The assay, purification and properties of vaccinia virus-induced uncoating protein

Vaccinia virus cores prepared in vitro can be uncoated by incubation with extracts of cells infected with vaccinia virus, as determined by the conversion of the genome to DNase susceptibility. This uncoating activity had all the characteristics of the corresponding in vivo activity and of the agent responsible for non-genetic reactivation. Thus, it was not induced by heat-inactivated virus, nor was it produced in the presence of inhibitors of RNA or protein synthesis. The uncoating protein induced by cowpox virus will uncoat vaccinia virus cores. The uncoating protein was concentrated from infected cell extracts by ultrafiltration and purified by gel filtration and ion-exchange and affinity chromatography. It was characterized as a trypsin-like protease with a mol. wt. of 23,050. Cores treated with the purified uncoating protein had an altered sedimentation rate but no differences between treated and untreated cores were detected by electron microscopy or polyacrylamide gel electrophoresis.