Characterization of a 105-kDa Polypeptide Encoded in Gene 1 of the Human Coronavirus HCV 229E

Gene 1 of the human coronavirus HCV 229E encompasses approximately 20.7 kb and contains two overlapping open reading frames, ORF 1a and ORF 1b. The downstream ORF 1b is expressed by a mechanism involving (−1) ribosomal frameshifting. Translation of mRNA 1, which is thought to be equivalent to the viral genomic RNA, results in the synthesis of two large polyproteins, pp1a and pp1ab. These polyproteins contain motifs characteristic of papain-like and 3C-like proteinases, RNA-dependent RNA polymerases, helicases, and metal-binding proteins. In this study, we have produced pp1ab-specific monoclonal antibodies and have used them to detect an intracellular, 105-kDa viral polypeptide that contains the putative RNA polymerase domain. Furthermore, usingtranscleavage assays with bacterially expressed HCV 229E 3C-like proteinase, we have demonstrated that the 105-kDa polypeptide is released from pp1ab by cleavage at the dipeptide bonds Gln-4068/Ser-4069 and Gln-4995/Ala-4996. These data contribute to the characterization of coronavirus 3C-like proteinase-mediated processing of pp1ab and provide the first identification of an HCV 229E ORF 1ab-encoded polypeptide in virus-infected cells.