The multistep proteolytic maturation pathway utilized by vaccinia virus P4a protein: a degenerate conserved cleavage motif within core proteins.

The most abundant vaccinia virus (VV) core protein found within the virion is protein 4a, which represents approximately 14% of the particle's dry weight. The 4a protein is synthesized as a 102.5-kDa precursor, which is proteolytically processed to a 62-kDa product concomitant with virion assembly. To identify the pathway by which P4a is converted into 4a, immunological reagents which are specific for subregions of the P4a precursor were developed and used in concert with peptide mapping and protein sequencing procedures. The results obtained suggest that the 891 amino acid P4a precursor is cleaved at two locations, between residues 614 and 615 and 697 and 698. Both the large amino-terminal 4a protein (residues 1-614) and the carboxy-terminal-derived 23-kDa protein (residues 698-891) become major virion constituents. The location and fate of the small internal peptide (residues 615-697) is not known. Interestingly, an analysis of the predicted amino acid sequences at the sites of cleavage within the P4a precursor indicated the presence of an Ala-Gly decreases Thr motif flanking the 697-698 site and an Ala-Gly decreases Ser motif flanking the 614-615 site. Since both of these signals are quite similar to the Ala-Gly decreases Ala signal previously identified as the cleavage point within the VV P4b and P25K core protein precursors (VanSlyke et al., 1991.J. Gen. Virol. 72, 411-416), this suggests that processing of all three core protein precursors may be coordinately linked and/or catalyzed by the same proteinase during viral assembly.