP. The catalytic site has similarity to those of the histidine triad (HIT) family of nucleotide-binding proteins. Due to the close proximity of the grooves and clefts, we investigated the possibility that the RNA-binding activity of the groove promoted the insertion of the 5′-triphosphate moiety of the RNA into the cleft, and the subsequent hydrolysis of its γ-β phosphoanhydride bond. Our results show that NSP2 hydrolyzes the γP from RNAs and NTPs through Mg²⁺-dependent activities that proceed with similar reaction velocities, that require the catalytic His225 residue, and that produce a phosphorylated intermediate. Competition assays indicate that although both substrates enter the active site, RNA is the preferred substrate due to its higher affinity for the octamer. The RNA triphosphatase (RTPase) activity of NSP2 may account for the absence of the 5′-terminal γP on the (−) strands of the double-stranded RNA genome segments. This is the first report of a HIT-like protein with a multifunctional catalytic site, capable of accommodating both NTPs and RNAs during γP hydrolysis.

Hyperphosphorylation of the rotavirus NSP5 protein is independent of serine 67, [corrected] NSP2, or [corrected] the intrinsic insolubility of NSP5 is regulated by cellular phosphatases

The NSP5 protein is required for viroplasm formation during rotavirus infection and is hyperphosphorylated into 32- to 35-kDa isoforms. Earlier studies reported that NSP5 is not hyperphosphorylated without NSP2 coexpression or deleting the NSP5 N terminus and that serine 67 is essential for NSP5 hyperphosphorylation. In this report, we show that full-length NSP5 is hyperphosphorylated in the absence of NSP2 or serine 67 and demonstrate that hyperphosphorylated NSP5 is predominantly present in previously unrecognized cellular fractions that are insoluble in 0.2% sodium dodecyl sulfate. The last 68 residues of NSP5 are sufficient to direct green fluorescent protein into insoluble fractions and cause green fluorescent protein localization into viroplasm-like structures; however, NSP5 insolubility was intrinsic and did not require NSP5 hyperphosphorylation. When we mutated serine 67 to alanine we found that the NSP5 mutant was both hyperphosphorylated and insoluble, identical to unmodified NSP5, and as a result serine 67 is not required for NSP5 phosphorylation. Interestingly, treating cells with the phosphatase inhibitor calyculin A permitted the accumulation of soluble hyperphosphorylated NSP5 isoforms. This suggests that soluble NSP5 is constitutively dephosphorylated by cellular phosphatases and demonstrates that hyperphosphorylation does not direct NSP5 insolubility. Collectively these findings indicate that NSP5 hyperphosphorylation and insolubility are completely independent parameters and that analyzing insoluble NSP5 is essential for studies assessing NSP5 phosphorylation. Our results also demonstrate the involvement of cellular phosphatases in regulating NSP5 phosphorylation and indicate that in the absence of other rotavirus proteins, domains on soluble and insoluble NSP5 recruit cellular kinases and phosphatases that coordinate NSP5 hyperphosphorylation.

Coupling of Rotavirus Genome Replication and Capsid Assembly

The Reoviridae family represents a diverse collection of viruses with segmented double-stranded (ds)RNA genomes, including some that are significant causes of disease in humans, livestock, and plants. The genome segments of these viruses are never detected free in the infected cell but are transcribed and replicated within viral cores by RNA-dependent RNA polymerase (RdRP). Insight into the replication mechanism has been provided from studies on Rotavirus, a member of the Reoviridae whose RdRP can specifically recognize viral plus (+) strand RNAs and catalyze their replication to dsRNAs in vitro. These analyses have revealed that although the rotavirus RdRP can interact with recognition signals in (+) strand RNAs in the absence of other proteins, the conversion of this complex to one that can support initiation of dsRNA synthesis requires the presence and partial assembly of the core capsid protein. By this mechanism, the viral polymerase can carry out dsRNA synthesis only when capsid protein is available to package its newly made product. By preventing the accumulation of naked dsRNA within the cell, the virus avoids triggering dsRNA-dependent interferon signaling pathways that can induce expression and activation of antiviral host proteins.

