Asp-285 of the metal-tetracycline/H+ antiporter of Escherichia coli is essential for substrate binding.

The transposon Tn10-encoded metal-tetracycline/H+ antiporter (TetA(B)) was preferentially photolabeled when [3H]tetracycline was irradiated in the presence of energized inverted membrane vesicles containing the TetA protein. The degree of labeling depended on the duration of irradiation and the energization of the membrane. Photolabeling was not observed in vesicles containing the Asp-285 --> Asn mutant TetA protein, indicating that Asp-285 participates in the substrate binding or the step(s) prior to substrate binding.