The glutamine transporter SNAT3 (SLC38A3) is known to be pH-sensitive with its optimum at extracellular pH of 8.0 (Fei et al., 2000, J. Biol. Chem., 275(31), 23707–17, Bröer et al., 2002, J. Physiol., 539(1), 3–14). The substrate sprectrum of this transporter includes glutamine, asparagine and histidine. In the heterologous expression system of Xenopus laevis oocytes we could show a new type of enzymatic interaction of carbonic anhydrase isoform II (CAII) with the transport protein, which results in a modification of the transport-associated ion conductance in CO₂/HCO₃^--buffered solution (Weise et al., 2007, J. Gen. Physiol., 130, 302–315). Furthermore, we could show that transport-associated conductivity of SNAT3 in CO₂/HCO₃^--buffered solution depends on the organic substrate: Asparagine causes larger inward currents though smaller transport activity than glutamine. Functional studies with other carbonic anhydrases support our findings with CAII: We found a suppression of the ion conductance associated with glutamine transport, mediated by the catalytic action of CAI. Surprisingly, the asparagine-induced associated ion conductance was neither affected by CAII nor by CAI when co-expressed with SNAT3. The 5 His-residues present in CA II that serve as a proton highway during catalysis (Miles et al., 2000, J.Biol.Chem.,274(18),12193–6), are missing in CA I, which is also catalytically less active than CA II. The CAII-mutant H64A, in which the H⁺-shuttling histidine residue at position 64 is exchanged by alanine, but which still has significant catalytic activity, was also able to suppress the glutamine-induced cation conductance, as CAII wild-type did. CAIII, a catalytically less active cytosolic isoform, showed, however, partial suppression of the transport-associated ion conductance of SNAT3 compared to CAII or CAII-H64A. The recovery of CAIII-mediated suppression of conductance by application of CA-inhibitor ethoxyzolamide (EZA) was incomplete, because CAIII appears to be largely EZA-insensitive. In summary, SNAT3 interacts with different isoforms of cytosolic CA, and this interaction modulates the transport-associated ion conductance, which appears to be substrate-and CA-isoform-dependent.

Supported by the GRK 845 of the DFG, and the LSP 'Membrantransport'.