The NH2-terminal half of the Tn10-specified tetracycline efflux protein TetA contains a dimerization domain.

The 43.1-kDa tetracycline-cation/proton antiporter TetA from Tn10 comprises two equal-sized domains, alpha and beta (amino-terminal and carboxyl-terminal halves, respectively). An inactivating mutation in the alpha domain can complement a mutation on a second polypeptide in the beta domain to restore partial tetracycline resistance in bacterial cells, suggesting that intermolecular interactions permit this transport protein to act as a multimer. In the present studies, multimer formation was examined in mixtures of dodecylmaltoside extracts of membranes from Escherichia coli cells containing different TetA derivatives. TetA, TetA alpha, and TetA beta were each fused genetically to a six-histidine carboxyl-terminal tail. The ability of these fusions, immobilized on a nickel affinity column, to bind wild type TetA or other Tet fusions was determined. An interaction between alpha domains on different polypeptides which resulted in multimerization was seen. The binding was specific for Tet protein and did not occur with other membrane proteins or another polyhistidine fusion protein. No alpha-beta interactions were detected by this method, although they are postulated to occur in the intact cell based on the alpha-beta genetic complementations. A dimeric model for TetA having intermolecular alpha-alpha and alpha-beta interactions is presented.