The intracellular Na(+)/H(+) exchanger NHE7 effects a Na(+)-coupled, but not K(+)-coupled proton-loading mechanism in endocytosis.

Vesicular H(+)-ATPases and ClC-chloride transporters are described to acidify intracellular compartments, which also express the highly conserved Na(+)/H(+) exchangers NHE6, NHE7, and NHE9. Mutations of these exchangers cause autism-spectrum disorders and neurodegeneration. NHE6, NHE7, and NHE9 are hypothesized to exchange cytosolic K(+) for H(+) and alkalinize vesicles, but this notion has remained untested in K(+) because their intracellular localization prevents functional measurements. Using proton-killing techniques, we selected a cell line that expresses wild-type NHE7 at the plasma membrane, enabling measurement of the exchanger's transport parameters. We found that NHE7 transports Li(+) and Na(+), but not K(+), is nonreversible in physiological conditions and is constitutively activated by cytosolic H(+). Therefore, NHE7 acts as a proton-loading transporter rather than a proton leak. NHE7 mediates an acidification of intracellular vesicles that is additive to that of V-ATPases and that accelerates endocytosis. This study reveals an unexpected function for vesicular Na(+)/H(+) exchangers and provides clues for understanding NHE-linked neurological disorders.