Histidine 225, a residue of the NhaA-Na+/H+ antiporter of Escherichia coli is exposed and faces the cell exterior

Cysteine residues were found nonessential in the mechanism of the NhaA antiporter activity of Escherichia coli. The functional C-less NhaA has provided the groundwork to study further histidine 225 of NhaA which has previously been suggested to play an important role in the activation of NhaA at alkaline pH (Rimon, A., Gerchman, Y., Olami, Y., Schuldiner, S. and Padan, E. (1995) J. Biol. Chem. 270, 26813-26817). C-less H225C was constructed and shown to possess an antiporter activity 60% of that of C-less antiporter and a pH profile similar to that of both the C-less or wild-type antiporters. Remarkably, whereas neither the wild-type nor the C-less antiporters were affected by N-ethylmaleimide, C-less H225C was inhibited by this reagent. To determine the degree of alkylation of the antiporter protein by N-ethylmaleimide, antiporter derivatives tagged at their C termini with six histidines residues were constructed. Alkylation of C-less H225C was measured by labeling of everted membrane vesicles with [C-14]N-ethylmaleimide, affinity purification of the His-tagged antiporter, and determination of the radioactivity of the purified protein. This assay showed that H225C is alkylated to a much higher level than any of the native cysteinyl residues of NhaA reaching saturation at alkyl/NhaA stoichiometry of 1. The wild-type derivative showed at least 10-fold less alkylation even at higher concentrations, suggesting that H225C resides in a domain that is much more exposed to N-ethylmaleimide than the native cysteinyl residues of NhaA. Since H225C residues both in right-side out and in side-out membrane vesicles were quantitatively alkylated by N-ethylmaleimide this assay was used to determine the accessibility of H225C to other SH reagents by titrating the H225C left free to react with N-ethylmaleimide, following exposure of the membranes to the reagents. Furthermore, since membrane-impermeant probes can react with residues in membrane-embedded protein only if accessible to the medium containing the reagent, the assay was used to determine the membrane topology of H225C. As expected for a membrane-permeant probe, p-chloromercuribenzoate reacted with H225C as efficiently as N-ethylmaleimide in both membrane orientations. Similar results were obtained with methanethiosulfonate ethylammonium supporting the recent observations that this probe is membrane-permeant. On the other hand, both membrane-impermeant reagents p-chloromercuribenzosulfonate and methanethiosulfonate ethyl-trimethyl ammonium bromide reacted with H225C 10-fold more in right-side out than in inside-out vesicles, and p-chloromercuribenzosulfonate also blocked completely the H225C in intact cells. These results strongly suggest that H225C is exposed at the periplasmic face of the membrane.