Diverse residues of intracellular loop 5 of the Na+/H+ exchanger modulate proton sensing, expression, activity and targeting.

Abstract The mammalian Na+/H+ exchanger isoform 1 (NHE1) is an integral membrane protein that regulates intracellular pH (pH i) by removing a single intracellular proton in exchange for one extracellular sodium ion. It is involved in cardiac hypertrophy and ischemia reperfusion damage to the heart and elevation of its activity is a trigger for breast cancer metastasis. NHE1 has an extensive 500 amino acid N-terminal membrane domain that mediates transport and consists of 12 transmembrane segments connected by intracellular and extracellular loops. Intracellular loops are hypothesized to modulate the sensitivity to pH i. In this study, we characterized the structure and function of intracellular loop 5 (IL5), specifically amino acids 431–443. Mutation of eleven residues to alanine caused partial or nearly complete inhibition of transport; notably, mutation of residues L432, T433, I436, N437, R440 and K443 demonstrated these residues had critical roles in NHE1 function independent of effects on targeting or expression. The nuclear magnetic resonance (NMR) solution spectra of the IL5 peptide in a membrane mimetic sodium dodecyl sulfate solution revealed that IL5 has a stable three-dimensional structure with substantial alpha helical character. NMR chemical shifts indicated that K438 was in close proximity with W434. Overall, our results show that IL5 is a critical, intracellular loop with a propensity to form an alpha helix, and many residues of this intracellular loop are critical to proton sensing and ion transport. Graphical abstract Unlabelled Image Highlights • Intracellular loop 5 (IL5) of the mammalian Na+/H+ exchanger isoform one was studied. • Mutation of 11 residues to Ala caused partial or near complete inhibition of NHE1. • Residues L432, T433, I436, N437, R440 and K443 had critical roles in activity of NHE1. • IL5 peptide in has a propensity towards alpha helical character. • NMR chemical shifts indicate that K438 is in close proximity with W434. [ABSTRACT FROM AUTHOR]