Expression cloning of two genes that together mediate organic solute and steroid transport in the liver of a marine vertebrate

Uptake of organic solutes and xenobiotics by mammalian cells is mediated by ATP-independent transporters, and four families of transporters have now b een identified. To search for novel organic solute transporters, a liver cD NA library from an evolutionarily primitive marine vertebrate, the little s kate Raja erinacea, was screened for taurocholate transport activity by usi ng Xenopus laevis oocytes. In contrast to the organic anion transporters id entified to date, a transport activity was identified in this library that required the coexpression of two distinct gene products, termed organic sol ute transporter alpha and beta (Ost alpha, Ost beta). Ost alpha cDNA encode s for a protein of 352 as and seven putative transmembrane (TM) domains. Os t beta contains 182 as and has at least one and perhaps two TM domains. The re is no significant sequence identity between Ost alpha and Ost beta, and only low identity with sequences in the databases; however, Ost alpha bears a resemblance to some G protein-coupled receptors, and Ost beta exhibits 2 2% amino acid identity with the C-terminal TM and intracellular domains of protocadherin-gamma, a cell surface glycoprotein. Xenopus oocytes injected with the cRNA for both Osta and Ost beta, but not each separately, were abl e to take up taurocholate, estrone sulfate, digoxin, and prostaglandin E-2, but not p-aminohippurate or S-dinitrophenyl glutathione. Transport was sod ium-independent, saturable, and inhibited by organic anions and steroids, i ncluding the major skate bile salt, scymnol sulfate. These results identify an organic anion transporter composed of a putative seven-helix TM protein and an ancillary membrane polypeptide.