RNA interference of rotavirus segment 11 mRNA reveals the essential role of NSP5 in the virus replicative cycle

Rotavirus genomes contain 11 double-stranded (ds) RNA segments. Genome segment 11 encodes the non-structural protein NSP5 and, in some strains, also NSP6. NSP5 is produced soon after viral infection and localizes in cytoplasmic viroplasms, where virus replication takes place. RNA interference by small interfering (si) RNAs targeted to genome segment 11 mRNA of two different strains blocked production of NSP5 in a strain-specific manner, with a strong effect on the overall replicative cycle: inhibition of viroplasm formation, decreased production of other structural and non-structural proteins, synthesis of viral genomic dsRNA and production of infectious particles. These effects were shown not to be due to inhibition of NSP6. The results obtained strengthen the importance of secondary transcription/translation in rotavirus replication and demonstrate that NSP5 is essential for the assembly of viroplasms and virus replication.

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.

RoXaN, a novel cellular protein containing TPR, LD, and zinc finger motifs, forms a ternary complex with eukaryotic initiation factor 4G and rotavirus NSP3

Rotavirus mRNAs are capped but not polyadenylated, and viral proteins are translated by the cellular translation machinery. This is accomplished through the action of the viral nonstructural protein NSP3, which specifically binds the 3' consensus sequence of viral mRNAs and interacts with the eukaryotic translation initiation factor eIF4G I. To further our understanding of the role of NSP3 in rotavirus replication, we looked for other cellular proteins capable of interacting with this viral protein. Using the yeast two-hybrid assay, we identified a novel cellular protein-binding partner for rotavirus NSP3. This 110-kDa cellular protein, named RoXaN (rotavirus X protein associated with NSP3), contains a minimum of three regions predicted to be involved in protein-protein or nucleic acid-protein interactions. A tetratricopeptide repeat region, a protein-protein interaction domain most often found in multiprotein complexes, is present in the amino-terminal region. In the carboxy terminus, at least five zinc finger motifs are observed, further suggesting the capacity of RoXaN to bind other proteins or nucleic acids. Between these two regions exists a paxillin leucine-aspartate repeat (LD) motif which is involved in protein-protein interactions. RoXaN is capable of interacting with NSP3 in vivo and during rotavirus infection. Domains of interaction were mapped and correspond to the dimerization domain of NSP3 (amino acids 163 to 237) and the LD domain of RoXaN (amino acids 244 to 341). The interaction between NSP3 and RoXaN does not impair the interaction between NSP3 and eIF4G I, and a ternary complex made of NSP3, RoXaN, and eIF4G I can be detected in rotavirus-infected cells, implicating RoXaN in translation regulation.

Rotavirus NSP2 interferes with the core lattice protein VP2 in initiation of minus-strand synthesis

The rotavirus nonstructural protein NSP2 self-assembles into stable octameric structures that possess nonspecific affinity for single-stranded (ss)RNA and RNA-RNA helix-destabilizing and NTPase activities. Furthermore, NSP2 is a component of replication intermediates with replicase activity and plays a critical role in the packaging and replication of the segmented dsRNA genome of rotavirus. To better understand the function of the protein in genome replication, we examined the effect that purified recombinant NSP2 had on the synthesis of dsRNA by the open core replication system. The results showed that NSP2 inhibited the synthesis of dsRNA from viral mRNA in vitro, in a concentration-dependent manner. The inhibition was overcome by adding increasing amounts of viral mRNA or nonviral ssRNA to the system, indicating that the inhibition was mediated by the nonspecific RNA-binding activity of NSP2. Further analysis revealed that NSP2 interfered with the ability of the open core proteins, GTP, and viral mRNA to form the initiation complex for (-) strand synthesis. Additional experiments indicated that NSP2 did not perturb recognition of viral mRNA by the viral RNA polymerase VP1, but rather interfered with the function of VP2, a protein that is essential for (-) strand initiation and dsRNA synthesis and that forms the T = 1 lattice of the virion core. In contrast to initiation, NSP2 did not inhibit (-) strand elongation. Collectively, the findings provide evidence that the temporal order of interaction of RNA-binding proteins with viral mRNA is a crucial factor impacting the formation of replication intermediates.

