Identification of membrane domains of the Na+/H+ antiporter (NhaA) protein from Helicobacter pylori required for ion transport and pH sensing

The Na+/H+ antiporter from Helicobacter pylori (HP NhaA) is normally active within the pH range 6.0-8.5. In contrast, the NhaA from Escherichia coli (EC NhaA) is active only within the alkaline pH range 7.5-8.5. We studied structures of HP NhaA involved in ion transport and pH sensing by analyzing mutants with defects in NhaA activity. The 36 mutants were classified into three types. The first type exhibited very low or null activity at all pH levels and had amino acid substitutions in the transmembrane segments (TM) 4, 5, 10, and 11, implicating these TMs in ion transport. The second type, which had amino acid substitutions at Met-138, Phe-144, and Lys-347 in TM 4 and 10, exhibited very low antiporter activity at acidic pH but had significantly higher activity at alkaline pH. These results imply that TM 4 (Met-138 and Phe-144) and 10 (Lys-347) are involved in supporting transport activity at acidic pH, in addition to their essential role in the overall transport mechanism. The third type of mutant exhibited very low antiporter activity at alkaline pH but relatively normal activity at acidic pH and had amino acid substitutions in loop 7 (a hydrophilic region between TM 7 and 8) as well as in TM 8, suggesting that these regions are involved in antiporter activation at alkaline pH. Three revertants that suppress a Lys-347 mutation were identified. Two of three suppressor mutations were located in loops 2 and 4, suggesting a functional interaction between these regions (loops 2 and 4 and TM 10). Thus, HP NhaA activity may be modulated by two independent factors that are dependent on pH: an activation mechanism at acidic pH, which is regulated by residues within TM 4 and 10 and another mechanism functioning at alkaline pH regulated by residues within loop 7 and TM 8.