The Solute Carrier 26 Family of Proteins in Epithelial Ion Transport

Abstract

Transepithelial Cl−and HCO3−transport is critically important for the function of all epithelia and, when altered or ablated, leads to a number of diseases, including cystic fibrosis, congenital chloride diarrhea, deafness, and hypotension ( 78 , 111 , 119 , 126 ). HCO3−is the biological buffer that maintains acid-base balance, thereby preventing metabolic and respiratory acidosis ( 48 ). HCO3−also buffers the pH of the mucosal layers that line all epithelia, protecting them from injury ( 2 ). Being a chaotropic ion, HCO3−is essential for solubilization of ions and macromolecules such as mucins and digestive enzymes in secreted fluids. Most epithelia have a Cl−/HCO3exchange activity in the luminal membrane. The molecular nature of this activity remained a mystery for many years until the discovery of SLC26A3 and the realization that it is a member of a new family of Cl−and HCO3−transporters, the SLC26 family ( 73 , 78 ). This review will highlight structural features, the functional diversity, and several regulatory aspects of the SLC26 transporters.