Reovirus sigma NS and mu NS proteins form cytoplasmic inclusion structures in the absence of viral infection

Reovirus replication occurs in the cytoplasm of infected cells and culminates in the formation of crystalline arrays of progeny virions within viral inclusions. Two viral nonstructural proteins, sigma NS and micro NS, and structural protein sigma 3 form protein-RNA complexes early in reovirus infection. To better understand the minimal requirements of viral inclusion formation, we expressed sigma NS, mu NS, and sigma 3 alone and in combination in the absence of viral infection. In contrast to its concentration in inclusion structures during reovirus replication, sigma NS expressed in cells in the absence of infection is distributed diffusely throughout the cytoplasm and does not form structures that resemble viral inclusions. Expressed sigma NS is functional as it complements the defect in temperature-sensitive, sigma NS-mutant virus tsE320. In both transfected and infected cells, mu NS is found in punctate cytoplasmic structures and sigma 3 is distributed diffusely in the cytoplasm and the nucleus. The subcellular localization of mu NS and sigma 3 is not altered when the proteins are expressed together or with sigma NS. However, when expressed with micro NS, sigma NS colocalizes with mu NS to punctate structures similar in morphology to inclusion structures observed early in viral replication. During reovirus infection, both sigma NS and mu NS are detectable 4 h after adsorption and colocalize to punctate structures throughout the viral life cycle. In concordance with these results, sigma NS interacts with mu NS in a yeast two-hybrid assay and by coimmunoprecipitation analysis. These data suggest that sigma NS and mu NS are the minimal viral components required to form inclusions, which then recruit other reovirus proteins and RNA to initiate viral genome replication.

Rotavirus nonstructural protein NSP5 interacts with major core protein VP2

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.

Rotavirus protein involved in genome replication and packaging exhibits a HIT-like fold

Rotavirus, the major cause of life-threatening infantile gastroenteritis, is a member of the Reoviridae. Although the structures of rotavirus and other members of the Reoviridae have been extensively studied, little is known about the structures of virus-encoded non-structural proteins that are essential for genome replication and packaging. The non-structural protein NSP2 of rotavirus, which exhibits nucleoside triphosphatase, single-stranded RNA binding, and nucleic-acid helix-destabilizing activities, is a major component of viral replicase complexes. We present here the X-ray structure of the functional octamer of NSP2 determined to a resolution of 2.6 A. The NSP2 monomer has two distinct domains. The amino-terminal domain has a new fold. The carboxy-terminal domain resembles the ubiquitous cellular histidine triad (HIT) group of nucleotidyl hydrolases. This structural similarity suggests that the nucleotide-binding site is located inside the cleft between the two domains. Prominent grooves that run diagonally across the doughnut-shaped octamer are probable locations for RNA binding. Several RNA binding sites, resulting from the quaternary organization of NSP2 monomers, may be required for the helix destabilizing activity of NSP2 and its function during genome replication and packaging.

RNA-binding activity of the rotavirus phosphoprotein NSP5 includes affinity for double-stranded RNA

Phosphoprotein NSP5 is a component of replication intermediates that catalyze the synthesis of the segmented double-stranded RNA (dsRNA) rotavirus genome. To study the role of the protein in viral replication, His-tagged NSP5 was expressed in bacteria and purified by affinity chromatography. In vitro phosphorylation assays showed that NSP5 alone contains minimal autokinase activity but undergoes hyperphosphorylation when combined with the NTPase and helix-destabilizing protein NSP2. Hence, NSP2 mediates the hyperphosphorylation of NSP5 in the absence of other viral or cellular proteins. RNA-binding assays demonstrated that NSP5 has unique nonspecific RNA-binding activity, recognizing single-stranded RNA and dsRNA with similar affinities. The possible functions of the RNA-binding activities of NSP5 are to cooperate with NSP2 in the destabilization of RNA secondary structures and in the packaging of RNA and/or to prevent the interferon-induced dsRNA-dependent activation of the protein kinase PKR.

