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 10928 and 11493. Coexpression, deletion, and mutagenesis studies have demonstrated that this protein is encoded by ORF 1a from nucleotide 10915 to 11544 and is released from the 1a polyprotein by the 3C-like proteinase-mediated proteolysis. A previously predicted Q-S (Q(3462)S(3463)) dipeptide bond encoded by the IBV sequence from nucleotide 10912 to 10917 is identified as the N-terminal cleavage site, and a Q-N (Q(3672)N(3673)) dipeptide bond encoded by the IBV sequence between nucleotides 11542 and 11547 is identified as the C-terminal cleavage site of the 24-kDa polypeptide.