Synaptic transmission of glutamatergic or GABAergic synapses benefits from the recycling of glutamate via the glutamate/glutamine cycle between neurons and glial cells. The export of glutamine from glial cells involves the transporter SNAT3 (SLC38A3; Deitmer et al., 2003, J. Neurochem. 87, 127- 135). In this study, we have investigated a possible interaction between SNAT3 and carbonic anhydrase II (CAII), as described for other acid/base-coupled membrane transporters (Sterling et al., 2001, J. Biol. Chem. 276, 47886-94). We heterologously expressed rat SNAT3 in oocytes of Xenopus, and measured membrane current in voltage-clamp, intracellular pH with ion- selective microelectrodes and fluxes of glutamine in the oocytes. There was no enhanced electroneutral transport of amino acid mediated by SNAT3, if CAII was injected or co-expressed in the oocytes. However, the non-stoichiometrical cation conductance associated with SNAT3 was suppressed in CO2/HCO3- buffer by CAII, both at pH 7.4 and pH 7.9. The effect of CAII on the cation conductance was reversed by blocking the catalytic activity of CA by ethoxyzolamide (10 mM). CAII activity in oocytes was confirmed by intracellular pH measurements and by mass spectrometry. Our results reveal a new type of enzymatic interaction of CAII with a transport protein, which results in a modification of its associated conductance.

Supported by the DFG (De 231/16-4) and GRK 845.