PHAGE-P22 TAILSPIKE PROTEIN - CRYSTAL-STRUCTURE OF THE HEAD-BINDING DOMAIN AT 2.3 ANGSTROM, FULLY REFINED STRUCTURE OF THE ENDORHAMNOSIDASEAT 1.56 ANGSTROM RESOLUTION, AND THE MOLECULAR-BASIS OF O-ANTIGEN RECOGNITION AND CLEAVAGE

The tailspike protein of Salmonella phage P22 is a viral adhesion prot ein with both receptor binding and destroying activities. It recognise s the O-antigenic repeating units of cell surface lipopolysaccharide o f serogroup A, B and D1 as receptor, but also inactivates its receptor by endoglycosidase (endorhamnosidase) activity. In the final step of bacteriophage P22 assembly six homotrimeric tailspike molecules are no n-covalently attached to the DNA injection apparatus, mediated by thei r N-terminal, head-binding domains. We report the crystal structure of the head-binding domain of P22 tailspike protein at 2.3 Angstrom reso lution, solved with a recombinant telluromethionine derivative and non -crystallographic symmetry averaging. The trimeric dome-like structure is formed by two perpendicular beta-sheets of five and three strands, respectively in each subunit and caps a three-helix bundle observed i n the structure of the C-terminal receptor binding and cleaving fragme nt, reported here after full refinement at 1.56 Angstrom resolution. I n the central part of the receptor binding fragment, three parallel be ta-helices of 13 complete turns are associated side-by-side, while the three polypeptide strands merge into a single domain towards their C termini, with close interdigitation at the junction to the beta-helix part. Complex structures with receptor fragments from S. typhimurium, S. enteritidis and S. typhi253Ty determined at 1.8 Angstrom resolution are described in detail. Insertions into the beta-helix form the O-an tigen binding groove, which also harbours the active site residues Asp 392, Asp395 and Glu359. In the intact structure of the tailspike prote in, head-binding and receptor-binding parts are probably linked by a f lexible hinge whose function may be either to deal with shearing force s on the exposed, 150 Angstrom long tailspikes or to allow them to ben d during the infection process. (C) 1997 Academic Press Limited.