IDENTIFICATION, EXPRESSION, AND PROCESSING OF AN 87-KDA POLYPEPTIDE ENCODED BY ORF 1A OF THE CORONAVIRUS INFECTIOUS-BRONCHITIS VIRUS

Nucleotide sequence analysis has shown previously that the genomic-length mRNA (mRNA 1) of the coronavirus infectious bronchitis virus (IBV) contains two large open reading frames (ORFs), 1a and 1b, with the potential to encode polyproteins of approximately 441 and 300 kDa, respectively. We have characterized the specificity of a set of region-specific antisera raised against the 5'-portion of ORF la by immunoprecipitation of in vitro-synthesized, C-terminally truncated la polypeptides and used these antisera to detect virus-specific proteins in IBV-infected vero cells. Two antisera, which had specificity for IBV sequences from nucleotides 710 to 2079 and 1355 to 2433, respectively, immunoprecipitated a polypeptide of approximately 87 kDa from IBV-infected Vero cells. In vitro translation of ORF la sequence terminating at nucleotide 5763 did not produce this protein unless the in vitro translation products were incubated with Vero cell sin extracts prepared from either IBV-infected or mock-infected Vero cells, However, processing of the 87-kDa protein was also observed when the same region was expressed in Vero cells using the vaccinia virus/T7 expression system. This observation indicates that the 87-kDa polypeptide is encoded within the 5'-most 3000 nucleotides of mRNA I and that it might be cleaved from the la polyprotein by viral and cellular proteinases. (C) 1995 Academic Press, Inc.