Probing the structure of rotavirus NSP4: a short sequence at the extreme C terminus mediates binding to the inner capsid particle

Rapid production of recombinant rotaviruses by overexpression of NSP2 and NSP5 genes with modified nucleotide sequences

Reverse genetics systems for rotaviruses (RV) facilitate the generation of genetically engineered RVs by transfection of 11 plasmids encoding 11 genomic viral RNA segments. In addition to viral genome expression, overexpression of NSP2 and NSP5 has been used to increase the rescue efficiency of recombinant RVs. Here, we showed that the overexpression of nucleotide sequence-modified NSP2 and NSP5 enabled the rapid and efficient production of recombinant RVs. Using improved reverse genetics, we established a reverse genetics system for human and bovine RV clinical isolates, as well as laboratory strains of bovine RV (NCDV and UK) and porcine RV (Gottfried). In addition, we rescued low-replicating recombinant RVs carrying a mutant NSP4 lacking the double-layered particle-binding domain, which was deficient in the efficient production of mature virions. These advancements in reverse genetics enabled the generation of molecular clones of RV clinical isolates and recombinant RVs harboring critical amino acid mutations, offering a versatile platform for investigating RV biology and pathogenesis.IMPORTANCERecombinant rotavirus (RV) synthesis via reverse genetics relies on both the viral propagation capacity and the efficiency of the experimental system. Since the establishment of our reverse genetics system, several enhancements have been implemented to augment the rescue efficiency. Nevertheless, challenges persist in generating RV clinical strains and recombinant viruses with low replication capacities. Notably, this improved reverse genetics system successfully facilitated the establishment of molecular clones of human and bovine RV clinical isolates. Fecal samples from patients with RV typically harbor quasi-species or, occasionally, multiple genotypes of RV. In the present study, we performed the genetic sequencing of clinical viral strains during the early propagation stages in cultured cells. Subsequently, infectious viruses were synthesized, allowing the characterization of circulating viruses in nature. This approach provides valuable insights into the genetic diversity and dynamics of RV populations and contributes to a more comprehensive understanding of viral pathogenesis and evolution.

Identification of a cooperative effect between amino acids 169 and 174 in the rotavirus NSP4 double-layered particle-binding domain

Segmented RNA viruses are capable of exchanging genome segments via reassortment as a means of immune evasion and to maintain viral fitness. Reassortments of single-genome segments are common among group A rotaviruses. Multiple instances of co-reassortment of two genome segments, GS6(VP6) and GS10(NSP4), have been documented in surveillance. Specifically, a division between NSP4 genotypes has been observed in the NSP4 double-layered particle (DLP)-binding domain. A previously hypothesized mechanism for this co-reassortment has been suggested to be the interaction between VP6 and NSP4 during DLP transport from viroplasms for particle maturation. In this study, we used sequence analysis, RNA secondary structure prediction, molecular dynamics and reverse genetics to form a hypothesis regarding the role of the NSP4 DLP-binding domain. Sequence analysis showed that the polarity of NSP4 DLP-binding domain amino acids 169 and 174 is clearly divided between E1 and E2 NSP4 genotypes. Viruses with E1 NSP4s had 169A/I or 169S/T with 174S. E2 NSP4s had 169R/K and 174A. RNA secondary structure prediction showed that mutation in both 545 (aa169) and 561 (aa174) causes global structure remodelling. Molecular dynamics showed that the NSP4/VP6 interaction stability is increased by mutating both aa positions 169 and 174. Using reverse genetics, we showed that an R169I mutation alone does not prevent rescue. Conversely, 174A to 174S prevented rescue, and rescue could be returned by combining 174S with 169I. When compared to rSA11 NSP4-wt, both rSA11 NSP4-R169I and rSA11 NSP4-R169I/A174S had a negligible but significant reduction in titre at specific time points. This study suggests that amino acid 174 of NSP4 may be essential in maintaining the VP6/NSP4 interaction required for DLP transport. Our results suggest that maintenance of specific polarities of amino acids at positions 169 and 174 may be required for the fitness of rotavirus field strains.

Whole-genome analysis of human group A rotaviruses in 1980s Japan and evolutionary assessment of global Wa-like strains across half a century

Historically, the Wa-like strains of human group A rotavirus (RVA) have been major causes of gastroenteritis. However, since the 2010s, the circulation of non-Wa-like strains has been increasingly reported, indicating a shift in the molecular epidemiology of RVA. Although understanding RVA evolution requires the analysis of both current and historical strains, comprehensive pre-1980's sequencing data are scarce globally. We determined the whole-genome sequences of representative strains from six RVA gastroenteritis outbreaks observed at an infant home in Sapporo, Japan, between 1981 and 1989. These outbreaks were mainly caused by G1 or G3 Wa-like strains, resembling strains from the United States in the 1970s-1980s and from Malawi in the 1990s. Phylogenetic analysis of these infant home strains, together with Wa-like strains collected worldwide from the 1970s to 2020, revealed a notable trend: pre-2010 strains diverged into multiple lineages in many genomic segments, whereas post-2010 strains tended to converge into a single lineage. However, Bayesian skyline plot indicated near-constant effective population sizes from the 1970s to 2020, and selection pressure analysis identified positive selection only at amino acid 75 of NSP2. These results suggest that evidence supporting the influence of rotavirus vaccines, introduced globally since 2006, on Wa-like RVA molecular evolution is lacking at present, and phylogenetic analysis may simply reflect natural fluctuations in RVA molecular evolution. Evaluating the long-term impact of RV vaccines on the molecular evolution of RVA requires sustained surveillance.

