title: Structure-based functional study reveals multiple roles of transmembrane segment IX and loop VIII-IX in NhaA Na+/H+ antiporter of Escherichia coli at physiological pH.
abstract: The three-dimensional crystal structure of Escherichia coli NhaA determined at pH 4 provided the first structural insights into the mechanism of antiport and pH regulation of a Na(+)/H(+) antiporter. However, because NhaA is activated at physiological pH (pH 6.5-8.5), many questions pertaining to the active state of NhaA have remained open including the structural and physiological roles of helix IX and its loop VIII-IX. Here we studied this NhaA segment (Glu(241)-Phe(267)) by structure-based biochemical approaches at physiological pH. Cysteine-scanning mutagenesis identified new mutations affecting the pH dependence of NhaA, suggesting their contribution to the "pH sensor." Furthermore mutation F267C reduced the H(+)/Na(+) stoichiometry of the antiporter, and F267C/F344C inactivated the antiporter activity. Tests of accessibility to [2-(trimethylammonium)ethyl]methanethiosulfonate bromide, a membrane-impermeant positively charged SH reagent with a width similar to the diameter of hydrated Na(+), suggested that at physiological pH the cytoplasmic cation funnel is more accessible than at acidic pH. Assaying intermolecular cross-linking in situ between single Cys replacement mutants uncovered the NhaA dimer interface at the cytoplasmic side of the membrane; between Leu(255) and the cytoplasm, many Cys replacements cross-link with various cross-linkers spanning different distances (10-18 A) implying a flexible interface. L255C formed intermolecular S-S bonds, cross-linked only with a 5-A cross-linker, and when chemically modified caused an alkaline shift of 1 pH unit in the pH dependence of NhaA and a 6-fold increase in the apparent K(m) for Na(+) of the exchange activity suggesting a rigid point in the dimer interface critical for NhaA activity and pH regulation.
authors.affiliation: Department of Biological Chemistry, Alexander Silberman Institute of Life Sciences, Hebrew University of Jerusalem, 91904 Jerusalem, Israel.
authors.forename: Tzvi
authors.initials: T
authors.lastname: Tzubery
authors.forename: Abraham
authors.initials: A
authors.lastname: Rimon
authors.forename: Etana
authors.initials: E
authors.lastname: Padan
mesh_terms: D019943:Amino Acid Substitution; D004926:Escherichia coli / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D017354:Point Mutation; D017433:Protein Structure, Secondary / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000502:physiology; D017923:Sodium-Hydrogen Exchangers / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D013329:Structure-Activity Relationship
chemical_list: D029968:Escherichia coli Proteins; C102839:NhaA protein, E coli; D017923:Sodium-Hydrogen Exchangers