Complete nucleotide sequence of a group A avian rotavirus genome and a comparison with its counterparts of mammalian rotaviruses

The nucleotide sequences encoding four structural proteins (VP1-4) and six nonstructural proteins (NSP1-6) of avian rotavirus PO-13 were determined. Based on the results of earlier sequencing studies [Ito et al., 1995, Sequence analysis of cDNA for the VP6 protein of group A avian rota viruses. Arch. Vriol. 140, 605-612; Rohwedder et al., 1997, Chicken rotavirus Ch-1 shows a second type of avian VP6 gene, Virus Genes 15, 65-71; Rohwedder et al., 1997, Bovine rotavirus 993/83 shows a third subtype of avian VP7 protein, Virus Genes 14, 147-151], determination of PO-13 genome sequence has been completed. The PO-13 genome is 18845 nucleotides in length. It is 290 nucleotides longer than the genome of SA11. The amino acid sequence homology between PO-13 and mammalian rotaviruses ranged from 76-77% (VP1) to 16-18% (NSP1). The features of gene and amino acid sequence were compared with those of the corresponding protein of mammalian rotaviruses. Based on results of the phylogenetic analyses of NSP1, we speculate that an ancestral rotavirus could have separated into groups A, B and C rotaviruses at an early evolutionary stage and that group A rotavirus separated into mammalian and avian rotaviruses with host evolution.

Identification and characterization of the helix-destabilizing activity of rotavirus nonstructural protein NSP2

The rotavirus nonstructural protein NSP2 self-assembles into homomultimers, binds single-stranded RNA nonspecifically, possesses a Mg2+-dependent nucleoside triphosphatase (NTPase) activity, and is a component of replication intermediates. Because these properties are characteristics of known viral helicases, we examined the possibility that this was also an activity of NSP2 by using a strand displacement assay and purified bacterially expressed protein. The results revealed that, under saturating concentrations, NSP2 disrupted both DNA-RNA and RNA-RNA duplexes; hence, the protein possesses helix-destabilizing activity. However, unlike typical helicases, NSP2 required neither a divalent cation nor a nucleotide energy source for helix destabilization. Further characterization showed that NSP2 displayed no polarity in destabilizing a partial duplex. In addition, helix destabilization by NSP2 was found to proceed cooperatively and rapidly. The presence of Mg2+ and other divalent cations inhibited by approximately one-half the activity of NSP2, probably due to the increased stability of the duplex substrate brought on by the cations. In contrast, under conditions where NSP2 functions as an NTPase, its helix-destabilizing activity was less sensitive to the presence of Mg2+, suggesting that in the cellular environment the two activities associated with the protein, helix destabilization and NTPase, may function together. Although distinct from typical helicases, the helix-destabilizing activity of NSP2 is quite similar to that of the sigmaNS protein of reovirus and to the single-stranded DNA-binding proteins (SSBs) involved in double-stranded DNA replication. The presence of SSB-like nonstructural proteins in two members of the family Reoviridae suggests a common mechanism of unwinding viral mRNA prior to packaging and subsequent minus-strand RNA synthesis.

NTP binding and phosphohydrolase activity associated with purified bluetongue virus non-structural protein NS2

The bluetongue virus ssRNA-binding protein, NS2, is a phosphoprotein that forms viral inclusion bodies in infected cells. Recombinant NS2 was expressed in the baculovirus expression system and purified to homogeneity from insect cells. Purified NS2 bound nucleosides. Further investigation revealed that the protein bound ATP and GTP and could hydrolyse both nucleosides to their corresponding NMPs, with a higher efficiency for the hydrolysis of ATP. The increased efficiency of hydrolysis of ATP correlated with a higher binding affinity of NS2 for ATP than GTP. Ca(2+), Mg(2+) and Mn(2+) were able to function as the required divalent cation in the reactions. The phosphohydrolase activity was not sensitive to ouabain, an inhibitor of cellular ATPases, suggesting that this activity was not the result of a cellular contaminant.

