Acid/base-coupled transport processes across cell membranes include a variety of carriers, some of which are important for cellular pH regulation or cellular import and export of metabolites, such as lactate, amino acids and others. These transport processes are believed to support energetic supply in nearly all organs, including brain, muscle, and kidney. The monocarboxylate transporter isoforms MCT1 and MCT4 play a crucial role in the shuttling of lactate between glycolytic and oxidative cell types in muscle and brain, and NBCe1 is involved in acid/base regulation and proton buffering in epithelial and glial cells. The catalytic activity of carbonic anhydrase isoform II can support transport activity of some of these acid-/base transporting membrane proteins, an interaction coined 'transport metabolon'. Using heterologous protein expression in oocytes of the frog Xenopus laevis, we have found that CAII enhances transport activity of the H+-coupled MCT1 and 4, and of the electrogenic sodium-bicarbonate cotransporter NBCe1 (Becker & Deitmer 2007, 2008). While the interaction between NBCe1 and CAII requires the catalytic activity of CAII, transport via MCT1 and MCT4 is enhanced by CAII independent of its catalytic activity. Binding and H+ shuttling appear to be crucial for the interaction between CAII and MCT isoforms, since CAI and CAIII and different binding and shuttling mutants of CAII do not enhance lactate transport. In contrast, NBCe1 transport activity is supported also by CAI, CAII, CAIII and binding and shuttling mutants of CAII, but not by the catalytically inactive mutant CAII-V143Y. Our results indicate different modes of enzyme – transporter interaction, pointing to some versatile modulation of transport activity by CA isoforms. Supported by the Deutsche Forschungsgemeinschaft Becker, H.M. & Deitmer, J.W. 2007. J Biol Chem 282, 13508-21 Becker, H.M. & Deitmer, J.W. 2008. J Biol Chem 283, 21655-67