Molecular cloning, primary structure, and characterization of two members of the mammalian electroneutral sodium-(potassium)-chloride cotransporter family expressed in kidney

Electrically silent Na⁺-(K⁺)-Cl^- transporter systems are present in a wide variety of cells and serve diverse physiological functions. In chloride secretory and absorbing epithelia, these cotransporters provide the chloride entry mechanism crucial for transcellular chloride transport. We have isolated cDNAs encoding the two major electroneutral sodium-chloride transporters present in the mammalian kidney, the bumetanide-sensitive Na⁺- K⁺-Cl^- symporter and thiazide-sensitive Na⁺-Cl^- cotransporter, and have characterized their functional activity in Xenopus laevis oocytes. Despite their differing sensitivities to bumetanide and thiazides and their different requirements for potassium, these ~115-kDa proteins share significant sequence similarity (~60%) and exhibit a topology featuring 12 potential membrane-spanning helices flanked by long non-hydrophobic domains at the NH₂ and COOH termini. Northern blot analysis and in situ hybridization indicate that these transporters are expressed predominantly in kidney with an intrarenal distribution consistent with their recognized functional localization. These proteins establish a new family of Na⁺-(K⁺)-Cl^- cotransporters.