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

Background: E colicin proteins have three functional domains, each of which is implicated in one of the stages of killing Escherichia coli cells: rece ptor binding, translocation and cytotoxicity. The central (R) domain is res ponsible for receptor-binding activity whereas the N-terminal (T) domain me diates translocation, the process by which the C-terminal cytotoxic domain is transported from the receptor to the site of its cytotoxicity. The trans location 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 d omain of colicin E9 and TolB, an essential component of the tol-dependent t ranslocation system in E. coli, using the yeast two-hybrid system. The crys tal structure of TolB, a procaryotic tryptophan-aspartate (WD) repeat prote in, reveals an N-terminal alpha+beta domain based on a five-stranded mixed beta sheet and a C-terminal six-bladed beta-propeller domain. Conclusions: The results suggest that the TolB-box residues of the T domain of colicin ES interact with the beta-propeller domain of TolB. The protein -protein interactions of other beta-propeller-containing proteins, the yeas t yPrp4 protein and G proteins, are mediated by the loops or outer sheets o f the propeller blades. The determination of the three-dimensional structur e 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 p rotein-protein interaction of TolB and the T domain of E colicins.