Mutations of Arg440 and Gly455/Gly456 oppositely change pH sensing of Na+/H+ exchanger 1

To identify important amino acid residues involved in intracellular pH (pH(i)) sensing of Na+/H+ exchanger 1, we produced single-residue substitution mutants in the region of the exchanger encompassing the putative 11 th transmembrane segment (TM11) and its adjacent intracellular (intracellular loop (IL) 5) and extracellular loops (extracellular loop 6). Substitution of Arg(440) in IL5 with other residues except positively charged Lys caused a large shift in pHi dependence of Na-22(+) uptake to an acidic side, whereas substitution of Gly(455) or Gly(456) within the highly conserved glycine-rich sequence of TM11 shifted pHi dependence to an alkaline side. The observed alkaline shift was larger with substitution of Gly(455) with residues with increasing sizes, suggesting the involvement of the steric effect. Interestingly, mutation of Arg440 (114401)) abolished the ATP depletion-induced acidic shift in pHi dependence of Na-22(+) uptake as well as the cytoplasmic alkalinization induced by various extracellular stimuli, whereas with that of Gly455 (G455Q) these functions were preserved. These mutant exchangers did not alter apparent affinities for extracellular transport substrates Na+ and H+ and the inhibitor 5-(N-ethyl-N-isopropyl)amiloride. These results suggest that positive charge at Arg440 is required for normal pHi sensing, whereas mutation-induced perturbation of the TM11 structure may be involved in the effects of Gly mutations. Thus, both Arg(440) in IL5 and Gly residues in the conserved segment of TM11 appear to constitute important elements for proper functioning of the putative "pH(i) sensor" of Na+/H+ exchanger 1.