Cloning of the human thiamine transporter, a member of the folate transporter family

We have isolated a cDNA from human placenta, which, when expressed heterolo gously in mammalian cells, mediates the transport of the water-soluble vita min thiamine. The cDNA codes for a protein of 497 amino acids containing 12 putative transmembrane domains, Northern blot analysis indicates that this transporter is widely expressed in human tissues. When expressed in HeLa c ells, the cDNA induces the transport of thiamine (K-i = 2.5 +/- 0.6 mu M) i n a Na+-independent manner, The cDNA-mediated transport of thiamine is stim ulated by an outwardly directed H+ gradient. Substrate specificity assays i ndicate that the transporter is specific to thiamine, Even though thiamine is an organic cation, the cDNA-induced thiamine transport is not inhibited by other organic cations. Similarly, thiamine is not a substrate for the kn own members of mammalian organic cation transporter family. The thiamine tr ansporter gene, located on human chromosome 1q24, consists of 6 exons and i s most likely the gene defective in the metabolic disorder, thiamine-respon sive megaloblastic anemia. At the level of amino acid sequence, the thiamin e transporter is most closely related to the reduced-folate transporter and thus represents member of the folate transporter family.