Proximity of periplasmic loops in the metal-tetracycline/H+ antiporter of Escherichia coli observed on site-directed chemical cross-linking

Our precious study on second-site suppressor mutations of the Tn10-encoded metal-tetracycline/H+ anti-porter suggested that Leu(30) and Ala(354), loca ted in periplasmic loop 1-2 and 11-12, respectively, are conformationally l inked to each other (Kawabe, T., and Yamaguchi, A. (1999) FEES Lett. 457, 1 69-173), To determine the spatial proximity of these two residues, crosslin king gel-shift assays of the L30C/A354C double mutant were performed after the mutant had been oxidized with Cu2+/o-phenanthroline. The results indica ted that Leu(30) and Ala(354) are close to each other but that Gly(62), whi ch is located in cytoplasmic loop 2-3, and Ala(354) are distant from each o ther, as a negative control. Then, a single Cys residue was introduced into each of the six periplasmic loop regions (P1-P6), and eleven double mutant s were constructed. Of these eleven double Cys mutants, the L30C/A354C and L30C/T235C mutants showed a mobility shift on oxidation, indicating that P1 is spatially close to P4 as well as P6, In contrast, the other nine mutant s, L30C/S92C, L30C/S156C, L30C/S296C, S92C/S296C, S92C/T235C, S92C/A354C, S 156C/T235C, S156C/S296C, and S156C/A354C, showed no mobility shift under ox idized conditions on intramolecular cross-linking. The S92C and S296C mutan ts showed dimerization on intermolecular cross-linking, indicating that P2 and P5 are located at the periphery of the helix bundle.