Characterization of bacteriophage P22 tailspike mutant proteins with altered endorhamnosidase and capsid assembly activities

The tailspike protein of Salmonella typhimurium phage P22 is a multifunctional homotrimer which is involved in the terminal reaction of phage assembly, the adsorption of the phage to susceptible cells, and the hydrolysis of the Salmonella O-antigen during the first steps of phage infection. The proteins made from 15 mutant tailspike structural genes carried on high level expression plasmids have been analyzed with respect to their in vivo stability, quaternary structure, capsid assembly activity, and enzymatic activity. Nine mutants synthesize tailspike proteins which fail to accumulate to any appreciable level in vivo, and thus these proteins are probably degraded. Four other altered proteins accumulate in vivo as soluble monomers. The remaining two altered proteins accumulate in vivo as stable trimers. Each of these two proteins is defective for at least one of the known functions of the tailspike protein. One is defective in the capsid assembly reaction and shows an unusual quaternary structural defect but is normal with respect to the enzymatic hydrolysis of O-antigen. The other is defective in the enzymatic hydrolysis of O-antigen but is normal with respect to its capsid assembly activity and quaternary structure. The known sequence changes which give rise to these altered proteins and those of previously identified mutants allow the description of possible functional and structural "domains" of this multifunctional protein.