Infection of a bacterium by a bacteriophage begins with binding of the phage particle to the bacterial cell surface, and most known bacteriophages utilize surface polysaccharides as receptors. The host range of a bacteriophage is determined by the spectrum of ligands to which its tail fiber or tailspike proteins can bind. Some bacteriophages have multiple tailspikes/tail fibers but the function of these proteins during infection remains poorly understood. The receptor-binding apparatus of phage CBA120 contains four tailspike proteins TSP1 through TSP4 that contain evolutionary related domains as well as at least one additional unrelated tailspike and a tail fiber. CBA120 was shown to infect Escherichia coli O157:H7 (a Shiga toxin–producing pathogenic serotype that can cause hemorrhagic colitis and hemolytic uremic syndrome in humans) and E. coli O78, but the function of its receptor-binding proteins in host specificity has not been understood.
In this work, the structure, the substrate specificity and the enzymatic activity of TSP1, TSP2, TSP3 and TSP4 were studied. Tailspike TSP2 was found to bind the O-specific polysaccharide (O-antigen) of the E. coli O157:H7 lipopolysaccharide, and to cleave its α-L-fucopyranosyl linkages to yield a tetrasaccharide monomer of the repeating unit. This puts forward a mechanistic model in which the phage uses its TSPs to clear its way through the polysaccharide layer in order to reach the cell surface. Treatment of E. coli O157:H7 cells with TSP2 makes them immune to phage CBA120 demonstrating that TSP2 must be a component of the phage particle for the infection to take place. Tailspikes TSP3 and TSP4 are also hydrolases that bind to and digest the O-antigens of E. coli O77 and O78, respectively. TSP1 likely expands the host range of CBA120 to Salmonella enterica. The X-ray crystal structures of TSP2, TSP3 and TSP4, and that of a complex of TSP2 with the E. coli O157-antigen repeating unit were determined and their active sites identified. A series of complex formation experiments was used to establish the architecture and assembly pathway of the hand-shaped TSP1-TSP2-TSP3-TSP4 complex, and it was found to have a similar organization to other multivalent tailspike complexes. The data obtained represent the biochemical basis for possible therapeutic application of TSP proteins from CBA120 and for production of conjugate vaccines against infections caused by enteric bacteria.