Cloning and expression of the Na+/H+ exchanger from Amphiuma RBCs: resemblance to mammalian NHE1

The cDNA encoding the Na+/H+ exchanger (NHE) from Amphiuma erythrocytes was cloned, sequenced, and found to be highly homologous to the human NHE1 iso form (hNHE1), with 79% identity and 89% similarity at the amino acid level. Sequence comparisons with other NHEs indicate that the Amphiuma tridactylu m NHE isoform 1 (atNHE1) is likely to be a phylogenetic progenitor of mamma lian NHE1. The atNHE1 protein, when stably transfected into the NHE-deficie nt AP-1 cell line (37), demonstrates robust Na+-dependent proton transport that is sensitive to amiloride but not to the potent NHE1 inhibitor HOE-694 . Interestingly, chimeric NHE proteins constructed by exchanging the amino and carboxy termini between atNHE1 and hNHE1 exhibited drug sensitivities s imilar to atNHE1. Based on kinetic, sequence, and functional similarities b etween atNHE1 and mammalian NHE1, we propose that the Amphiuma exchanger sh ould prove to be a valuable model for studying the control of pH and volume regulation of mammalian NHE1. However, low sensitivity of atNHE1 to the NH E inhibitor HOE-694 in both native Amphiuma red blood cells (RBCs) and in t ransfected mammalian cells distinguishes this transporter from its mammalia n homologue.