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

We have used a cDNA probe from a cloned rat liver Na+/taurocholate cot ransporting polypeptide (Ntcp) to screen a human liver cDNA library. A 1,599-bp cDNA clone that encodes a human Na+/taurocholate cotransport ing polypeptide (NTCP) was isolated. The human NTCP consists of 349 am ino acids (calculated molecular mass of 38 kD) and exhibits 77% amino acid homology with the rat Ntcp. In vitro translation experiments indi cate that the protein is glycosylated and has a molecular, weight simi lar to the rat Ntcp. Injection of in vitro transcribed cRNA into Yenop us laevis oocytes resulted in the expression of Na+-dependent taurocho late uptake. Saturation kinetics indicated that the human NTCP has a h igher affinity for taurocholate (apparent K-m = 6 mu M) than the previ ously cloned rat protein (apparent K-m = 25 mu M). NTCP-mediated tauro cholate uptake into oocytes was inhibited by all major bile acid deriv atives (100 mu M), bumetanide (500 mu M), and bromosulphophthalein (10 0 mu M). Southern blot analysis of genomic DNA from a panel of human/h amster somatic cell hybrids mapped the human NTCP gene to chromosome 1 4.