EXPRESSION CLONING AND CHARACTERIZATION OF THE HAMSTER ILEAL SODIUM-DEPENDENT BILE-ACID TRANSPORTER

Active uptake of bile acids from the lumen of the small intestine is m ediated hy an ileal Na+-dependent bile acid transport system. To ident ify components of this transport system, an expression cloning strateg y was employed to isolate a hamster ileal cDNA that exhibits bile, aci d transport activity. By Northern blot analysis, mRNA for the cloned t ransporter was readily detected in ileum and kidney but was absent fro m liver and proximal small intestine. The transporter cDNA encoded a 3 48-amino acid protein with seven potential transmembrane domains and t hree possible N-linked glycosylation sites. The amino acid sequence wa s 35% identical and 63% similar to the rat liver Na+/bile acid cotrans porter. After transfection into COS cells, the hamster cDNA transporte d taurocholate in a strict Na+-dependent fashion with an apparent K(m) of 33 muM. This taurocholate transport was inhibited by various bile acids but not by taurine or other organic anions. The Na+ dependence, saturability, and bile acid specificity of transport as well as the ti ssue specificity of mRNA expression strongly argue that the transporte r cDNA characterized in this study is the Na+/bile acid cotransporter described previously in ileum.