Identification of a novel voltage-driven organic anion transporter present at apical membrane of renal proximal tubule

A novel transport protein with the properties of voltage-driven organic anion transport was isolated from pig kidney cortex by expression cloning in Xenopus laevis oocytes. A cDNA library was constructed from size-fractionated poly(A)⁺ RNA and screened for p-aminohippurate (PAH) transport in high potassium medium. A 1856-base pair cDNA encoding a 467-amino acid peptide designated as OAT_v1 (voltage-driven organic anion transporter 1) was isolated. The predicted amino acid sequence of OAT_v1 exhibited 60-65% identity to those of human, rat, rabbit, and mouse sodium-dependent phosphate cotransporter type 1 (NPT1), although OAT_v1 did not transport phosphate. The homology of this transporter to known members of the organic anion transporter family (OAT family) was about 25-30%. OAT_v1-mediated PAH transport was affected by the changes in membrane potential. The transport was Na⁺-independent and enhanced at high concentrations of extracellular potassium and low concentrations of extracellular chloride. Under the voltage clamp condition, extracellularly applied PAH induced outward currents in oocytes expressing OAT_v1. The current showed steep voltage dependence, consistent with the voltage-driven transport of PAH by OAT_v1. The PAH transport was inhibited by various organic anions but not by organic cations, indicating the multispecific nature of OAT_v1 for anionic compounds. This transport protein is localized at the apical membrane of renal proximal tubule, consistent with the proposed localization of a voltage-driven organic anion transporter. Therefore, it is proposed that OAT_v1 plays an important role to excrete drugs, xenobiotics, and their metabolites driven by membrane voltage through the apical membrane of the tubular epithelial cells into the urine.