Spectroscopic analysis of the bacterially expressed head domain of rotavirus VP6

The rotavirus capsid protein VP6 forms the middle of three protein layers and is responsible for many critical steps in the viral life cycle. VP6 as a structural protein can be used in various applications including as a subunit vaccine component. The head domain of VP6 (VP6H) contains key sequences that allow the protein to trimerize and that represent epitopes that are recognized by human antibodies in the viral particle. The domain is rich in β-sheet secondary structures. Here, VP6H was solubilised from bacterial inclusion bodies and purified using a single affinity chromatography step. Spectral (far-UV circular dichroism and intrinsic tryptophan fluorescence) analysis revealed that the purified domain had native-like secondary and tertiary structures. The domain could maintain structure up to 44°C during thermal denaturation following which structural changes result in an intermediate forming and finally irreversible aggregation and denaturation. The chemical denaturation with urea and guanidinium hydrochloride produces intermediates that represent a loss in the cooperativity. The VP6H domain is stable and can fold to produce its native structure in the absence of the VP6 base domain but cannot be defined as an independent folding unit.

Rotavirus Particle Disassembly and Assembly In Vivo and In Vitro

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

Rotavirus-Induced Lipid Droplet Biogenesis Is Critical for Virus Replication

A variety of pathogens, including viruses, bacteria and parasites, target cellular lipid droplets for their replication. Rotaviruses (RVs) infect the villous epithelium of the small intestine and are a major cause of acute gastroenteritis in infants and young children worldwide. RVs induce and require lipid droplets for the formation of viroplasms, sites of virus genome replication, and nascent particle assembly. Here we review the role of lipid droplets in RV replication. Inhibitors of fatty acid synthesis or chemicals that interfere with lipid droplet homeostasis decrease the number and size of viroplasms and the yield of infectious virus. We used a genetically engineered RV, delayed in viroplasm assembly, to show an early interaction of RV nonstructural protein NSP2 and the lipid droplet-associated protein phospho-PLIN1. The interaction between NSP2 and phospho-PLIN1 suggests that we have identified part of the mechanism of RV-induced lipid droplet formation. These studies demonstrate that RV is an excellent model to dissect the cellular process of lipid droplet formation and to determine how RV induces and usurps lipid droplet biogenesis to form viroplasm/lipid droplets for virus replication.

Non-structural Enterotoxin (NSP4) Gene based Molecular Characterization of Caprine and Ovine Rotavirus A, India

Rotavirus A (RVA) causes viral gastroenteritis in humans and animals, including calves, piglets, and foals. The current study reports the genetic characterization of the full-length enterotoxin gene, NSP4, from caprine and ovine species. Upon characterizing eight full-length NSP4 genes by sequencing, it was found that the four caprine and three ovine RVAs NSP4 genes are of E2 genotype and the sole ovine RVA isolate was found to be of E1 genotype. In the sequence and phyloanalysis of the NSP4 gene the seven E2 genotypes clustered with bovine, human, and caprine isolates from India and Bangladesh, respectively. The E1 genotype of ovine RVA was closer to human RVA isolate from India. The nucleotide per cent identity analysis revealed that all E2 genotype strains of caprine and ovine species ranged from 88.4% to 90.4% and it was found common to both the reference human RVA isolates DS-1 and AU-1. Whereas, the E1 genotype ovine strain clustered with human RVA isolates with 93.1% nucleotide per cent identity. The RVA strains circulating in caprine and ovine populations may share a common origin which is usually found in artiodactyl species because humans share a common dwelling with animals. Future studies are needed to confirm these findings of their relationship with humans and large animals.

Rotavirus Replication: Gaps of Knowledge on Virus Entry and Morphogenesis

In 1973, rotaviruses A (RVAs) were discovered as major causative agents of acute gastroenteritis in infants and young children worldwide. The infectious RV virion is an icosahedral particle composed of three concentric protein layers surrounding the 11 double-stranded (dsRNA) segments. An in vitro replication system for RVs in permanent cell lines was developed in 1982 and expanded to replication in intestinal organoids in 2015. However, the details of rotavirus (RV) entry into cells and particle maturation mechanisms at the molecular level remain incompletely understood. Slowing down human RVA replication in cell culture on ice allowed morphological visualization of virus particle entry and the assembly of triple-layered particles (virion). Although RVAs are non-enveloped viruses, after virus attachment to the cell membrane, the virus enters the cell by perforating the plasma membrane by a fusion mechanism involving VP5* of the cleaved VP4 protein, as the alternative virus entry route besides the receptor-mediated endocytosis which is generally accepted. After assembling double-layered particles (DLPs) in viroplasm or cytoplasm, they appear to be connected with the endoplasmic reticulum (ER) membrane and become coated with outer capsid proteins (VP4 and VP7) in a coating process. The perforation of the ER membrane is caused by an unknown mechanism following interaction between non-structural protein 4 (NSP4) and the inner capsid protein VP6 of the DLPs. The coating process is closely related to the formation of a hetero-oligomeric complex (NSP4, VP4 and VP7). These lines of evidence suggest the existence of novel mechanisms of RV morphogenesis.

