Cloning and characterization of a Na⁺-driven anion exchanger (NDAE1). A new bicarbonate transporter

Regulation of intra- and extracellular ion activities (e.g. H⁺, Cl^-, Na⁺) is key to normal function of the central nervous system, digestive tract, respiratory tract, and urinary system. With our cloning of an electrogenic Na⁺/HCO₃^- cotransporter (NBC), we found that NBC and the anion exchangers form a bicarbonate transporter superfamily. Functionally three other HCO₃^- transporters are known: a neutral Na⁺/HCO₃^- cotransporter, a K⁺/HCO₃^- cotransporter, and a Na⁺-dependent Cl^--HCO₃^- exchanger. We report the cloning and characterization of a Na⁺-coupled Cl^--HCO₃^- exchanger and a physiologically unique bicarbonate transporter superfamily member. This Drosophila cDNA encodes a 1030-amino acid membrane protein with both sequence homology and predicted topology similar to the anion exchangers and NBCs. The mRNA is expressed throughout Drosophila development and is prominent in the central nervous system. When expressed in Xenopus oocytes, this membrane protein mediates the transport of Cl^-, Na⁺, H⁺, and HCO₃^- but does not require HCO₃^-. Transport is blocked by the stilbene 4,4'-diisothiocyanodihydrostilbene- 2,2'-disulfonates and may not be strictly electroneutral. Our functional data suggest this Na⁺ driven anion exchanger (NDAE1) is responsible for the Na⁺-dependent Cl^--HCO₃^- exchange activity characterized in neurons, kidney, and fibroblasts. NDAE1 may be generally important for fly development, because disruption of this gene is apparently lethal to the Drosophila larva.