Identification of a 24-kDa polypeptide processed from the coronavirus infectious bronchitis virus 1a polyprotein by the 3C-like proteinase and determination of its cleavage sites.

We report here the identification of a 24-kDa polypeptide in IBV-infected Vero cells by immunoprecipitation with a region-specific antiserum raised in rabbits against the IBV sequence encoded between nucleotides 10,928 and 11,493. Coexpression, deletion, and mutagenesis studies have demonstrated that this protein is encoded by ORF 1a from nucleotide 10,915 to 11,544 and is released from the 1a polyprotein by the 3C-like proteinase-mediated proteolysis. A previously predicted Q-S (Q3462S3463) dipeptide bond encoded by the IBV sequence from nucleotide 10,912 to 10,917 is identified as the N-terminal cleavage site, and a Q-N (Q3672N3673) dipeptide bond encoded by the IBV sequence between nucleotides 11,542 and 11,547 is identified as the C-terminal cleavage site of the 24-kDa polypeptide.