Structural studies of the contractile tail sheath protein of bacteriophage T4. 1. Conformational change of the tail sheath upon contraction as probed by differential chemical modification.

Differential chemical modifications of tyrosine residues of the tail sheath protein, gp18, were performed to elucidate the structural change of the tail sheath upon contraction. Tyrosine residues of monomeric gp18, extended tail sheath, and contracted tail sheath were nitrated by tetranitromethane, and the modified tyrosine residues in each state of the sheath protein were identified by peptide mapping and amino acid sequence analyses of the isolated peptides. Of 31 tyrosine residues in gp18 monomer or in the extended sheath, 12 or 13 residues (Tyr63 and/or -73, -225, -254, -270, -304, -455, -460, -493, -532, -535, -569, and -590) were modified. When photo-CIDNP difference spectra were measured with monomeric gp18, two peaks, which are due to highly exposed tyrosine residues on the molecular surface of gp18, were observed. These two peaks disappeared when the monomeric gp18 was nitrated. With contracted sheath, however, only eight tyrosine residues (Tyr225, -254, -270, -455, -460, -493, -532, and -535) were nitrated on the contracted sheath. Chemical modification of cysteine residues by sulfhydryl group specific reagent ABD-F [(4-aminosulfonyl)-7-fluoro-2,1,3-benzoxadiazole] revealed that, among five cysteine residues, Cys377, Cys477, and Cys607 have a sulfhydryl group. Cys402 and Cys406 were modified only under reducing conditions, which strongly suggested the presence of a disulfide bond between these two residues.