IDENTIFICATION OF A NOVEL MEMBRANE TRANSPORTER ASSOCIATED WITH INTRACELLULAR MEMBRANES BY PHENOTYPIC COMPLEMENTATION IN THE YEAST SACCHAROMYCES-CEREVISIAE

A partial mouse cDNA was isolated by its ability to functionally compl ement a thymidine transport deficiency in plasma membranes of the yeas t, Saccharomyces cerevisiae, The full-length cDNA encoded a previously unidentified 27-kDa protein (mouse transporter protein (MTP)) with fo ur predicted transmembrane-spanning domains, MTP mRNA was detected in cells of several mammalian species, and its predicted protein sequence exhibited near identity (98%) with that of a human cDNA (HUMORF13), M TP and its homologs evidently reside in an intracellular membrane comp artment because a protein (about 24 kDa) that was recognized by MTP-sp ecific antibodies was observed in a subcellular fraction of rat hepato cytes enriched for Gels membranes. Deletion of the hydrophilic C termi nus of MTP, which encompassed two putative signal motifs for intracell ular localization (Tyr-X-X-hydrophobic amino acid), allowed expression of recombinant protein (MTP Delta C) in plasma membranes of Xenopus l aevis oocytes, MTP Delta C-expressing oocytes exhibited greater fragil ity than nonexpressing oocytes, and those that survived the experiment al manipulations were capable of mediated uptake of thymidine, uridine , and adenosine, Thymidine uptake by MTP Delta C-expressing oocytes wa s inhibited by thymine and dTMP. MTP may function in the transport of nucleosides and/or nucleoside derivatives between the cytosol and the lumen of an intracellular membrane-bound compartment.