The glutamine transporter SNAT3 (SN1; SLC38A3) is a major player in the glutamine efflux from astrocytes in the glutamate/glutamine cycle in the brain, and in glutamine uptake into hepatocytes and kidney epithelial cells. We heterologously expressed SNAT3 in frog oocytes, and measured membrane current, pH, and radio-labelled flux of glutamine in oocytes, and used site-directed mutagenesis. In addition to the extablished coupled sodium-glutamine cotransport/proton antiport, we found that there are three conductances associated with SNAT3. Two of these conductances are dependent on the presence of substrate (glutamine, asparagine), and one is independent of the amino acid substrate. At pH 7.4 or lower, the substrate-dependent conductance was carried by cations, mostly Na+, and at pH 8.4 by H+. Site-directed mutagenesis of threonine 380 to alanine suppressed the cation conductance, but not the H+ conductance. A substrate-independent conductance associated with SNAT3 became apparent at pH 8.4 in the absence of external Na+, which is carried by H+ and K+, but is unaffected by replacement of chloride. The different conductances found to be associated with SNAT3 suggest that the coupling of the ion transport to substrate translocation is incomplete.

Supported by the DFG (De 231/16-2).