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J. Biol. Chem., Vol. 283, Issue 12, 7379-7389, March 21, 2008

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Characterization of Thi9, a Novel Thiamine (Vitamin B₁) Transporter from Schizosaccharomyces pombe^*

Christian Vogl, Cornelia M. Klein¹, Angelika F. Batke², M. Ernst Schweingruber, and Jürgen Stolz³

From the Lehrstuhl für Zellbiologie und Pflanzenphysiologie, Universität Regensburg, Universitätsstrasse 31, D-93040 Regensburg, Germany and the Institute of Cell Biology, University of Berne, Baltzerstrasse 4, CH-3012 Bern, Switzerland

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, -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.

Received for publication, October 4, 2007 , and in revised form, January 16, 2008.

^* This work was supported by Deutsche Forschungsgemeinschaft Grant STO434/2-1. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ Current address: Max F. Perutz Laboratories, Medizinische Universität Wien, Institut für Medizinische Biochemie, Dr. Bohr-Gasse 9/2, A-1030 Wien, Austria.

² Current address: Lehrstuhl für Genetik, Universität Regensburg, Universitätsstrasse 31, D-93040 Regensburg, Germany.

³ To whom correspondence should be addressed: Lehrstuhl für Ernährungsphysiologie, Technische Universität München, Am Forum 5, D-85350 Freising-Weihenstephan, Germany. Tel.: 49-8161-712359; Fax: 49-8161-713999; E-mail: stolz{at}wzw.tum.de.

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