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J. Biol. Chem., Vol. 280, Issue 48, 39809-39817, December 2, 2005

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The Monocarboxylate Transporter Homolog Mch5p Catalyzes Riboflavin (Vitamin B₂) Uptake in Saccharomyces cerevisiae^*

From the Lehrstuhl für Zellbiologie und Pflanzenphysiologie, Universität Regensburg, Universitätsstrasse 31, Regensburg D-93040, Germany

Riboflavin is a water-soluble vitamin (vitamin B₂) required for the production of the flavin cofactors FMN and FAD. Mammals are unable to synthesize riboflavin and need a dietary supply of the vitamin. Riboflavin transport proteins operating in the plasma membrane thus have an important role in the absorption of the vitamin. However, their sequences remained elusive, and not a single eukaryotic riboflavin transporter is known to date. Here we used a genetic approach to isolate MCH5, a Saccharomyces cerevisiae gene with homology to mammalian monocarboxylate transporters, and characterize the protein as a plasma membrane transporter for riboflavin. This conclusion is based on the suppression of riboflavin biosynthetic mutants (rib mutants) by overexpression of MCH5 and by synthetic growth defects caused by deletion of MCH5 in rib mutants. We also show that cellular processes in multiple compartments are affected by deletion of MCH5 and localize the protein to the plasma membrane. Transport experiments in S. cerevisiae and Schizosaccharomyces pombe cells demonstrate that Mch5p is a high affinity transporter (K_m = 17 µM) with a pH optimum at pH 7.5. Riboflavin uptake is not inhibited by protonophores, does not require metabolic energy, and operates by a facilitated diffusion mechanism. The expression of MCH5 is regulated by the cellular riboflavin content. This indicates that S. cerevisiae has a mechanism to sense riboflavin and avert riboflavin deficiency by increasing the expression of the plasma membrane transporter MCH5. Moreover, the other members of the MCH gene family appear to have unrelated functions.

Received for publication, May 6, 2005 , and in revised form, September 7, 2005.

^* This work was supported in part by Deutsche Forschungsgemeinschaft Grants SFB521/C7 and STO 434/2-1. 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 Supported by a stipend from Regensburg University.

² To whom correspondence should be addressed: Lehrstuhl für Zellbiologie und Pflanzenphysiologie, Universität Regensburg, Universitätsstr. 31, Regensburg D-93040, Germany. Tel.: 49-941-943-3005; Fax: 49-941-943-3352; E-mail: juergen.stolz{at}biologie.uni-regensburg.de.

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