Expression, assembly, and proteolytic processing of Helminthosporium victoriae 190S totivirus capsid protein in insect cells.

The dsRNA genome (5.2 kbp) of Helminthosporium victoriae 190S totivirus (Hv190SV) consists of two large overlapping open reading frames (ORFs). The 5' proximal ORF codes for the capsid protein (CP) and the 3' ORF codes for an RNA-dependent RNA polymerase. Although the capsid of Hv190SV is encoded by a single gene, it is composed of two major closely related polypeptides, either p88 and p83 or p88 and p78. Whereas p88 and p83 are phosphoproteins, p78 is nonphosphorylated. Expression of the CP ORF in insect cells generated both p78 and p88 which assembled into virus-like particles. The finding that p78, p83, and p88 share a common N-terminal amino acid sequence is consistent with the determination that N-terminal, but not C-terminal, CP deletions were incompetent for assembly. Evidence was obtained that p78 is derived from p88 via proteolytic cleavage at the C-terminus. Proteolytic processing may play a regulatory role in the virus life cycle since it leads to dephosphorylation of CP and a subsequent decrease in virion transcriptional activity.