COPII Vesicle Transport Is Required for Rotavirus NSP4 Interaction with the Autophagy Protein LC3 II and Trafficking to Viroplasms

Many viruses that replicate in the cytoplasm dramatically remodel and stimulate the accumulation of host cell membranes for efficient replication by poorly understood mechanisms. For rotavirus, a critical step in virion assembly requires the accumulation of membranes adjacent to virus replication centers called viroplasms. Early electron microscopy studies describe viroplasm-associated membranes as "swollen" endoplasmic reticulum (ER). We previously demonstrated that rotavirus infection initiates cellular autophagy and that membranes containing the autophagy marker protein LC3 and the rotavirus ER-synthesized transmembrane glycoprotein NSP4 traffic to viroplasms, suggesting that NSP4 must exit the ER. This study aimed to address the mechanism of NSP4 exit from the ER and determine whether the viroplasm-associated membranes are ER derived. We report that (i) NSP4 exits the ER in COPII vesicles, resulting in disrupted COPII vesicle transport and ER exit sites; (ii) COPII vesicles are hijacked by LC3 II, which interacts with NSP4; and (iii) NSP4/LC3 II-containing membranes accumulate adjacent to viroplasms. In addition, the ER transmembrane proteins SERCA and calnexin were not detected in viroplasm-associated membranes, providing evidence that the rotavirus maturation process of "budding" occurs through autophagy-hijacked COPII vesicle membranes. These findings reveal a new mechanism for rotavirus maturation dependent on intracellular host protein transport and autophagy for the accumulation of membranes required for virus replication.IMPORTANCE In a morphogenic step that is exceedingly rare for nonenveloped viruses, immature rotavirus particles assemble in replication centers called viroplasms, and bud through cytoplasmic cellular membranes to acquire the outer capsid proteins for infectious particle assembly. Historically, the intracellular membranes used for particle budding were thought to be endoplasmic reticulum (ER) because the rotavirus nonstructural protein NSP4, which interacts with the immature particles to trigger budding, is synthesized as an ER transmembrane protein. This present study shows that NSP4 exits the ER in COPII vesicles and that the NSP4-containing COPII vesicles are hijacked by the cellular autophagy machinery, which mediates the trafficking of NSP4 to viroplasms. Changing the paradigm for rotavirus maturation, we propose that the cellular membranes required for immature rotavirus particle budding are not an extension of the ER but are COPII-derived autophagy isolation membranes.

The Guanine Nucleotide Exchange Factor GBF1 Participates in Rotavirus Replication

Cellular and viral factors participate in the replication cycle of rotavirus. We report that the guanine nucleotide exchange factor GBF1, which activates the small GTPase Arf1 to induce COPI transport processes, is required for rotavirus replication since knocking down GBF1 expression by RNA interference or inhibiting its activity by treatment with brefeldin A (BFA) or Golgicide A (GCA) significantly reduces the yield of infectious viral progeny. This reduction in virus yield was related to a block in virus assembly, since in the presence of either BFA or GCA, the assembly of infectious mature triple-layered virions was significantly prevented and only double-layered particles were detected. We report that the catalytic activity of GBF1, but not the activation of Arf1, is essential for the assembly of the outer capsid of rotavirus. We show that both BFA and GCA, as well as interfering with the synthesis of GBF1, alter the electrophoretic mobility of glycoproteins VP7 and NSP4 and block the trimerization of the virus surface protein VP7, a step required for its incorporation into virus particles. Although a posttranslational modification of VP7 (other than glycosylation) could be related to the lack of trimerization, we found that NSP4 might also be involved in this process, since knocking down its expression reduces VP7 trimerization. In support, recombinant VP7 protein overexpressed in transfected cells formed trimers only when cotransfected with NSP4.IMPORTANCE Rotavirus, a member of the family Reoviridae, is the major cause of severe diarrhea in children and young animals worldwide. Despite significant advances in the characterization of the biology of this virus, the mechanisms involved in morphogenesis of the virus particle are still poorly understood. In this work, we show that the guanine nucleotide exchange factor GBF1, relevant for COPI/Arf1-mediated cellular vesicular transport, participates in the replication cycle of the virus, influencing the correct processing of viral glycoproteins VP7 and NSP4 and the assembly of the virus surface proteins VP7 and VP4.

Molecular characterisation of the NSP4 gene of group A human rotavirus G2P[4] strains circulating in São Paulo, Brazil, from 1994 and 2006 to 2010

Group A human rotaviruses (HuRVA) are causative agents of acute gastroenteritis. Six viral structural proteins (VPs) and six nonstructural proteins (NSPs) are produced in RV-infected cells. NSP4 is a diarrhoea-inducing viral enterotoxin and NSP4 gene analysis revealed at least 15 (E1-E15) genotypes. This study analysed the NSP4 genetic diversity of HuRVA G2P[4] strains collected in the state of São Paulo (SP) from 1994 and 2006-2010 using reverse transcription-polymerase chain reaction, sequencing and phylogenetic analysis. Forty (97.6%) G2P[4] strains displayed genotype E2; one strain (2.4%) displayed genotype E1. These results are consistent with the proposed linkage between VP4/VP7 (G2P[4]) and the NSP4 (E2) genotype of HuRVA. NSP4 phylogenetic analysis showed distinct clusters, with grouping of most strains by their genotype and collection year, and most strains from SP were clustered together with strains from other Brazilian states. A deduced amino acid sequence alignment for E2 showed many variations in the C-terminal region, including the VP4-binding domain. Considering the ability of NSP4 to generate host immunity, monitoring NSP4 variations, along with those in the VP4 or VP7 protein, is important for evaluating the circulation and pathogenesis of RV. Finally, the presence of one G2P[4]E1 strain reinforces the idea that new genotype combinations emerge through reassortment and independent segregation.

