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

Encased within the 280 kb genome in the capsid of the giant myovirus fKZ 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 fKZ head and the complex inner body. The identification of 39 cleavage sites in 19 fKZ head proteins indicates cleavage of many prohead proteins forms a major morphogenetic step in fKZ head maturation. The fKZ 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 fKZ 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 3565% by cleavages making their mature form similar (2631 kDa), while their precursors are dissimilar (3688 kDa). Together the six abundant proteins sum to the estimated mass of the inner body (1520 MDa). The identification of these proteins is important for future studies on the composition and function of the inner body.