Characterization of rotavirus NSP2/NSP5 interactions and the dynamics of viroplasm formation

Design and selection of an intrabody library produced de-novo for the non-structural protein NSP5 of rotavirus

Intracellular antibodies or intrabodies have great potential in protein knockout strategies for intracellular antigens. We applied the Intracellular Antibody Capture Technology for the direct selection in yeast of a mouse scFv library (V(L)-V(H) format) constructed from animals immunised with recombinant non-structural protein NSP5 of Rotavirus. We selected five different intracellular antibodies (ICAbs), which specifically recognize Delta2, an NSP5 deletion mutant used as bait. The anti-NSP5 ICAbs were well expressed both in yeast and mammalian cells as cytoplasmic or nuclear-tagged forms. By immunofluorescence and co-immunoprecipitation assays we characterised the intracellular interaction of the five anti-NSP5 ICAbs with the co-expressed antigens.

Carboxyl-proximal regions of reovirus nonstructural protein muNS necessary and sufficient for forming factory-like inclusions

Mammalian orthoreoviruses are believed to replicate in distinctive, cytoplasmic inclusion bodies, commonly called viral factories or viroplasms. The viral nonstructural protein muNS has been implicated in forming the matrix of these structures, as well as in recruiting other components to them for putative roles in genome replication and particle assembly. In this study, we sought to identify the regions of muNS that are involved in forming factory-like inclusions in transfected cells in the absence of infection or other viral proteins. Sequences in the carboxyl-terminal one-third of the 721-residue muNS protein were linked to this activity. Deletion of as few as eight residues from the carboxyl terminus of muNS resulted in loss of inclusion formation, suggesting that some portion of these residues is required for the phenotype. A region spanning residues 471 to 721 of muNS was the smallest one shown to be sufficient for forming factory-like inclusions. The region from positions 471 to 721 (471-721 region) includes both of two previously predicted coiled-coil segments in muNS, suggesting that one or both of these segments may also be required for inclusion formation. Deletion of the more amino-terminal one of the two predicted coiled-coil segments from the 471-721 region resulted in loss of the phenotype, although replacement of this segment with Aequorea victoria green fluorescent protein, which is known to weakly dimerize, largely restored inclusion formation. Sequences between the two predicted coiled-coil segments were also required for forming factory-like inclusions, and mutation of either one His residue (His570) or one Cys residue (Cys572) within these sequences disrupted the phenotype. The His and Cys residues are part of a small consensus motif that is conserved across muNS homologs from avian orthoreoviruses and aquareoviruses, suggesting this motif may have a common function in these related viruses. The inclusion-forming 471-721 region of muNS was shown to provide a useful platform for the presentation of peptides for studies of protein-protein association through colocalization to factory-like inclusions in transfected cells.

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.

Effects of intrabodies specific for rotavirus NSP5 during the virus replicative cycle

Intracellular antibodies or intrabodies (ICAbs) have great potential in protein knockout strategies for intracellular antigens. In this study, they have been used to investigate the role of the rotavirus non-structural protein NSP5 in the virus replication cycle. Intracellular antibody-capture technology was used to select single-chain Fv format (scFv) ICAbs against an NSP5 mutant. Five different specific ICAbs were selected and expressed in MA104 cells, in the scFv format, as cytoplasmic- and nuclear-tagged forms. By confocal microscopy, it was found that three of these ICAbs recognized the full-length wild-type NSP5 specifically, forming antigen-specific aggresomes in the cytoplasm of cotransfected cells. Expression of the ICAbs in rotavirus-infected cells largely reduced the assembly of viroplasms and cellular cytopathic effect. Replication of dsRNA was partially inhibited, despite there being no reduction in virus titre. These results demonstrate for the first time a key role for NSP5 during the virus replicative cycle.

Uncoupling substrate and activation functions of rotavirus NSP5: phosphorylation of Ser-67 by casein kinase 1 is essential for hyperphosphorylation

Rotavirus NSP5 is a nonstructural protein that localizes in viroplasms of virus-infected cells. NSP5 interacts with NSP2 and undergoes a complex posttranslational hyperphosphorylation, generating species with reduced PAGE mobility. Here we show that NSP5 operates as an autoregulator of its own phosphorylation as a consequence of two distinct activities of the protein: substrate and activator. We developed an in vivo hyperphosphorylation assay in which two NSP5 mutant constructs are cotransfected. One of them, fused to an 11-aa tag, served as substrate whereas the other was used to map NSP5 domains required for activation. The activation and substrate activity could be uncoupled, demonstrating a hyperphosphorylation process in trans between the activator and substratum. This process involved dimerization of the two components through the 18-aa C-terminal tail. Phosphorylation of Ser-67 within the SDSAS motif (amino acids 63-67) was required to trigger hyperphosphorylation by promoting the activation function. We present evidence of casein kinase 1alpha being the protein kinase responsible for this key step as well as for the consecutive ones leading to NSP5 hyperphosphorylation.