Monocarboxylate transporters (MCT; SLC16) are carriers of high-energy metabolites like lactate and pyruvate and different isoforms are expressed in a wide range of cells and tissues. MCT isoforms 1-4 have been reported to co-transport 1 monocarboxylate and 1 H⁺. In the brain, MCT1 and MCT4 are expressed in astrocytes, where they have been suggested to be responsible for the export of lactate to provide neurons with the metabolite during increased activity. Carbonic anhydrase (CA) is an ubiquitous enzyme catalysing the equilibration of carbon dioxide, H⁺ and bicarbonate. It has been shown, that CA can support transport activity of acid-/base transporting membrane proteins, an interaction coined 'transport metabolon'. We have recently shown that transport activity of MCT1, heterologously expressed in Xenopus oocytes, is supported by CAII (Becker & Deitmer 2008). Remarkably, this augmentation was independent of the catalytic activity of CAII, but appears to involve interaction between the C-term of MCT1 and the N-term of CAII. In the present study, we have investigated possible interactions of different CAII mutants and CAIII with MCT1 and 4 when expressed in Xenopus oocytes to further elucidate the mechanism of this transport metabolon. Transport activity of both isoforms is enhanced by CAII, independent of CAII catalytic activity, with the highest augmentation at low extracellular pH and low lactate concentrations. Co-expression of MCT1 or 4 with CAIII or CAII-H64A, both missing a histidine at position 64, known to act as intramolecular proton shuttle, suppressed the enhancement of MCT transport activity by CAII. Our results suggest that the interaction between MCT1 / MCT4 and CAII might depend on intramolecular proton shuttle within CAII. However, this transport metabolon is independent of the enzyme´s catalytic activity, but highly sensitive to changes in lactate and proton concentrations.

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Becker, H.M. & Deitmer, J.W. 2008. J Biol Chem 283, 21655-67.