Structure and function of phage receptor-binding proteins

The host range of a bacteriophage is determined by the spectrum of ligands to which its tail fiber and tailspike proteins can bind. In addition to binding, tailspikes degrade or modify the surface polysaccharides of the bacterial cell thus creating a passageway for the phage to reach the cell surface. Bacteriophages that carry multiple tailspike and tail fibers are thought to have a wider host range than their less endowed relatives. In that aspect, Viunalike bacteriophages (distant relatives of phage T4) are champions with up to six different tailspikes and/or tail fibers per virus particle, several of which form a branched or hand-like structure. However, little is known about the actual host range for any of these bacteriophages. CBA120 is a rare representative of viunalike bacteriophages in that it specifically targets many pathogenic Escherichia coli O157:H7 strains and does not infect common E. coli B and K12 strains. It carries four tailspike proteins (TSP1 through TSP4, orf210-orf213), an additional fiber (orf215) and a VrlC-like tailspike or tail fiber (orf209) on the particle. In this talk, we will discuss the structure and function of TSP2, TSP3 and TSP4 proteins. TSP2, TSP3 and TSP4 are hydrolases that digest the O157, O77 and O78 exopolysaccharides, respectively. TSP2 is responsible for E. coli O157:H7 infection and its crystal structure in complex with the repeating unit of O157 polysaccharide will be presented. We will also discuss the assembly pathway of the TSP1-TSP2-TSP3-TSP4 complex and present data showing that TSP4 is responsible for attachment of the entire complex to the baseplate. We will also discuss the structure, function, and assembly of the TSP proteins of phage G7C. These TSPs contain CBA120-like particle-binding modules but different enzymatic domains.