Molecular cloning, expression, and chromosomal localization of two isoforms of the AE3 anion exchanger from human...

Cl-/HCO3- exchange contributes to regulation of pHi and [Cl-] in cardiac muscle, with possible effects on excitability and contractility. We have isolated human heart cDNAs, which encode two isoforms of the anion exchanger AE3. These clones share long portions of common sequence but have different 5' ends encoding distinct amino-terminal amino acid sequences. The longer AE3 polypeptide of 1232 amino acids, bAE3, displays nearly 96% amino acid sequence identity to the rat and mouse AE3 "brain isoforms." The shorter cAE3 polypeptide of 1034 amino acids in length corresponds to the rat AE3 "cardiac isoform." The unique N-terminal 73 amino acids of the cAE3 sequence are less well conserved between rat and human. Northern blot analysis with isoform-specific probes revealed the presence of both cAE3 and bAE3 mRNAs in human heart tissue. Both AE3 protein isoforms were overexpressed in Chinese hamster ovary cells and detected by immunoblot with antipeptide antibodies. Immunoblot studies of human cardiac membranes detected only cAE3 polypeptides, which were apparently not susceptible to enzymatic deglycosylation. Injection into Xenopus oocytes of cRNAs encoding either cAE3 or bAE3 produced increased 36Cl- uptake into the oocytes, confirming the ability of both AE3 isoforms to transport Cl-. The human AE3 gene was localized to chromosome 2. AE3 may provide a new pharmacologic target for antiarrhythmic and cardioprotective drugs.