MOLECULAR-CLONING, EXPRESSION, AND CHROMOSOMAL LOCALIZATION OF 2 ISOFORMS OF THE AE3 ANION-EXCHANGER FROM HUMAN HEART

Cl-/HCO3- exchange contributes to regulation of pH, and [Cl-] in cardi ac muscle, with possible effects on excitability and contractility. We have isolated human heart cDNAs, which encode two isoforms of the ani on 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, disp lays nearly 96% amino acid sequence identity to the rat and mouse AE3 ''brain isoforms.'' The shorter cAE3 polypeptide of 1034 amino acids i n length corresponds to the rat AE3 ''cardiac isoform.'' The unique N- terminal 73 amino acids of the cAE3 sequence are less well conserved b etween rat and human. Northern blot analysis with isoform-specific pro bes revealed the presence of both cAE3 and bAE3 mRNAs in human heart t issue. Both AE3 protein isoforms were overexpressed in Chinese hamster ovary cells and detected by immunoblot with antipeptide antibodies. I mmunoblot studies of human cardiac membranes detected only cAE3 polype ptides, which were apparently not susceptible to enzymatic deglycosyla tion. Injection into Xenopus oocytes of cRNAs encoding either cAE3 or bAE3 produced increased Cl-36(-) uptake into the oocytes, confirming t he ability of both AE3 isoforms to transport Cl-. The human AE3 gene w as localized to chromosome 2. AE3 may provide a new pharmacologic targ et for antiarrhythmic and cardioprotective drugs.