Second-site suppressor mutations for the Arg70 substitution mutants of the Tn10-encoded metal-tetracycline/H+ antiporter of Escherichia coli

Role of the charge interaction between Arg(70) and Asp(120) in the Tn10-encoded metal-tetracycline/H(+) antiporter of Escherichia coli

We reported that the positive charge of Arg(70) is mandatory for tetracycline transport activity of Tn10-encoded metal-tetracycline/H(+) antiporter (TetA(B)) (Someya, Y., and Yamaguchi, A. (1996) Biochemistry 35, 9385-9391). Arg(70) may function through a charge-pairing with a negatively charged residue in close proximity. Therefore, we mutated Asp(66) and Asp(120), which are only two negatively charged residues located close to Arg(70) in putative secondary structure of TetA(B) and highly conserved throughout transporters of the major facilitator superfamily. Site-directed mutagenesis studies revealed that Asp(66) is essential, but Asp(120) is important for TetA(B) function. Surprisingly, when Asp(120) was replaced by a neutral residue, the R70A mutant recovered tetracycline resistance and transport activity. There was no such effect in the Asp(66) mutation. The charge-exchanged mutant, R70D/D120R, also showed significant drug resistance and transport activity (about 50% of the wild type), although the R70D mutant had absolutely no activity, and the D120R mutant retained very low activity (about 10% of the wild type). Both the R70C and D120C mutants were inactivated by N-ethylmaleimide. Mercuric ion (Hg(2+)), which gives a positive charge to a SH group of a Cys residue through mercaptide formation, had an opposite effect on the R70C and D120C mutants. The activity of the R70C mutant was stimulated by Hg(2+); however, on the contrary, the D120C mutant was partially inhibited. On the other hand, the R70C/D120C double mutant was almost completely inactivated by Hg(2+), probably because the side chains at positions 70 and 120 are bridged with Hg(2+). The close proximity of positions 70 and 120 were confirmed by disulfide cross-linking formation of the R70C/D120C double mutant when it was oxidized by copper-(1,10-phenanthroline). These results indicate that the positive charge of Arg(70) requires the negative charge of Asp(120) for neutralization, probably for properly positioning transmembrane segments in the membrane.

Cysteine-scanning mutagenesis around transmembrane segment VI of Tn10-encoded metal-tetracycline/H(+) antiporter

Each amino acid in putative transmembrane helix VI and its flanking regions, from Ser-156 to Thr-185, of a Cys-free mutant of the Tn10-encoded metal-tetracycline/H(+) antiporter (TetA(B)) was individually replaced by Cys. All of the cysteine-scanning mutants showed a normal level of tetracycline resistance except for the S156C mutant, which showed moderate resistance, indicating that there is no essential residue located in this region. All 20 mutants from S159C to W178C showed no reactivity with N-ethylmaleimide (NEM), whereas the mutants of the flanking regions from S156C to H158C and F179C to T185C were highly or moderately reactive with NEM. These results indicate that like transmembrane helices III and IX, the transmembrane helix VI comprising residues Ser-159-Trp-178 is totally embedded in the hydrophobic environment.

Site-directed chemical modification of cysteine-scanning mutants as to transmembrane segment II and its flanking regions of the Tn10-encoded metal-tetracycline/H+ antiporter reveals a transmembrane water-filled channel

Cysteine-scanning mutants, E32C to G62C, of the metal-tetracycline/H+ antiporter were constructed in order to precisely determine the membrane topology around putative transmembrane segment II. None of the mutants lost the ability to confer tetracycline resistance, indicating that the cysteine mutation in each mutant did not alter the protein conformation. [14C]N-Ethylmaleimide (NEM) binding to these cysteine mutants in isolated membranes was then investigated. The peptide chain of this region passes through the membrane at least once because residues 36 and 65 are exposed on the outside and inside surfaces of the membrane, respectively (Kimura, T., Ohnuma, M., Sawai, T., and Yamaguchi, A. (1997) J. Biol. Chem. 272, 580-585). However, there was no continuous segment in which all of the introduced cysteine residues showed almost no reactivity with [14C]NEM. The proportion of the unbound positions in the second half downstream from position 45 was 55% (10/18), which was clearly higher than that in the first half (21%; 3/14), suggesting that the second half is a transmembrane segment. Positions reactive to NEM appear periodically in the second half. They are located on one side of the helical wheel, suggesting that this side of the transmembrane helix faces a water-filled channel. The cysteine mutants as to the reactive positions in the second half were severely inactivated by NEM except for the P59C mutant, whereas the A40C mutant was the only one inactivated by NEM in the first half. These results suggest that the water-filled channel along this helical region may be a substrate translocation pathway.