The structure of TolB, an essential component of the tol-dependent translocation system, and its protein–protein interaction with the translocation domain of colicin E9

AbstractBackground: E colicin proteins have three functional domains, each of which is implicated in one of the stages of killing Escherichia coli cells: receptor binding, translocation and cytotoxicity. The central (R) domain is responsible for receptor-binding activity whereas the N-terminal (T) domain mediates translocation, the process by which the C-terminal cytotoxic domain is transported from the receptor to the site of its cytotoxicity. The translocation of enzymatic E colicins like colicin E9 is dependent upon TolB but the details of the process are not known.Results: We have demonstrated a protein–protein interaction between the T domain of colicin E9 and TolB, an essential component of the tol-dependent translocation system in E. coli, using the yeast two-hybrid system. The crystal structure of TolB, a procaryotic tryptophan–aspartate (WD) repeat protein, reveals an N-terminal α+β domain based on a five-stranded mixed β sheet and a C-terminal six-bladed β-propeller domain.Conclusions: The results suggest that the TolB-box residues of the T domain of colicin E9 interact with the β-propeller domain of TolB. The protein–protein interactions of other β-propeller-containing proteins, the yeast yPrp4 protein and G proteins, are mediated by the loops or outer sheets of the propeller blades. The determination of the three-dimensional structure of the T domain–TolB complex and the isolation of mutations in TolB that abolish the interaction with the T domain will reveal fine details of the protein–protein interaction of TolB and the T domain of E colicins