The C-terminal domain of rotavirus NSP5 is essential for its multimerization, hyperphosphorylation and interaction with NSP6

Rotavirus NSP5 is a non-structural phosphoprotein with putative autocatalytic kinase activity, and is present in infected cells as various isoforms having molecular masses of 26, 28 and 30-34 kDa. We have previously shown that NSP5 forms oligomers and interacts with NSP6 in yeast cells. Here we have mapped the domains of NSP5 responsible for these associations. Deletion mutants of the rotavirus YM NSP5 were constructed and assayed for their ability to interact with full-length NSP5 and NSP6 using the yeast two-hybrid assay. The homomultimerization domain was mapped to the 20 C-terminal aa of the protein, which have a predicted alpha-helical structure. A deletion mutant lacking the 10 C-terminal aa (DeltaC10) failed to multimerize both in yeast cells and in an in vitro affinity assay. When transiently expressed in MA104 cells, NSP5 became hyperphosphorylated (30-34 kDa isoforms). In contrast, the DeltaC10 mutant produced forms equivalent to the 26 and 28 kDa species, but was poorly hyperphosphorylated, suggesting that multimerization is important for this proposed activity of the protein. The interaction domain with NSP6 was found to be present in the 35 C-terminal aa of NSP5, overlapping the multimerization domain of the protein, and suggesting that NSP6 might have a regulatory role in the self-association of NSP5. NSP6 was also found to interact with wild-type NSP5, but not with its mutant DeltaC10, in cells transiently transfected with plasmids encoding these proteins, confirming the relevance of the 10 C-terminal aa for the formation of the heterocomplex.

Multimers formed by the rotavirus nonstructural protein NSP2 bind to RNA and have nucleoside triphosphatase activity

The nonstructural protein NSP2 is a component of rotavirus replication intermediates and accumulates in cytoplasmic inclusions (viroplasms), sites of genome RNA replication and the assembly of subviral particles. To better understand the structure and function of the protein, C-terminally His-tagged NSP2 was expressed in bacteria and purified to homogeneity. In its purified form, the protein did not exist as a monomer but rather was present as an 8S-10S homomultimer consisting of 6 +/- 2 subunits of recombinant NSP2 (rNSP2). As shown by gel mobility shift assays, the rNSP2 multimers bound to RNA in discrete cooperative steps to form higher-order RNA-protein complexes. The RNA-binding activity of the rNSP2 multimers was determined to be nonspecific and to have a strong preference for single-stranded RNA over double-stranded RNA, for which it displayed little affinity. Enzymatic analysis revealed that rNSP2 possessed an associated nucleoside triphosphatase (NTPase) activity in vitro, which in the presence of Mg(2+) catalyzed the hydrolysis of each of the four NTPs to NDPs with equal efficiency. Evidence indicating that the hydrolysis of NTP resulted in the covalent linkage of the gamma-phosphate to rNSP2 was obtained. Additional experiments showed that NSP2 expressed transiently in MA014 cells is phosphorylated. We propose that NSP2 functions as a molecular motor, catalyzing the packaging of viral mRNA into core-like replication intermediates through the energy derived from its NTPase activity.

