CLONING AND FUNCTIONAL EXPRESSION OF A COMPLEMENTARY-DNA ENCODING A MAMMALIAN NUCLEOSIDE TRANSPORT PROTEIN

Expression screening in Xenopus oocytes was used to isolate a cDNA fro m rat jejunal epithelium encoding a Na+-dependent nucleoside transport protein (named cNT1). The cDNA sequence of cNT1 predicts a protein of 648 amino acids (relative molecular mass 71,000) with 14 potential tr ansmembrane domains. Data base searches indicate significant sequence similarity to the NUPC proton/nucleoside symporter of Escherichia coli , There is no sequence similarity between cNT1 and proteins of mammali an origin. Functionally, cNT1 exhibited the transport characteristics of the nucleoside transport system cit (selective for pyrimidine nucle osides and adenosine) and accepted both 3'-azido-3'-deoxythymidine (AZ T) and 2',3'-dideoxycytidine (ddC) as permeants (K-m = 0.49 and 0.51 m M, respectively). The demonstration of transport of AZT by cNT1 expres sed in Xenopus oocytes provides the first direct evidence that AZT ent ers cells by transporter-mediated processes, as well as by passive dif fusion. Consistent with the tissue distribution of system cit transpor t activity, transcripts for cNT1 were detected in kidney as well as je junum. cNT1 therefore belongs to a potential new gene family and may b e involved in the intestinal absorption and renal handling of pyrimidi ne nucleoside analogs used to treat acquired immunodeficiency syndrome (AIDS).