Proteolytic maturation of vaccinia virus core proteins: identification of a conserved motif at the N termini of the 4b and 25K virion proteins.

Three structural proteins (4a, 4b and 25K) located within the virion core of vaccinia virus are cleavage products of precursor polypeptides (P4a, P4b and P25K) synthesized late in viral infection. Pulse-chase labelling experiments revealed that cleavage of the core proteins lags considerably behind precursor synthesis and that processing requires continuous protein synthesis. The N-terminal sequences of 4b and 25K, but not 4a, were determined by microsequencing core proteins isolated from purified virions. Comparison of these data with the predicted amino acid sequence of P4b and P25K revealed a conserved Ala-Gly-Ala motif flanking the apparent N termini of both proteins, as well as several additional sequence similarities between the P4b and P25K precursors both upstream and downstream of the putative cleavage site. The Ala-Gly-Ala tripeptide signal was also found in the same region of the amino acid sequences of the homologous proteins of fowlpox virus.