Chimeric Na⁺/H⁺ exchangers: An epithelial membrane-bound N-terminal domain requires an epithelial cytoplasmic C-terminal domain for regulation by protein kinases

All cloned members of the mammalian Na⁺/H⁺ exchanger gene family encode proteins that consist of two functionally distinct domains: a membrane-bound N terminus and a cytoplasmic C terminus, which are required for ion transport and regulation of transport, respectively. Despite their similarity in structure, three members of this family, designated NHE1, NHE2, and NHE3, exhibit different kinetic mechanisms in response to growth factors and protein kinases. For instance, growth factors stimulate NHE1 by a change in the affinity constant for intracellular H⁺, K'(H(i)/⁺), and regulate NHE2 and NHE3 by a change in V(max). We have constructed chimeric Na⁺/H⁺ exchangers by exchanging the N and C termini among three cloned rabbit Na⁺/H⁺ exchangers (NHE1 to NHE3) to determine which domain is responsible for the above V(max)-vs.-K'(H(i)/⁺) effect of the Na⁺/H⁺ isoforms. All of the chimeras had functional exchange activity and basal kinetic properties similar to those of wild-type exchangers. Studies with serum showed that the N terminus is responsible for the V(max)-vs.-K'(H(i)/⁺) stimulation of the Na⁺/H^- exchanger isoforms. Moreover, phorbol 12-myristate 13-acetate and fibroblast growth factor altered Na⁺/H⁺ exchange only in chimeras that had an epithelial N-terminal domain Matched with an epithelial C-terminal domain. Therefore, the protein kinase-induced regulation of Na⁺/H⁺ exchangers is mediated through a specific interaction between the N- and C-termini, which is restricted so that epithelial N- and epithelial C-terminal portions of the exchangers are required for regulation.