NHX

NHX refers to a family of Na+/H+ exchangers, transmembrane transport proteins that mediate the exchange of sodium ions for protons across cellular membranes. Members are found in bacteria, fungi, plants, and animals, with plant NHX proteins playing a prominent role in ion homeostasis and salt tolerance. In plants, NHX proteins are mainly localized to the vacuolar (tonoplast) membrane where they sequester Na+ (and to a lesser extent K+) in the vacuole, contributing to osmotic balance and pH regulation under saline conditions. A subset of NHX family members resides in the endomembrane system, including the Golgi apparatus and endosomes, where they help regulate vesicle trafficking and luminal pH.

NHX proteins function as cation/proton antiporters, using the electrochemical proton gradient generated by H+-ATPases or H+-PPases to drive the antiport of Na+ (and sometimes K+) for H+. They typically contain multiple transmembrane segments with cytosolic N- and C- termini that influence activity and trafficking. Specificity and regulatory properties vary among isoforms and across species.

In plants, NHX activity contributes to salt tolerance by reducing cytosolic Na+ levels and maintaining vacuolar osmotic potential, enabling cells to retain water under stress. Mutants lacking functional NHX proteins often display increased salt sensitivity and altered vacuolar pH, while overexpression in some crops has been investigated as a strategy to improve salinity tolerance, with outcomes that depend on genetic context and environmental conditions.

NHX is part of the CPA2 family of cation/proton antiporters. The best-studied plant NHX proteins are implicated in vacuolar Na+/H+ exchange and endomembrane pH regulation, and ongoing research seeks to clarify isoform-specific roles and interactions with signaling pathways.