Abstract
The production of functional nidovirus replication-transcription complexes involves extensive proteolytic processing by virus-encoded proteases. In this study, we characterized the viral main protease (M(pro)) of the type species, White bream virus (WBV), of the newly established genus Bafinivirus (order Nidovirales, family Coronaviridae, subfamily Torovirinae). Comparative sequence analysis and mutagenesis data confirmed that the WBV M(pro) is a picornavirus 3C-like serine protease that uses a Ser-His-Asp catalytic triad embedded in a predicted two-β-barrel fold, which is extended by a third domain at its C terminus. Bacterially expressed WBV M(pro) autocatalytically released itself from flanking sequences and was able to mediate proteolytic processing in trans. Using N-terminal sequencing of autoproteolytic processing products we tentatively identified Gln↓(Ala, Thr) as a substrate consensus sequence. Mutagenesis data provided evidence to suggest that two conserved His and Thr residues are part of the S1 subsite of the enzyme's substrate-binding pocket. Interestingly, we observed two N-proximal and two C-proximal autoprocessing sites in the bacterial expression system. The detection of two major forms of M(pro), resulting from processing at two different N-proximal and one C-proximal site, in WBV-infected epithelioma papulosum cyprini cells confirmed the biological relevance of the biochemical data obtained in heterologous expression systems. To our knowledge, the use of alternative M(pro) autoprocessing sites has not been described previously for other nidovirus M(pro) domains. The data presented in this study lend further support to our previous conclusion that bafiniviruses represent a distinct group of viruses that significantly diverged from other phylogenetic clusters of the order Nidovirales.