Mutational analysis of the intramembranous H10 loop of yeast Nhx1 reveals a critical role in ion homoeostasis and vesicle trafficking

Yeast Nhx1 [Na⁺(K⁺)/H⁺ exchanger 1] is an intracellular Na⁺(K⁺)/H⁺ exchanger, localizing to the late endosome where it is important for ion homoeostasis and vesicle trafficking. Phylogenetic analysis of NHE (Na⁺/H⁺ exchanger) sequences has ident-ified orthologous proteins, including HsNHE6 (human NHE6), HsNHE7 and HsNHE9 of unknown physiological role. These appear distinct from well-studied mammalian plasma membrane isoforms (NHE1–NHE5). To explore the differences between plasma membrane and intracellular NHEs and understand the link between ion homoeostasis and vesicle trafficking, we examined the consequence of replacing residues in the intramembranous H10 loop of Nhx1 between transmembrane segments 9 and 10. The critical role for the carboxy group of Glu³⁵⁵ in ion transport is consistent with the invariance of this residue in all NHEs. Surprisingly, residues specifically conserved in the intracellular isoforms (such as Phe³⁵⁷ and Tyr³⁶¹) could not be replaced with closely similar residues (leucine and phenylalanine) found in the plasma membrane isoforms without loss of function, revealing unexpected side chain specificity. The trafficking phenotypes of all Nhx1 mutants, including hygromycin-sensitivity and missorting of carboxypeptidase Y, were found to directly correlate with pH homoeostasis defects and could be proportionately corrected by titration with weak base. The present study demonstrates the importance of the H10 loop of the NHE family, highlights the differences between plasma membrane and intracellular isoforms and shows that trafficking defects are tightly coupled with pH homoeostasis.