The sodium-coupled neutral amino acid transporter SNAT2 mediates an anion leak conductance that is differentially inhibited by transported substrates

The sodium-coupled neutral amino acid transporter SNAT2 mediates cellular uptake of glutamine and other small, neutral amino acids. Here, we report the existence of a leak anion pathway associated with SNAT2. The leak anion conductance was increased by, but did not require the presence of, extracellular sodium. The transported substrates L-alanine, L-glutamine, and α-(methylamino)isobutyrate inhibited the anion leak conductance, each with different potency. A transporter with the mutation H-304A did not catalyze alanine transport but still catalyzed anion leak current, demonstrating that substrate transport is not required for anion current inhibition. Both the substrate and Na⁺ were able to bind to the SNAT2_H-304A transporter normally. The selectivity sequence of the SNAT2_H-304A anion conductance was SCN^- ≫ NO₃^- > I^- > Br^- > Cl^- > Mes^-. Anion flux mediated by the more hydrophobic anion SCN^- was not saturable, whereas nitrate flux demonstrated saturation kinetics with an apparent K_m of 29 mM. SNAT2, which belongs to the SLC38 family of transporters, has to be added to the growing number of secondary, Na ⁺-coupled transporters catalyzing substrate-gated or leak anion conductances. Therefore, we can speculate that such anion-conducting pathways are general features of Na⁺-transporting systems.