Proteolytic processing of the polyprotein encoded by ORF1b of the coronavirus infectious bronchitis virus (IBV)

We present here evidence demonstrating that four previously predicted Q-S(G) cleavage sites, encoded by the IBV sequences from nucleotide 15129 to 15134, 16929 to 16934, 18492 to 18497, and 19506 to 19511, respectively, can be recognised and trans-cleaved by the 3C-like proteinase. Five mature products with sizes of approximately 100 kDa, 65 kDa, 63 kDa, 42 kDa and 35 kDa are released from the ORF1b polyprotein by the 3C-like proteinase-mediated cleavage at these positions. Meanwhile, expression of plasmids containing only the ORF1b region showed no autocleavage of the polyproptein encoded, suggesting that the 3C-like proteinase may be the sole proteinase involved in processing of the Ib polyprotein. These data may therefore represent a complete processing map of the polyprotein encoded by ORF Ib of mRNA1.