Viroporins: structure, function and potential as antiviral targets

The channel-forming activity of a family of small, hydrophobic integral membrane proteins termed 'viroporins' is essential to the life cycles of an increasingly diverse range of RNA and DNA viruses, generating significant interest in targeting these proteins for antiviral development. Viroporins vary greatly in terms of their atomic structure and can perform multiple functions during the virus life cycle, including those distinct from their role as oligomeric membrane channels. Recent progress has seen an explosion in both the identification and understanding of many such proteins encoded by highly significant pathogens, yet the prototypic M2 proton channel of influenza A virus remains the only example of a viroporin with provenance as an antiviral drug target. This review attempts to summarize our current understanding of the channel-forming functions for key members of this growing family, including recent progress in structural studies and drug discovery research, as well as novel insights into the life cycles of many viruses revealed by a requirement for viroporin activity. Ultimately, given the successes of drugs targeting ion channels in other areas of medicine, unlocking the therapeutic potential of viroporins represents a valuable goal for many of the most significant viral challenges to human and animal health.

Rotaviruses: Extraction and Isolation of RNA, Reassortant Strains, and NSP4 Protein

Rotavirus (RV) contains 11 double-stranded RNA segments that encode for twelve structural and nonstructural proteins. The separation and isolation of viral RNA is a necessary precursor for many experimental techniques and can be useful for rapid RV RNA typing and sequencing of different rotavirus strains. The segmented genome enables RV to recombine easily. These recombinant viruses are essential for many purposes, including generation of potential vaccine strains. Rotavirus gene 10 expresses the viral enterotoxin, NSP4, which has been the focus of several studies due to the influence of NSP4 on rotavirus replication, morphogenesis, and pathogenesis. This unit will describe the isolation and separation of viral RNAs, the production characterization of recombinant RV in culture, and the expression and isolation of NSP4 in mammalian and insect cells.

Sequence and structural analyses of NSP4 proteins from human group A rotavirus strains detected in Tunisia

BACKGROUND

The NSP4 protein of group A rotavirus (RVA) has been recognized as a viral enterotoxin and plays important roles in viral pathogenesis and morphogenesis. Domains involved in structural and functional interactions have been proposed mainly based on the simian SA11 strain.

METHODS

NSP4 has been classified into 15 different genotypes (E1-E15), and the aim of this study was to analyze the sequences of 46 RVA strains in order to determine the aminoacid (aa) differences between E1 and E2 genotypes. Another aspect was to characterize the structural and physicochemical properties of these strains.

RESULTS

Comparison of deduced aa sequences of the NSP4 protein showed that divergences between NSP4 genotypes E1 and E2 were mostly observed in the VP4-binding, the interspecies variable domain (ISVD) and the double-layered particle (DLP) binding domains. Interestingly, uncommon variations in residues 131 and 138, which are known to be important aa in pathogenesis, were found in one unusual animal derived strain belonging to the E2 genotype. Concerning the structural aspect, no significant differences were noted.

CONCLUSION

The presence of punctual aa variations in the NSP4 genotypes may indicate that NSP4 mutates mainly via accumulation of point mutations.

Rotaviral enterotoxin nonstructural protein 4 targets mitochondria for activation of apoptosis during infection

Viruses have evolved to encode multifunctional proteins to control the intricate cellular signaling pathways by using very few viral proteins. Rotavirus is known to express six nonstructural and six structural proteins. Among them, NSP4 is the enterotoxin, known to disrupt cellular Ca(2+) homeostasis by translocating to endoplasmic reticulum. In this study, we have observed translocation of NSP4 to mitochondria resulting in dissipation of mitochondrial membrane potential during virus infection and NSP4 overexpression. Furthermore, transfection of the N- and C-terminal truncated NSP4 mutants followed by analyzing NSP4 localization by immunofluorescence microscopy identified the 61-83-amino acid region as the shortest mitochondrial targeting signal. NSP4 exerts its proapoptotic effect by interacting with mitochondrial proteins adenine nucleotide translocator and voltage-dependent anion channel, resulting in dissipation of mitochondrial potential, release of cytochrome c from mitochondria, and caspase activation. During early infection, apoptosis activation by NSP4 was inhibited by the activation of cellular survival pathways (PI3K/AKT), because PI3K inhibitor results in early induction of apoptosis. However, in the presence of both PI3K inhibitor and NSP4 siRNA, apoptosis was delayed suggesting that the early apoptotic signal is initiated by NSP4 expression. This proapoptotic function of NSP4 is balanced by another virus-encoded protein, NSP1, which is implicated in PI3K/AKT activation because overexpression of both NSP4 and NSP1 in cells resulted in reduced apoptosis compared with only NSP4-expressing cells. Overall, this study reports on the mechanism by which enterotoxin NSP4 exerts cytotoxicity and the mechanism by which virus counteracts it at the early stage for efficient infection.

