Phage P22 tailspike protein
The tailspike protein of Enterobacteria phage P22 mediates the recognition and adhesion between the bacteriophage and the surface of Salmonella enterica cells. It is anchored within the viral coat and recognizes the O-antigen portion of the lipopolysaccharide on the outer-membrane of Gram-negative bacteria. It possesses endoglycanase activity, serving to shorten the length of the O-antigen during infection.
History
The initial interest in tailspike proteins was in the study of the effect the mutations on protein folding. Some mutations affect the folding efficiency of the protein but have no effect on the final native structure. Other mutations have been identified that lead to a temperature sensitive phenotype. Reconstitution experiments have demonstrated that the in vitro folding process closely mirrors the in vivo folding pathway. It has been further been demonstrated that folding yields in vitro decrease strongly with increasing temperature.Function
O-antigen binding
P22TSP recognizes the O-antigen polysaccharide of LPS serotypes A, B, or D1. The serotypes correspond to species S. Typhimurium, S. Enteritidis, and S. Paratyphi A. These carbohydrates share the same main chain trisaccharide repeating unit alpha-D-mannose--alpha-L-rhamnose--alpha-D-galactose-, but each have a different 2,6-dideoxyhexose substituent at C-3 of the mannose.In vivo, P22TSP binds as a homotrimer and one phage particle can carry up to 6 tailspikes. P22TSP can bind multivalently, leading to an essentially irreversible attachment. It was shown that a minimum of two repeating units or an octasaccharide is required for binding. The TSP is also capable of binding longer fragments with similar affinity.
Endoglycosidase activity
P22TSP has endorhamnosidase activity and cleaves the glycosidic bond of the rhamnose group, producing an octasaccharide product. Two aspartic acids and one glutamic acid in the active site have been strongly linked to enzymatic activity. Different biological functions for this cleavage have been proposed. Cleavage could facilitate access to the membrane. or allow the phage to find the optimal position for infection.Role in DNA injection
It has been demonstrated that cleavage of the O-antigen is necessary for DNA ejection by the phage. It has been proposed that P22TSP binding positions the phage to inject its DNA.Structure
P22TSP is a homotrimeric structural protein consisting of 666 amino acids. It is noncovalently bound to the neck of the viral capsid. It has been crystallized in space group P213 and has one monomer in the asymmetric unit. The secondary structure of P22TSP is dominated by a parallel Beta helix comprising 13 complete turns. This structure is further characterized as a beta-solenoid domain.P22TSP is compOsed of two domains, each with distinct function. An N-terminal domains serves to bind to the phage particle and a C-terminal domains that interacts with the Salmonella surface. These two domains are connected by a flexible linker.
The binding site of P22TSP is located in the central part of the Beta helix. A deep cleft is formed by a 60-residue insertion on one side along with three smaller 5-25 residue insertions on the other.