SLC19A3 encodes a second thiamine transporter ThTr2

Recently, a new family of facilitative carriers has been cloned consisting of the reduced folate (SLC19A1) and the thiamine (SLC19A2) transporters. Despite a high level of sequence identity and similarity there is essentially no functional overlap between these carriers. The former transports folates and the latter thiamine. In this paper we describe the function of SLC19A3, another member of this transporter family most recently cloned, after transient transfection of the cDNA into HeLa cells. Uptake of [³H]thiamine, but not of methotrexate nor folic acid, was enhanced in SLC19A3 transfectants relative to vector control. Similarly, in the transfectants thiamine transport increased with an increase in pH with peak activity at pH ∼7.5. While [³H]thiamine uptake was markedly inhibited by nonlabeled thiamine it was not inhibited by several organic cations in 100-fold excess. Hence this carrier has a high degree of specificity for vitamin B₁. The data indicate that SLC19A3 has the characteristics of SLC19A2 (ThTr1) and represents a second thiamine transporter (ThTr2) in this family of facilitative carriers.