Characterization of a second cleavage site and demonstration of activity in trans by the papain-like proteinase of the murine coronavirus mouse hepatitis virus strain A59

The 21.7-kb replicase locus of mouse hepatitis virus strain A59 (MHV-A59) encodes several putative functional domains, including three proteinase domains. Encoded closest to the 5' terminus of this locus is the first papain-like proteinase (PLP-1) (S. C. Baker et al., J. Virol. 67:6056-6063, 1993; H.-J. Lee et al., Virology 180:567-582, 1991). This cysteine proteinase is responsible for the in vitro cleavage of p28, a polypeptide that is also present in MHV-A59-infected cells. Cleavage at a second site was recently reported for this proteinase (P. J. Bonilla et al., Virology 209:489-497, 1995). This new cleavage site maps to the same region as the predicted site of the C terminus of p65, a viral polypeptide detected in infected cells. In this study, microsequencing analysis of the radiolabeled downstream cleavage product and deletion mutagenesis analysis were used to identify the scissile bond of the second cleavage site to between Ala832 and Gly833. The effects of mutations between the P5 and P2' positions on the processing at the second cleavage site were analyzed. Most substitutions at the P4, P3, P2, and P2' positions were permissive for cleavage. With the exceptions of a conservative P1 mutation, Ala832Gly, and a conservative P5 mutation, Arg828Lys, substitutions at the P5, P1, and P1' positions severely diminished second-site proteolysis. Mutants in which the p28 cleavage site (Gly247 / Val248) was replaced by the Ala832 / Gly833 cleavage site and vice versa were found to retain processing activity. Contrary to previous reports, we determined that the PLP-1 has the ability to process in trans at either the p28 site or both cleavage sites, depending on the choice of substrate. The results from this study suggest a greater role by the PLP-1 in the processing of the replicase locus in vivo.