Avian Sarcoma Leukemia Virus Protease Linked to the AdjacentGagPolyprotein Is Enzymatically Active

The activity of avian sarcoma leukemia virus (ASLV) protease (PR) prior to its release from the precursor protein was determined by introducing mutations at the cleavage site between PR and the adjacent upstream nucleocapsid (NC) protein.GagDNA fragments containing these mutations were cloned into expression vectors and introduced intoEscherichia coliin which the ASLV proteins were expressed. The dipeptide NC-PR containing these mutations did not undergo autoprocessing when expressed in bacterial cells and the fused proteins were devoid of enzymatic activity. However, when the wholeGagpolyprotein containing these mutations was expressed in bacterial cells, other PR cleavage sites in the viralGagpolyprotein underwent normal cleavage, indicating that the release of free PR is not a prerequisite for correct processing of the ASLVGagprecursor.