The Gag-Pol encoded proteinase of an avian retrovirus expressed in E. coli can produce a novel proteinase (PR + IleGly) that is two amino acids larger at its carboxy-terminal region than the major Gag proteinase (PR)

Abstract

Gag-Pol frameshift translational products of avian retroviruses (e.g., myeloblastosis associated virus, MAV) contain a putative proteinase species of 131 amino acids that maps between the NC/PR and the PR/RT processing sites. Expression in Escherichia coli of an in-frame PR precursor that contains the natural NC/PR processing site and is translationally terminated at the PR/RT site leads to formation of a Gag-Pol proteinase of the expected molecular size (131 amino acids) and a novel PR product of 126 amino acids. This product extends 2 amino acids downstream of the gag-encoded 124 amino acids, and its proteolytic cleavage is promoted by conditions favorable for enzyme catalysis, is blocked by a specific MAV proteinase inhibitor, and can be demonstrated also for corresponding peptide substrates. The new self-processing cleavage product is termed PR(+IleGly) and exhibits similar, but slower, catalytic parameters than those of the Gag PR.