Processing of the tobacco etch virus 49K protease requires autoproteolysis

The final products encoded by the tobacco etch virus genome arise by proteolytic cleavage of a single large polyprotein precursor. Processing of the polyprotein at several sites requires the activity of a viral protease of 49,000 molecular weight (49K). We have examined the excision of the 49K protease from polyproteins translated from defined RNA transcripts. Polyproteins containing an intact 49K protein were efficiently processed after synthesis in a rabbit reticulocyte lysate to yield the 49K product. Introduction of a single amino acid substitution (cysteine to alanine) at the putative active site of the 49K protease abolished processing, indicating that the protease was excised from the polyprotein via an autocatalytic mechanism. Release of the 49K protease was determined to require autoproteolysis, since synthetic polyproteins which contained either or both 49K cleavage sites were processed poorly, if at all, in trans reactions. Protein microsequence analysis revealed that processing in vitro occurred between a glutamine-glycine dipeptide to generate the 49K amino terminus.