MOLECULAR-CLONING, CHROMOSOMAL LOCALIZATION, AND FUNCTIONAL-CHARACTERIZATION OF A HUMAN LIVER NA+ BILE-ACID COTRANSPORTER

We have used a cDNA probe from a cloned rat liver Na+/taurocholate cotransporting polypeptide (Ntcp) to screen a human liver cDNA library. A 1,599-bp cDNA clone that encodes a human Na+/taurocholate cotransporting polypeptide (NTCP) was isolated. The human NTCP consists of 349 amino acids (calculated molecular mass of 38 kD) and exhibits 77% amino acid homology with the rat Ntcp. In vitro translation experiments indicate that the protein is glycosylated and has a molecular, weight similar to the rat Ntcp. Injection of in vitro transcribed cRNA into Yenopus laevis oocytes resulted in the expression of Na+-dependent taurocholate uptake. Saturation kinetics indicated that the human NTCP has a higher affinity for taurocholate (apparent K-m = 6 mu M) than the previously cloned rat protein (apparent K-m = 25 mu M). NTCP-mediated taurocholate uptake into oocytes was inhibited by all major bile acid derivatives (100 mu M), bumetanide (500 mu M), and bromosulphophthalein (100 mu M). Southern blot analysis of genomic DNA from a panel of human/hamster somatic cell hybrids mapped the human NTCP gene to chromosome 14.