The putative monocarboxylate permeases of the yeast Saccharomyces cerevisiae do not transport monocarboxylic acids across the plasma membrane

We have characterized the monocarboxylate permease family of Saccharomyces cerevisiae comprising five proteins. We could not find any evidence that th e monocarboxylate transporter-homologous (Mch) proteins of S. cerevisiae ar e involved in the uptake or secretion of monocarboxylates such as lactate, pyruvate or acetate across the plasma membrane. A yeast mutant strain delet ed for all five MCH genes exhibited no growth defects on monocarboxylic aci ds as the sole carbon and energy sources. Moreover, the uptake and secretio n rates of monocarboxylic acids were indistinguishable from the wildtype st rain. Additional deletion of the JEN1 lactate transporter gene completely b locked uptake of lactate and pyruvate. However, uptake of acetate was not e ven affected after the additional deletion of the gene YHL008c, which had b een proposed to code for an acetate transporter. The mch1-5 mutant strain s howed strongly reduced biomass yields in aerobic glucose-limited chemostat cultures, pointing to the involvement of Mch transporters in mitochondrial metabolism. Indeed, intracellular localization studies indicated that at le ast some of the Mch proteins reside in intracellular membranes. However, py ruvate uptake into isolated mitochondria was not affected in the mch1-5 mut ant strain. It is concluded that the yeast monocarboxylate transporter-homo logous proteins perform other functions than do their mammalian counterpart s. Copyright (C) 2001 John Wiley & Sons, Ltd.