Site-directed mutagenesis of Avibirnavirus VP4 gene

Avibirnavirus VP4 Protein Is a Phosphoprotein and Partially Contributes to the Cleavage of Intermediate Precursor VP4-VP3 Polyprotein

Birnavirus-encoded viral protein 4 (VP4) utilizes a Ser/Lys catalytic dyad mechanism to process polyprotein. Here three phosphorylated amino acid residues Ser538, Tyr611 and Thr674 within the VP4 protein of the infectious bursal disease virus (IBDV), a member of the genus Avibirnavirus of the family Birnaviridae, were identified by mass spectrometry. Anti-VP4 monoclonal antibodies finely mapping to phosphorylated (p)Ser538 and the epitope motif ⁵³⁰PVVDGIL⁵³⁶ were generated and verified. Proteomic analysis showed that in IBDV-infected cells the VP4 was distributed mainly in the cytoskeletal fraction and existed with different isoelectric points and several phosphorylation modifications. Phosphorylation of VP4 did not influence the aggregation of VP4 molecules. The proteolytic activity analysis verified that the pTyr611 and pThr674 sites within VP4 are involved in the cleavage of viral intermediate precursor VP4-VP3. This study demonstrates that IBDV-encoded VP4 protein is a unique phosphoprotein and that phosphorylation of Tyr611 and Thr674 of VP4 affects its serine-protease activity.

Crystal structures of yellowtail ascites virus VP4 protease: trapping an internal cleavage site trans acyl-enzyme complex in a native Ser/Lys dyad active site

Yellowtail ascites virus (YAV) is an aquabirnavirus that causes ascites in yellowtail, a fish often used in sushi. Segment A of the YAV genome codes for a polyprotein (pVP2-VP4-VP3), where processing by its own VP4 protease yields the capsid protein precursor pVP2, the ribonucleoprotein-forming VP3, and free VP4. VP4 protease utilizes the rarely observed serine-lysine catalytic dyad mechanism. Here we have confirmed the existence of an internal cleavage site, preceding the VP4/VP3 cleavage site. The resulting C-terminally truncated enzyme (ending at Ala(716)) is active, as shown by a trans full-length VP4 cleavage assay and a fluorometric peptide cleavage assay. We present a crystal structure of a native active site YAV VP4 with the internal cleavage site trapped as trans product complexes and trans acyl-enzyme complexes. The acyl-enzyme complexes confirm directly the role of Ser(633) as the nucleophile. A crystal structure of the lysine general base mutant (K674A) reveals the acyl-enzyme and empty binding site states of VP4, which allows for the observation of structural changes upon substrate or product binding. These snapshots of three different stages in the VP4 protease reaction mechanism will aid in the design of anti-birnavirus compounds, provide insight into previous site-directed mutagenesis results, and contribute to understanding of the serine-lysine dyad protease mechanism. In addition, we have discovered that this protease contains a channel that leads from the enzyme surface (adjacent to the substrate binding groove) to the active site and the deacylating water.

Critical roles of glucocorticoid-induced leucine zipper in infectious bursal disease virus (IBDV)-induced suppression of type I Interferon expression and enhancement of IBDV growth in host cells via interaction with VP4

Infectious bursal disease (IBD) is an acute, highly contagious, and immunosuppressive avian disease caused by IBD virus (IBDV). Although IBDV-induced immunosuppression has been well established, the underlying exact molecular mechanism for such induction is not very clear. We report here the identification of IBDV VP4 as an interferon suppressor by interaction with the glucocorticoid-induced leucine zipper (GILZ) in host cells. We found that VP4 suppressed the expression of type I interferon in HEK293T cells after tumor necrosis factor alpha (TNF-α) treatment or Sendai virus (SeV) infection and in DF-1 cells after poly(I·C) stimulation. In addition, the VP4-induced suppression of type I interferon could be completely abolished by knockdown of GILZ by small interfering RNA (siRNA). Furthermore, knockdown of GILZ significantly inhibited IBDV growth in host cells, and this inhibition could be markedly mitigated by anti-alpha/beta interferon antibodies in the cell cultures (P < 0.001). Thus, VP4-induced suppression of type I interferon is mediated by interaction with GILZ, a protein that appears to inhibit cell responses to viral infection.

Characterization of cleavage sites and protease activity in the polyprotein precursor of Japanese marine aquabirnavirus and expression analysis of generated proteins by a VP4 protease activity in four distinct cell lines

A polyprotein precursor NH(2)-pVP2-VP4-VP3-COOH is encoded in genomic segment A of members of the family Birnaviridae. By N-terminal sequencing analysis, primary cleavage sites of a marine birnavirus (MABV) polyprotein were identified as Ala(508) downward arrow Ser(509) and Ala(734) downward arrow Ser(735), where the cleavage motif was the same as that of infectious pancreatic necrosis virus (IPNV). However, further VP4 and VP3 cleavages occurred at novel sites. Ser(633) and Lys(674) mutations affected the cleavage activity by site-directed mutagenesis. Additional catalytic residues including Ile(543) and Val(686) were MABV-specific. As shown by electron microscopy, pVP2 and further cleaved VP3s (fcVP3s) could not form virus-like particles (VLPs). This suggests that VP3 is necessary for VLP formation. By Western blot analysis of the VP3 expression, fcVP3s were found in RSBK-2 cells and FHM cells, while VP3 was cleaved less in EPC cells, suggesting that fcVP3s might merely be a degraded form. Alternatively, if fcVP3s play functional roles other than in viral assembly, the further VP3 cleavage is, at least, not restricted in FHM cells. Strangely, VP3 was not completely further cleaved in CHSE-214 cells despite the fact that this cell line has a potential proteolytic factor, implying that complicated factors are associated with the further VP3 cleavage.