Monocarboxylate transporters (MCT; SLC16), of which 14 isoforms have been described, are carriers of high-energy metabolites like lactate and pyruvate (Halestrap & Meredith, 2004, Pflügers Arch 447, 619–28). MCT isoforms 1–4 have been reported to co-transport 1 monocarboxylate and 1 proton in an electroneutral transport mode with different substrate affinities. Carbonic anhydrase (CA) is a ubiquitous enzyme catalysing the equilibration of carbon dioxide, protons and bicarbonate. It has been shown, that catalytic activity of different CA isoforms can support transport activity of acid-/base transporting membrane proteins, an interaction coined 'transport metabolon'. We have recently shown that the transport activity of MCT isoform 1 (MCT1), heterologously expressed in Xenopus oocytes, is supported by carbonic anhydrase II (CAII; Becker, Fecher-Trost, Hirnet, Sültemeyer & Deitmer, 2005, J Biol Chem 280, 39882–9; Becker & Deitmer, 2008, J Biol Chem 283, 21655–67). Remarkably, the support of MCT1 activity by CAII was independent of the catalytic activity of the enzyme, but appears to involve interaction between the C-terminal of MCT1 and the N-terminal of CAII, since either C-terminal truncation of MCT1 (Becker, Fecher-Trost, Hirnet, Sültemeyer & Deitmer, 2005, J Biol Chem 280, 39882–9) or mutation of the potential binding motif at the N-terminal of CAII (Becker & Deitmer, 2008, J Biol Chem 283, 21655–67), as found in the mutant CAII-HEX (Vince & Reithmeier, 2000, Biochemistry 39, 5527–33), suppressed the enhancing effect of CAII on MCT1 transport activity.

In the present study, we have investigated possible interactions of CAII with MCT2 and MCT4 when expressed in Xenopus oocytes. MCT2 transport activity is not affected by CAII. In contrast MCT4 activity is enhanced by CAII similar as MCT1, with the highest augmentation at low extracellular pH and low lactate concentrations. For the interaction with MCT4, catalytic activity of CAII is not required, as both, co-expression of MCT4 with wild-type CAII or the catalytically inactive mutant CAII-V143Y enhanced MCT4 activity. However, an N-terminal binding motif of CAII is essential to increase MCT4 activity, since CA isoform I and the mutant CAII-HEX, both lacking the potential N-terminal binding motif, do not augment lactate transport via MCT4. Our results indicate that the CAII interaction displays a specificity for different MCT isoforms, resulting in a substrate-dependent increase in transport activity of MCT isoform 4 by CAII, but not of MCT isoform 2.

Supported by GRK845 of the DFG, and the LSP 'Membrantransport'.