MEMBRANE POTENTIAL-GENERATING MALATE (MLEP) AND CITRATE (CITP) TRANSPORTERS OF LACTIC-ACID BACTERIA ARE HOMOLOGOUS PROTEINS - SUBSTRATE-SPECIFICITY OF THE 2-HYDROXYCARBOXYLATE TRANSPORTER FAMILY

Membrane potential generation via malate/lactate exchange catalyzed by the malate carrier (MleP) of Lactococcus lactis, together with the ge neration of a pH gradient via decarboxylation of malate to lactate in the cytoplasm, is a typical example of a secondary proton motive force -generating system, The mleP gene was cloned, sequenced, and expressed in a malolactic fermentation-deficient L. lactis strain. Functional a nalysis revealed the same properties as observed in membrane vesicles of a malolactic fermentation-positive strain. MleP belongs to a family of secondary transporters in which the citrate carriers from Leuconos toc mesenteroides (CitP) and Klebsiella pneumoniae (CitS) ass; found a lso. CitP, but not CitS, is also involved in membrane potential genera tion via electrogenic citrate/lactate exchange, MleP, CitP, and CitS w ere analyzed for their substrate specificity, The 2-hydroxycarboxylate motif R1R2COHCOOH, common to the physiological substrates, was found to be essential for transport although same 2-oxocarboxylates could be transported to a lesser extent. Clear differences ill substrate speci ficity among the transporters were observed because of different toler ances toward the R substituents at the C2 atom, Both MleP and CitP tra nsport a broad range of 2-hydroxycarboxylates with R substituents rang ing in size from two hydrogen atoms (glycolate) to acetyl and methyl g roups (citromalate) for MleP and two acetyl groups (citrate) for CitP. CitS was much less tolerant and transported only citrate and at a low rats citromalate. The substrate specificities are discussed in the co ntext of the physiological function of the transporters.