Abstract:
The virus-encoded proteins of tobacco etch virus (TEV), a plant potyvirus, arise by proteolytic processing of
a large polyprotein precursor. The TEV genome codes for two proteinases, a 49-kilodalton proteinase and
helper component proteinase (HC-Pro), which cleave the polyprotein at specific sites. The only known cleavage
event catalyzed by HC-Pro occurs at the HC-Pro carboxyl terminus. The proteolytic activity of HC-Pro was
analyzed by expression of the enzyme in bacterial and cell-free systems. The carboxyl-terminal domain of
HC-Pro exhibited proteolytic activity in Escherichia coli with a processing half-time of approximately 100 s.
The processing kinetics of HC-Pro expressed in vitro by cell-free transcription and translation was variable,
depending on the presence or absence of TEV polypeptide sequences at the amino terminus of the proteolytic
domain. Cleavage of the HC-Pro carboxyl terminus appeared to proceed exclusively by an autocatalytic
mechanism; the proteinase synthesized in vitro exhibited little or no proteolytic activity when reacted with the
HC-Pro cleavage site in trans or bimolecular reactions.
