CORONAVIRUS POLYPROTEIN PROCESSING

MHV gene 1 contains two ORFs in different reading frames. Translation proceeds through ORF la into ORF 1b via a translational frame-shift. O RF 1a potentially encodes three protease activities, two papain-like a ctivities and one poliovirus 3C-like activity. Of the three predicted activities, only the more amino terminal papain-like domain has been d emonstrated to have protease activity. ORF 1a polypeptides have been d etected in infected cells by the use of antibodies. The order of polyp eptides encoded from the 5' end of the ORF is p28, p65, p290. p290 is processed into p240 and p50. Processing of ORF1a polypeptides differs during cell free translation of genome RNA and in infected cells, sugg esting that different proteases may be active under different conditio ns. Two RNA negative mutants of MHV-A59 express greatly reduced amount s of p28 and p65 at the non-permissive temperature. These mutants may have defects in one or more viral protease activities. ORF 1b, highly conserved between MHV and IBV, potentially contains polymerase, helica se and zinc finger domains. None of these activities have yet been dem onstrated. ORF 1b polypeptides have yet been detected in infected cell s.