Characterization of Self-Processing Activities and Substrate Specificities of Porcine Torovirus 3C-Like Protease

Currently, the active-site residues and substrate specificities of 3C-like protease (3CL pro ) differ among nidoviruses, and the detailed catalytic mechanism remains largely unknown. Here, porcine torovirus (PToV) 3CL pro cleaves 12 sites in the polyproteins, including its N- and C-terminal self-processing sites. Unlike coronaviruses and arteriviruses, PToV 3CL pro employed His53 and Ser160 as the active-site residues that recognize a glutamine (Gln) at the P1 position. Surprisingly, mutations of P1-Gln impaired the C-terminal self-processing but did not affect N-terminal self-processing. The “noncanonical” substrate specificity for its N-terminal self-processing was attributed to the phenylalanine (Phe) residue at the P4 position in the N-terminal site. Furthermore, a double glycine (neutral) substitution at the putative P4-Phe-binding residues (P62G/L185G) abolished the cleavage activity of PToV 3CL pro suggested the potential hydrophobic force between the PToV 3CL pro and P4-Phe side chains.