NA-DEPENDENT AND NA+-INDEPENDENT BILE-ACID UPTAKE SYSTEMS IN THE LIVER()

Expression cloning in Xenopus laevis oocytes was used to clone the Na/taurocholate cotransporting polypeptide (Ntcp) and a Na+-independent organic anion transporting polypeptide (oatp) from rat liver. Ntcp is a glycoprotein of 362 amino acids, with a calculated molecular mass of 39 kD. It is predicted to span the membrane 7 times and is only expre ssed in differentiated hepatocytes of mammalian liver. Functionally, i t mediates selective Na+-dependent bile acid uptake. oatp is a glycopr otein of 670 amino acids with a calculated molecular mass of 74 kD whi ch is predicted to have 10 transmembrane domains. Functionally, oatp c an mediate chloride-dependent and -independent bromosulfophthalein (BS P) uptake as well as Na+-independent bile acid transport. Neither Ntcp nor oatp demonstrate any significant homologies between each other an d in comparison to other so far cloned transport proteins.