Characterization of a virus-specific proteolytic activity processing the gag precursor of the simian...

The proteolytic processing of the gag precursor polypeptide pr65gag of simian sarcoma-associated virus (SSAV) has been studied in vivo and in vitro. In SSAV-infected cells (i.e., in vivo) proteins of 52 and 38 kDa and the viral protein p30 could be immunoprecipitated with anti-p30 serum. This cleavage pattern is only in part imitated by in vitro cleavage of the isolated pr65gag with avian myeloblastosis virus (AMV) protease p15. However, in vitro incubation of isolated pr65gag with detergent-disrupted SSAV particles generated products identical in size to those found in vivo, i.e., proteins of 52 and 38 kDa and p30. The extent of cleavage is dependent on the concentration of the disrupted virions added to the incubation mixture. Studies with protease inhibitors suggest that the SSAV enzyme is a serine-type protease like that of other mammalian retroviruses and unlike the protease of avian viruses. The SSAV protease activity eluted from a molecular sieve column in a range of about 10-15 kDa reflecting the molecular weight of the murine leukemia virus (MuLV) protease (Mr = 13.5K). Thus, it appears that there is a close similarity between the proteolytic enzymes present in different mammalian retroviruses such as MuLV and SSAV.