EXPRESSION CLONING AND CHARACTERIZATION OF A RENAL ELECTROGENIC NA+ HCO3- COTRANSPORTER/

Bicarbonate transporters are the principal regulators of pH in animal cells, and play a vital role in acid-base movement in the stomach, pan creas, intestine, kidney, reproductive system and central nervous syst em. The functional family of HCO3- transporters includes Cl--HCO3- exc hangers, three Na+/HCO3- cotransporters(1-3), a K+/HCO3- cotransporter (4,5), and a Na+-driven Cl--HCO3- exchanger(6,7). Molecular informatio n is sparse on HCO3- transporters, apart from Cl--HCO3- exchangers ('a nion exchangers'), whose complementary DNAs were cloned several years ago(8-11). Attempts to done other HCO3- transporters, based on binding of inhibitors, protein purification or homology with anion exchangers , have so far been unsuccessful. Here we monitor the intracellular PH and membrane voltage in Xenopus oocytes to follow the expression of th e most electrogenic transporter known: the renal 1:3 electrogenic Na+/ HCO3- cotransporter from the salamander Ambystoma tigrinum. We now rep ort the successful cloning and characterization of a cDNA encoding a c ation-coupled HCO3- transporter. The encoded protein is 1,035 amino ac ids long with several potential membrane-spanning domains. We show tha t when it is expressed in Xenopus oocytes, this protein is electrogeni c, Na+ and HCO3- dependent, and blocked by the anion-transport inhibit or DIDS, and conclude that it is the renal electrogenic sodium bicarbo nate cotransporter (NBC).