Elucidation of the Rotavirus NSP4-Caveolin-1 and -Cholesterol Interactions Using Synthetic Peptides

Rotavirus (RV) NSP4, the first described viral enterotoxin, is a multifunctional glycoprotein that contributes to viral pathogenesis, morphogenesis, and replication. NSP4 binds both termini of caveolin-1 and is isolated from caveolae fractions that are rich in anionic phospholipids and cholesterol. These interactions indicate that cholesterol/caveolin-1 plays a role in NSP4 transport to the cell surface, which is essential to its enterotoxic activity. Synthetic peptides were utilized to identify target(s) of intervention by exploring the NSP4-caveolin-1 and -cholesterol interactions. NSP4_112–140 that overlaps the caveolin-1 binding domain and a cholesterol recognition amino acid consensus (CRAC) motif and both termini of caveolin-1 (N-caveolin-1_2–20,  _19–40 and C-caveolin-1_161–180) were synthesized. Direct fluorescence-binding assays were employed to determine binding affinities of the NSP4-caveolin-1 peptides and cholesterol. Intracellular cholesterol alteration revealed a redistribution of NSP4 and disintegration of viroplasms. These data further imply interruption of NSP4_112–140-N-caveolin-1_19–40 and cholesterol interactions may block NSP4 intracellular transport, hence enterotoxicity.

Structural insights into the coupling of virion assembly and rotavirus replication

Viral replication is rapid and robust, but it is far from a chaotic process. Instead, successful production of infectious progeny requires that events occur in the correct place and at the correct time. Rotaviruses (segmented double-stranded RNA viruses of the Reoviridae family) seem to govern their replication through ordered disassembly and assembly of a triple-layered icosahedral capsid. In recent years, high-resolution structural data have provided unprecedented insight into these events. In this Review, we explore the current understanding of rotavirus replication and how it compares to replication of other Reoviridae family members.

Rotavirus NSP4 is secreted from infected cells as an oligomeric lipoprotein and binds to glycosaminoglycans on the surface of non-infected cells

Background

Nonstructural glycoprotein 4 (NSP4) encoded by rotavirus is the only viral protein currently believed to function as an enterotoxin. NSP4 is synthesized as an intracellular transmembrane glycoprotein and as such is essential for virus assembly. Infection of polarized Caco-2 cells with rotavirus also results in the secretion of glycosylated NSP4 apparently in a soluble form despite retention of its transmembrane domain. We have examined the structure, solubility and cell-binding properties of this secreted form of NSP4 to further understand the biochemical basis for its enterotoxic function. We show here that NSP4 is secreted as discrete detergent-sensitive oligomers in a complex with phospholipids and demonstrate that this secreted form of NSP4 can bind to glycosaminoglycans present on the surface of a range of different cell types.

Methods

NSP4 was purified from the medium of infected cells after ultracentrifugation and ultrafiltration by successive lectin-affinity and ion exchange chromatography. Oligomerisation of NSP4 was examined by density gradient centrifugation and chemical crosslinking and the lipid content was assessed by analytical thin layer chromatography and flame ionization detection. Binding of NSP4 to various cell lines was measured using a flow cytometric-based assay.

Results

Secreted NSP4 formed oligomers that contained phospholipid but dissociated to a dimeric species in the presence of non-ionic detergent. The purified glycoprotein binds to the surface of various non-infected cells of distinct lineage. Binding of NSP4 to HT-29, a cell line of intestinal origin, is saturable and independent of divalent cations. Complementary biochemical approaches reveal that NSP4 binds to sulfated glycosaminoglycans on the plasma membrane.

Conclusion

Our study is the first to analyze an authentic (i.e. non-recombinant) form of NSP4 that is secreted from virus-infected cells. Despite retention of the transmembrane domain, secreted NSP4 remains soluble in an aqueous environment as an oligomeric lipoprotein that can bind to various cell types via an interaction with glycosaminoglycans. This broad cellular tropism exhibited by NSP4 may have implications for the pathophysiology of rotavirus disease.

Conformational Differences Unfold a Wide Range of Enterotoxigenic Abilities Exhibited by rNSP4 Peptides from Different Rotavirus Strains

NSP4 has been recognized as the rotavirus-encoded enterotoxin. However, a few studies failed to support its diarrheagenic activity. As recombinant NSP4 (rNSP4) peptides of different lengths were used in the limited number of studies, a comparison of relative diarrheagenic potential of NSP4 from different strains could not be possible. To better understand the diarrheagenic potential of NSP4 from different strains, in this report we have evaluated the enterotoxigenic activity of the deletion mutant ΔN72 that lacks the N-terminal 72 residues and the biologically relevant ΔN112 peptide which when derived from SA11 rotavirus strain were previously shown to be highly diarrheagenic in newborn mice. Detailed comparative analysis of biochemical and biophysical properties and diarrheagenic activity of the recombinant ΔN72 peptides from seventeen different strains under identical conditions revealed wide differences among themselves in their resistance to trypsin cleavage, thioflavin T (ThT) binding, multimerization and conformation without any correlation with their diarrhea inducing abilities. These results support our previously proposed concept for the requirement of a unique conformation for optimal biological functions conferred by cooperation between the N- and C-terminal regions of the cytoplasmic tail.

