Extensive proteolysis of head and inner body proteins by a morphogenetic protease in the giant Pseudomonas aeruginosa phage φKZ

Summary Encased within the 280 kb genome in the capsid of the giant myovirus φKZ is an unusual cylindrical proteinaceous ‘inner body’ of highly ordered structure. We present here mass spectrometry, bioinformatic and biochemical studies that reveal novel information about the φKZ head and the complex inner body. The identification of 39 cleavage sites in 19 φKZ head proteins indicates cleavage of many prohead proteins forms a major morphogenetic step in φKZ head maturation. The φKZ head protease, gp175, is newly identified here by a bioinformatics approach, as confirmed by a protein expression assay. Gp175 is distantly related to T4 gp21 and recognizes and cleaves head precursors at related but distinct S/A/G‐X‐E recognition sites. Within the φKZ head there are six high‐copy‐number proteins that are probable major components of the inner body. The molecular weights of five of these proteins are reduced 35–65% by cleavages making their mature form similar (26–31 kDa), while their precursors are dissimilar (36–88 kDa). Together the six abundant proteins sum to the estimated mass of the inner body (15–20 MDa). The identification of these proteins is important for future studies on the composition and function of the inner body.