The Na+/H+ antiporter cytoplasmic domain mediates growth factor signals and controls "H(+)-sensing"

The amiloride-sensitive Na+/H+ exchanger (NHE1 human isoform) is activated in response to diverse mitogenic and oncogenic signals presumably through phosphorylation. To get insight into the activating mechanism, a set of deletion mutants within the C-terminal cytoplasmic domain of NHE1 has been generated. These mutant forms expressed in antiporter-deficient fibroblasts revealed that deletion of the complete cytoplasmic domain (i) preserves amiloride-sensitive Na+/H+ exchange and activation by intracellular H+, (ii) reduces the affinity of the internal "H(+)-modifier site" in a manner mimicked by cellular ATP depletion, and (iii) abolishes growth factor-induced cytoplasmic alkalinization. We conclude that NHE1 can be separated into two distinct functional domains. One is an N-terminal transporter domain (T) that has all the features required to catalyze amiloride-sensitive Na+/H+ exchange with a built-in H(+)-modifier site. The other is a C-terminal cytoplasmic regulatory domain (R) that (i) determines the set point value of the exchanger and (ii) mediates growth factor signals by interacting with the "H(+)-sensor" in a phosphorylation-dependent manner.