The flexible C terminus of the rotavirus non-structural protein NSP4 is an important determinant of its biological properties

The rotavirus non-structural protein NSP4 functions as the viral enterotoxin and intracellular receptor for the double-layered particles (DLP). The full-length protein cannot be expressed and/or purified to homogeneity from bacterial or insect cells. However, a bacterially expressed and purified mutant lacking the N-terminal 72 aa (DeltaN72) was recently obtained from strains Hg18 and SA11 exhibiting approximately 17-20-, 150-200- and 13166-15800-fold lower DD50 (50% diarrhoea-inducing dose) values in suckling mice compared with that reported for the partially pure, full-length protein, a C-terminal M175I mutant and a synthetic peptide comprising aa 114-135, respectively, suggesting the requirement for a unique conformation for optimal functions of the purified protein. The stretch of approximately 40 aa from the C terminus of the cytoplasmic tail of the endoplasmic reticulum-anchored NSP4 is highly flexible and exhibits high sequence variation compared with the other regions, the significance of which in diarrhoea induction remain unresolved. Here, it was shown that every amino acid substitution or deletion in the flexible C terminus resulted in altered conformation, multimerization, trypsin resistance and thioflavin T (ThT) binding, and affected DLP binding and the diarrhoea-inducing ability of the highly diarrhoeagenic SA11 and Hg18 DeltaN72 in suckling mice. These studies further revealed that high ThT fluorescence correlated with efficient diarrhoea induction, suggesting the importance of an optimal ThT-recognizable conformation in diarrhoea induction by purified NSP4. These results based on biological properties provide a possible conformational basis for understanding the influence of primary sequence variations on diarrhoea induction in newborn mice by purified NSP4s that cannot be explained by extensive sequence analyses.

Design of a multiplex nested PCR for genotyping of the NSP4 from group A rotavirus

A novel PCR method was developed to discriminate amongst genotypes A-C of the rotavirus non-structural protein 4 (NSP4). Genotype-specific primers were designed that correctly identified the NSP4 genotype when evaluated as a multiplex PCR with cell culture adapted rotavirus strains. Rotavirus strains B223 SGIG6P6[1], NCDV SGIG6P6[1] and SA11 SGIG3P5B[2] were used as control for NSP4 genotype A; A34 SGIG5P14[23], Gottfried SGIIG4P2B[6] and Wa SGIIG1P1A[8] for NSP4 genotype B; RRV SGIG3P5B[3] for NSP4 genotype C. Subsequently, the same set of specific primers was used to genotype a set of 77 Swedish clinical samples. The results showed that all human clinical samples analyzed belong to the NSP4 genotype B and the VP6 subgroup II.

Epitope mapping and use of epitope-specific antisera to characterize the VP5* binding site in rotavirus SA11 NSP4

Rotavirus (RV) is the leading cause of infantile gastroenteritis worldwide. RV nonstructural protein 4 (NSP4), the first characterized viral enterotoxin, is a 28-kDa glycoprotein that has pleiotropic functions in RV infection and pathogenesis. NSP4 has multiple forms enabling it to perform its different functions. Dissecting such functions could be facilitated by use of epitope-specific antibodies. This work mapped the epitopes for the monoclonal antibody B4-2/55 and three polyclonal antisera generated against synthetic SA11 NSP4 peptides corresponding to residues 114-135, 120-147, and 150-175. The epitope for B4-2/55 mapped to residues 100-118, wherein residues E105, R108 and E111 are critical for antibody binding. Antiserum generated to two peptides (aa114-135 and aa120-147) with enterotoxin activity each recognize a single but distinct epitope. The epitope for the peptide antiserum to aa114-135 was mapped to residues 114-125 with highly conserved residues T117/T118, E120, and E122 being critical for antibody binding. The peptide antiserum to aa120-147 binds to NSP4 at residues 130-140 and residues Q137-T138 are critical for this epitope. Finally, the epitope for the antiserum to peptide aa150-175 mapped to residues 155-170, wherein residues E160 and E170 are critical for antibody binding. Knowledge of the binding sites of domain-specific antibodies can aid in further characterizing different functions of NSP4. To demonstrate this, we characterized the interaction between NSP4 and VP5() [K(D)=0.47 microM] and show that binding of NSP4 to VP5* is blocked by antibody to NSP4 aa114-135 and aa120-147, but not aa150-175. The use of single epitope-specific antibodies to differentially block functions of NSP4 is a feasible approach to determine the functional domain structure of this important RV virulence factor.

Transient expression of HPV16 E7 peptide (aa 44-60) and HPV16 L2 peptide (aa 108-120) on chimeric potyvirus-like particles using Potato virus X-based vector

The optimized expression of recombinant Potato virus A coat protein (ACP) carrying two different epitopes from Human papillomavirus type 16 (HPV16) was developed. Epitope derived from minor capsid protein L2 was expressed as N-terminal fusion with ACP while an epitope derived from E7 oncoprotein was fused to its C-terminus. The construct was cloned into Potato X potexvirus (PVX) based vector and transiently expressed in plants using Agrobacterium tumefaciens mediated inoculation. To increase the level of expressed protein the transgenic Nicotiana benthamiana plants expressing Potato virus A HC-Pro gene and transgenic Nicotiana tabacum, cv. Petit Havana SR1 carrying Potato virus A P3 protein gene were tested. Synergistic infection of host plants with PVX carrying the construct and Potato virus Y(O) (PVY(O)) increased the expression of L2ACPE7 in N. tabacum and in transgenic N. benthamiana carrying potyviral HC-Pro gene as compared to control plants infected with L2ACPE7 only.

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.

