Characterization of Thi9, a novel thiamine (Vitamin B1) transporter from Schizosaccharomyces pombe

Thiamine is an essential component of the human diet and thiamine diphosphate-dependent enzymes play an important role in carbohydrate metabolism in all living cells. Although the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe can derive thiamine from biosynthesis, both are also able to take up thiamine from external sources, leading to the down-regulation of the enzymes involved in its formation. We have isolated the S. pombe thiamine transporter Thi9 by genetic complementation of mutants defective in thiamine biosynthesis and transport. Thi9 localizes to the S. pombe cell surface and works as a high-affinity proton/thiamine symporter. The uptake of thiamine was reduced in the presence of pyrithiamine, oxythiamine, amprolium, and the thiazole part of thiamine, indicating that these compounds are substrates of Thi9. In pyrithiamine-resistant mutants, a conserved glutamate residue close to the first of the 12 transmembrane domains is exchanged by a lysine and this causes aberrant localization of the protein. Thiamine uptake is significantly increased in thiamine-deficient medium and this is associated with an increase in thi9(+) mRNA and protein levels. Upon addition of thiamine, the thi9(+) mRNA becomes undetectable within minutes, whereas the Thi9 protein appears to be stable. The protein is distantly related to transporters for amino acids, gamma-aminobutyric acid and polyamines, and not to any of the known thiamine transporters. We also found that the pyridoxine transporter Bsu1 has a marked contribution to the thiamine uptake activity of S. pombe cells.