INTERACTION OF SALMONELLA PHAGE-P22 WITH ITS O-ANTIGEN RECEPTOR STUDIED BY X-RAY CRYSTALLOGRAPHY

The O-antigenic repeating units of the Salmonella cell surface lipopol ysaccharides (serotypes A, B and D1) serve as receptors for phage P22, This initial binding step is mediated by the tailspike protein (TSP), which is present in six copies on the base plate of the phage, In add ition to the binding activity, TSP also displays a low endoglycolytic activity, cleaving the alpha(1,3)-O-glycosidic bond between rhamnose a nd galactose of the O-antigenic repeats, The crystal structure of TSP in complex with receptor fragments allowed to identify the receptor bi nding site for the octasaccharide product of the enzymatic action of T SP on delipidated LPS and the active site consisting of Asp392, Asp395 and Glu359. The structure comprises a large right-handed parallel bet a-helix of 13 turns. These fold independently in the trimer, whereas t he N-terminus forms a cap-like structure and the C-terminal parts of t he three polypeptide strands merge to a single common domain, In addit ion, TSP has served as model system for the folding of large, multisub unit proteins, Its folding pathway is influenced by a large number of point mutations, classified as lethal, temperature sensitive or genera l suppressor mutations, which influence the partitioning between aggre gation and the productive folding pathway.