Functional identity of a purified proximal tubule anion exchanger protein: Mediation of chloride/formate and chloride/bicarbonate exchange

Based on the transport activities and inhibitor sensitivities, different functional modes of anion exchangers have been identified in the kidney proximal tubule including chloride/formate, chloride/oxalate, chloride/hydroxyl, and chloride/bicarbonate exchange. There is little information on the molecular structure and properties of the protein(s) involved in these processes. Previously, using stilbene affinity matrix and Pac Q chromatography, we partially purified a protein with anion exchange properties in brush border membranes (BBM) isolated from rabbit kidney proximal tubules. This protein has a molecular weight of 162 kDa. When reconstituted into liposomes. the fraction containing the 162 kDa protein demonstrated Cl-/Cl- exchange activity. In the current experiments, the 162 kDa protein was purified to homogeneity using a combination of affinity, ion exchange, and size exclusion chromatography. This protein has binding affinity for known inhibitors of anion exchangers. When reconstituted in liposomes, the 162 kDa protein showed anion exchange activity as assayed by Cl-36(-)/Cl- exchange. Functional studies in liposomes reconsti tuted with the purified 162 kDa protein revealed that this protein mediates the transport of Cl-/formate and Cl-/HCO3-. The Cl-/formate and Cl-/HCO3- exchange activities in the reconstituted liposomes were inhibited in the presence of DIDS and furosemide, two known inhibitors of renal anion exchangers. We conclude that Cl-/formate exchange and and Cl-/HCO3- exchange in kidney proximal tubules are mediated via the same protein. This protein is distinct from the known anion exchanger proteins (AE1, AE2, and AE3) and map represent another isoform from this family of transporters.