The rotavirus enterotoxin NSP4 directly interacts with the caveolar structural protein caveolin-1

Rotavirus nonstructural protein 4 (NSP4) is known to function as an intracellular receptor at the endoplasmic reticulum (ER) critical to viral morphogenesis and is the first characterized viral enterotoxin. Exogenously added NSP4 induces diarrhea in rodent pups and stimulates secretory chloride currents across intestinal segments as measured in Ussing chambers. Circular dichroism studies further reveal that intact NSP4 and the enterotoxic peptide (NSP4(114-135)) that is located within the extended, C-terminal amphipathic helix preferentially interact with caveola-like model membranes. We now show colocalization of NSP4 and caveolin-1 in NSP4-transfected and rotavirus-infected mammalian cells in reticular structures surrounding the nucleus (likely ER), in the cytosol, and at the cell periphery by laser scanning confocal microscopy. A direct interaction between NSP4 residues 112 to 140 and caveolin-1 was determined by the Pro-Quest yeast two-hybrid system with full-length NSP4 and seven overlapping deletion mutants as bait, caveolin-1 as prey, and vice versa. Coimmunoprecipitation of NSP4-caveolin-1 complexes from rotavirus-infected mammalian cells demonstrated that the interaction occurs during viral infection. Finally, binding of caveolin-1 from mammalian cell lysates to Sepharose-bound, NSP4-specific synthetic peptides confirmed the yeast two-hybrid data and further delineated the binding domain to amino acids 114 to 135. We propose that the association of NSP4 and caveolin-1 contributes to NSP4 intracellular trafficking from the ER to the cell surface and speculate that exogenously added NSP4 stimulates signaling molecules located in caveola microdomains.

N- and C-terminal cooperation in rotavirus enterotoxin: novel mechanism of modulation of the properties of a multifunctional protein by a structurally and functionally overlapping conformational domain

Rotavirus NSP4 is a multifunctional endoplasmic reticulum (ER)-resident nonstructural protein with the N terminus anchored in the ER and about 131 amino acids (aa) of the C-terminal tail (CT) oriented in the cytoplasm. Previous studies showed a peptide spanning aa 114 to 135 to induce diarrhea in newborn mouse pups with the 50% diarrheal dose approximately 100-fold higher than that for the full-length protein, suggesting a role for other regions in the protein in potentiating its diarrhea-inducing ability. In this report, employing a large number of methods and deletion and amino acid substitution mutants, we provide evidence for the cooperation between the extreme C terminus and a putative amphipathic alpha-helix located between aa 73 and 85 (AAH73-85) at the N terminus of DeltaN72, a mutant that lacked the N-terminal 72 aa of nonstructural protein 4 (NSP4) from Hg18 and SA11. Cooperation between the two termini appears to generate a unique conformational state, specifically recognized by thioflavin T, that promoted efficient multimerization of the oligomer into high-molecular-mass soluble complexes and dramatically enhanced resistance against trypsin digestion, enterotoxin activity of the diarrhea-inducing region (DIR), and double-layered particle-binding activity of the protein. Mutations in either the C terminus, AAH73-85, or the DIR resulted in severely compromised biological functions, suggesting that the properties of NSP4 are subject to modulation by a single and/or overlapping highly sensitive conformational domain that appears to encompass the entire CT. Our results provide for the first time, in the absence of a three-dimensional structure, a unique conformation-dependent mechanism for understanding the NSP4-mediated pleiotropic properties including virus virulence and morphogenesis.

Single-chain variable fragment (scFv) antibodies against rotavirus NSP4 enterotoxin generated by phage display

The rotavirus non-structural NSP4 protein causes membrane destabilization as well as an increase in intracellular calcium levels in eukaryotic cells and induces diarrhea in young mice, acting as a viral enterotoxin. In this study the phage display technique was used to generate a panel of single-chain variable fragment (scFv) antibodies specific for the NSP4 protein of the human rotavirus strain Wa from a human semi-synthetic scFv library. After several rounds of panning and selection on NSP4 adsorbed to polystyrene tubes, individual scFv were isolated and characterised by fingerprinting and by sequencing the VH and VL genes. The isolated scFv antibodies specifically recognize NSP4 in enzyme immunoassay and in Western blot. Four truncated forms of the NSP4 protein were constructed which allowed us to map the binding region of the selected scFv antibodies to the C-terminal portion of NSP4. The isolated scFv antibodies constitute valuable tools to analyse the mechanisms of NSP4 functions.

Rotavirus enterotoxin NSP4 binds to the extracellular matrix proteins laminin-beta3 and fibronectin

Rotavirus is the most important cause of viral gastroenteritis and dehydrating diarrhea in young children. Rotavirus nonstructural protein 4 (NSP4) is an enterotoxin that was identified as an important agent in symptomatic rotavirus infection. To identify cellular proteins that interact with NSP4, a two-hybrid technique with Saccharomyces cerevisiae was used. NSP4 cDNA, derived from the human rotavirus strain Wa, was cloned into the yeast shuttle vector pGBKT7. An intestinal cDNA library derived from Caco-2 cells cloned into the yeast shuttle vector pGAD10 was screened for proteins that interact with NSP4. Protein interactions were confirmed in vivo by coimmunoprecipitation and immunohistochemical colocalization. After two-hybrid library screening, we repeatedly isolated cDNAs encoding the extracellular matrix (ECM) protein laminin-beta3 (amino acids [aa] 274 to 878) and a cDNA encoding the ECM protein fibronectin (aa 1755 to 1884). Using deletion mutants of NSP4, we mapped the region of interaction with the ECM proteins between aa 87 and 145. Deletion analysis of laminin-beta3 indicated that the region comprising aa 726 to 875 of laminin-beta3 interacts with NSP4. Interaction of NSP4 with either laminin-beta3 or fibronectin was confirmed by coimmunoprecipitation. NSP4 was present in infected enterocytes and in the basement membrane (BM) of infected neonatal mice and colocalized with laminin-beta3, indicating a physiological interaction. In conclusion, two-hybrid screening with NSP4 yielded two potential target proteins, laminin-beta3 and fibronectin, interacting with the enterotoxin NSP4. The release of NSP4 from the basal side of infected epithelial cells and the subsequent binding to ECM proteins localized at the BM may signify a new mechanism by which rotavirus disease is established.

