Polymerization of bacteriophage T4 tail sheath protein mutants truncated at the C-termini

Gene 18 of bacteriophage T4 encodes the contractile protein of the tail she ath. Previous work has shown that the full-length recombinant gene product (gp) 18 of 658 amino acid residues assembles in Escherichia coli cells into a long polysheath structure. However, the gp18 mutants truncated at the N- termini form insoluble aggregates similar to inclusion bodies. In this stud y, six plasmid vectors expressing the recombinant gp18 proteins truncated a t the C-termini have been constructed. The C Delta 58, C Delta 129, C Delta 152, C[g1]72, C Delta 48, and C Delta 287 proteins contain 600, 529, 506, 486, 410, and 371 residues of the full-length gp18 molecule, respectively. All the recombinant proteins were soluble and, except for the C Delta 287 m utant, were assembled into polysheath-related structures. Electron microsco py of negatively stained purified proteins was performed and the resulting images were analyzed by computing their Fourier transforms. The C Delta 58 and C Delta 129 mutants, in addition to forming common contracted-type poly sheath structures, assembled into thinner filaments that we called "noncont racted polysheaths" (NCP). The C Delta 152, C Delta 172, and C Delta 248 pr oteins assembled into the NCP type only. Image processing showed that the N CP filaments significantly differ from both extended sheaths of T4 particle and polysheaths. The structure of the NCP filaments might correspond to th e transitional helices postulated by Moody (J. Mol. Biol., 1973, 80, 613-63 6) that appeared during the process of tail contraction. Our results sugges t that a short region at the C-terminus of the C Delta 129 protein determin es the contractile properties of the gp18 molecule. The shortest, the C Del ta 287 protein, does not assemble into regular structures, thus indicating that a sequence's stretch at the C-end of the C Delta 248 mutant might be r esponsible for polymerization of gp18. (C) 1999 Academic Press.