Activation of the Mason-Pfizer monkey virus protease within immature capsids in vitro

For all retroviruses, the completion of the viral budding process correlate s with the activation of the viral protease by an unknown mechanism, and, a s the structural (Gag) polyproteins are cleaved by the viral protease, matu ration of the immature virus-like particle into an infectious virion. Unlik e most retroviruses, the Mason-Pfizer monkey virus Gag polyproteins assembl e into immature capsids within the cytoplasm of the cell before the viral b udding event. The results reported here describe a unique experimental syst em in which Mason-Pfizer monkey virus immature capsids are removed from the cell, and the protease is activated in vitro by the addition of a reducing agent. The cleavage of the protease from the precursor form is a primary e vent, which proceeds with a half time of 14 min, and is followed by authent ic processing of the Gag polyproteins. Activity of the viral protease in vi tro depends on pH, with an increase in catalytic rates at acidic and neutra l pH. The initiation of protease activity within immature capsids in vitro demonstrates that viral protease activity is sensitive to oxidation-reducti on conditions, and that the viral protease can be activated in the absence of viral budding.