Expression and purification of polyhistidine-tagged rotavirus NSP4 proteins in insect cells

The rotavirus nonstructural NSP4 protein, a transmembrane endoplasmic reticulum-specific glycoprotein, has been described as the first viral enterotoxin. Purified NSP4 or a peptide corresponding to NSP4 residues 114-135 induces diarrhea in young mice. NSP4 has a membrane-destabilizing activity and causes an increase in intracellular calcium levels and chloride secretion by a calcium-dependent signalling pathway in eucaryotic cells. In this study, four recombinant baculoviruses were generated expressing the rotavirus NSP4 glycoprotein from the human strains Wa and Ito, the porcine strain OSU, and the simian strain SA11, which belong to two different NSP4 genotypes, A and B. The recombinant glycoproteins, expressed as polyhistidine-tagged molecules, were analyzed by Western blotting and immunoprecipitation. Newborn mice responded with diarrhea after inoculation with each of the recombinant NSP4 proteins.

Evidence for genetic linkage between the gene segments encoding NSP4 and VP6 proteins in common and reassortant human rotavirus strains

NSP4-encoding genes of 78 human rotavirus strains of common or reassortant genotypes were characterized by reverse transcription-PCR followed by sequencing and phylogenetic analysis. It was found that all the human strains characterized clustered into only two of the five known NSP4 genotypes. Linkage between NSP4 genotypes and VP6 subgroups was 100%, NSP4 genotype A being linked to VP6 of subgroup I (SGI) and NSP4 of genotype B being linked to VP6 of SGII. The diversity among the NSP4- and VP6-encoding genes was significantly less than that among the VP7 and VP4 genes in cocirculating human rotavirus strains. Whereas G and P types appear to be shared among different animal species and humans, the NSP4- and VP6-encoding genes appear to segregate according to their host of origin, suggesting that these two proteins may be host restriction determinants. The NSP4-VP6 association may be structurally determined during rotavirus replication (morphogenesis).

C-terminal domains implicated in the functional surface expression of potassium channels

A short C-terminal domain is required for correct tetrameric assembly in some potassium channels. Here, we show that this domain forms a coiled coil that determines not only the stability but also the selectivity of the multimerization. Synthetic peptides comprising the sequence of this domain in Eag1 and other channels are able to form highly stable tetrameric coiled coils and display selective heteromultimeric interactions. We show that loss of function caused by disruption of this domain in Herg1 can be rescued by introducing the equivalent domain from Eag1, and that this chimeric protein can form heteromultimers with Eag1 while wild-type Erg1 cannot. Additionally, a short endoplasmic reticulum retention sequence closely preceding the coiled coil plays a crucial role for surface expression. Both domains appear to co-operate to form fully functional channels on the cell surface and are a frequent finding in ion channels. Many pathological phenotypes may be attributed to mutations affecting one or both domains.

Sequential analysis of nonstructural protein NSP4s derived from Group A avian rotaviruses

We determined the NSP4 sequences of turkey rotavirus strains Ty-1 and Ty-3 and a chicken rotavirus, strain Ch-1, and compared these sequences with those of a pigeon rotavirus, strain PO-13, and mammalian rotaviruses. The turkey strains and PO-13 were found to be closely related (90-97% homologies). Ch-1 NSP4 was distinctly different from other avian rotavirus NSP4s, with 78-79% homologies. The NSP4 sequences of avian rotaviruses were found to be 6-7 amino acids shorter than those of all mammalian strains and to have considerably low identities (31-37%) with them. Therefore, it seems highly likely that the NSP4 genes of avian rotaviruses are classified into two NSP4 genotypes distinct from those of mammalian rotaviruses. The enterotoxin domain in NSP4 is conserved in terms of its sequential and structural properties despite extremely low homologies in the full lengths of NSP4s in avian and mammalian rotaviruses.

Crystal structure of the oligomerization domain of NSP4 from rotavirus reveals a core metal-binding site

During the maturation of rotaviral particles, non-structural protein 4 (NSP4) plays a critical role in the translocation of the immature capsid into the lumen of the endoplasmic reticulum. Full-length NSP4 and a 22 amino acid peptide (NSP4(114-135)) derived from this protein have been shown to induce diarrhea in young mice in an age-dependent manner, and may therefore be the agent responsible for rotavirally-induced symptoms. We have determined the crystal structure of the oligomerization domain of NSP4 which spans residues 95 to 137 (NSP4(95-137)). NSP4(95-137) self-associates into a parallel, tetrameric coiled-coil, with the hydrophobic core interrupted by three polar layers occupying a and d-heptad positions. Side-chains from two consecutive polar layers, consisting of four Gln123 and two of the four Glu120 residues, coordinate a divalent cation. Two independent structures built from MAD-phased data indicated the presence of a strontium and calcium ion bound at this site, respectively. This metal-binding site appears to play an important role in stabilizing the homo-tetramer, which has implications for the engagement of NSP4 as an enterotoxin.