Molecular characterization of human group C rotavirus genes 6, 7 and 9

Genes 6, 7 and 9 of human group C rotavirus 'Bristol' strain, encoding non-structural proteins (NSP) 3, 1 and 2, respectively, were cloned and sequenced. Human group C rotavirus genome segment 6 is 1350 bp and contains a single ORF of 1231 nucleotides (encoding 402 amino acids). Genome segment 7 is 1270 bp and encodes a protein of 394 amino acids and genome segment 9 is 1037 bp and encodes a 312 amino acid protein. The human group C rotavirus genes 6, 7 and 9 showed 78, 67 and 88% sequence identity, respectively, to the corresponding porcine group C rotavirus genes. The derived protein sequences were compared with those of the porcine 'Cowden' group C and mammalian group A rotavirus strains. The human group C rotavirus NSP1 protein sequence is one amino acid longer than the porcine group C equivalent. In common with group A and porcine group C rotaviruses, the human group C rotavirus NSP1 protein has a zinc finger motif. Human group C rotavirus NSP2 has two hydrophobic heptad repeat regions, a basic, RNA-binding domain and a basic, proline-rich region. Human group C rotavirus NSP3 has both single- and double-stranded RNA-binding domains and several hydrophobic heptad repeat regions, one of which forms a leucine zipper. This work completes the molecular characterization of the non-structural proteins of a human group C rotavirus. Phylogenetic analysis of all the non-structural genes of group A, B and C rotaviruses suggests that these viruses have diverged at a constant rate from a common ancestor.

Rotavirus RNA-binding protein NSP3 interacts with eIF4GI and evicts the poly(A) binding protein from eIF4F

Most eukaryotic mRNAs contain a 5'cap structure and a 3'poly(A) sequence that synergistically increase the efficiency of translation. Rotavirus mRNAs are capped, but lack poly(A) sequences. During rotavirus infection, the viral protein NSP3A is bound to the viral mRNAs 3' end. We looked for cellular proteins that could interact with NSP3A, using the two-hybrid system in yeast. Screening a CV1 cell cDNA library allowed us to isolate a partial cDNA of the human eukaryotic initiation factor 4GI (eIF4GI). The interaction of NSP3A with eIF4GI was confirmed in rotavirus infected cells by co-immunoprecipitation and in vitro with NSP3A produced in Escherichia coli. In addition, we show that the amount of poly(A) binding protein (PABP) present in eIF4F complexes decreases during rotavirus infection, even though eIF4A and eIF4E remain unaffected. PABP is removed from the eIF4F complex after incubation in vitro with the C-terminal part of NSP3A, but not with its N-terminal part produced in E.coli. These results show that a physical link between the 5' and the 3' ends of mRNA is necessary for the efficient translation of viral mRNAs and strongly support the closed loop model for the initiation of translation. These results also suggest that NSP3A, by taking the place of PABP on eIF4GI, is responsible for the shut-off of cellular protein synthesis.

In vivo and in vitro phosphorylation of rotavirus NSP5 correlates with its localization in viroplasms

NSP5 (NS26), the product of rotavirus gene 11, is a phosphoprotein whose role in the virus replication cycle is unknown. To gain further insight into its function, we obtained monoclonal antibodies against the baculovirus-expressed protein. By immunoprecipitation and immunoblotting experiments, we showed that (i) NSP5 appears in many different phosphorylated forms in rotavirus-infected cells; (ii) immunoprecipitated NSP5 from rotavirus-infected cells can be phosphorylated in vitro by incubation with ATP; (iii) NSP5, produced either by transient transfection of rotavirus gene 11 or by infection by gene 11 recombinant vaccinia virus or baculovirus, can be phosphorylated in vivo and in vitro; (iv) NSP5 expressed in Escherichia coli is phosphorylated in vitro, and thus NSP5 is a potential protein kinase; and (v) NSP5 forms dimers and interacts with NSP2. The intracellular localization of NSP5 in the course of rotavirus infection and after transient expression in COS7 cells has also been investigated. In rotavirus-infected cells, NSP5 is localized in viroplasms, but it is widespread throughout the cytoplasm of transfected COS7 cells. NSP5 produced by transfected COS7 cells did not acquire the multiphosphorylated forms observed in rotavirus-infected COS7 cells. Thus, there is a tight correlation between the localization of NSP5 in the viroplasms and its protein kinase activity in vivo or in vitro. Our results suggest that cellular or viral cofactors are indispensable to fully phosphorylate NSP5 and to reach its intracellular localization.