Retroviral
proteinase(PR)-catalyzed cleavage of the viral Gag and Gag-
Pol polyproteins within the nascent virus particle is required for productive
viral infection. Kinetic characterization and specificity analyses have been reported for several retroviral PR using
oligopeptide substrates. In this study, we performed a comparative analysis of PR from avian, bovine, simian and human retroviruses using
polyproteins of human immunodeficiency virus (HIV) type 1 or avian leukosis virus as substrates.
Polyproteins were derived from immature virus-like particles purified from culture medium of transfected or recombinant baculovirus-infected cells. Specific cleavage to the correct size intermediate and end products occurred in the presence of
detergent and homologous PR. HIV-1 PR cleaved its Gag precursor to completion at a concentration of approximately 25 nM but cleaved the Gag-Pol precursor incompletely even at fourfold higher PR concentration. In contrast to the requirement for high ionic strength for
peptide cleavage reported previously, we found that
Gag protein cleavage by HIV-1 PR proceeded best at low ionic strength, for both of the
protein substrates tested. HIV-2 PR was approximately sixfold less active than HIV-1 PR. PR from avian myeloblastosis-associated virus (MAV) yielded efficient cleavage of the HIV-1
polyprotein only at concentrations above 1 microM. Both
enzymes were stimulated by high
salt and their cleavage products were identical or very similar to those of HIV-1 PR. A mutant of MAV PR engineered to cleave HIV-1
peptide substrates did not cleave the HIV-1
polyprotein at a concentration of 0.4 microM. The PR of Mason Pfizer monkey virus cleaved this
polyprotein very poorly, whereas PR of bovine leukemia virus cleaved it, albeit at different sites.