Heat cleavage of bacteriophage T4 gene 23 product produces two peptides previously identified as head proteins.

During studies on the intracellular protein pools of bacteriophage T4, we found that amber mutants in gene 23 blocked the synthesis of a 20-kilodalton (kDa) protein. Radiolabeled amino acid pulses showed that the protein appears at 8 min postinfection with kinetics similar to those of other major late species. Pulse-chase experiments demonstrated that the 20-kDa protein behaves like a primary product and also revealed a 29-kDa protein which, like other proteins cleaved during head assembly, appeared only after a long chase. Both species have been identified as constituents of the T4 head and have resisted previous efforts to identify their genetic origin. The dependence of the 20- and 29-kDa head proteins on the presence of gene 23 protein (gp23) and the observation that the sum of their masses equalled that of mature cleaved gp23 suggested that these two proteins were derived from this major capsid species. Evidence is presented demonstrating that heating samples before electrophoresis causes peptide bond cleavages in gp23, leading to the formation of the two peptides. As predicted by the results of Rittenhouse and Marcus (Anal. Biochem. 138:442-448, 1984), the cleavage occurs at Asp-336-Pro-337 and at two other Asp-Pro sites. Limited heat-induced proteolysis followed by two-dimensional gel analysis provided a peptide map of gp23 useful in the characterization of its assembly-related cleavages.