Symporter that cotransports specific neutral amino acids and sodium ions, coupled to an H(+) antiporter activity . Mainly participates in the glutamate-GABA-glutamine cycle in brain where it transports L-glutamine from astrocytes in the intercellular space for the replenishment of both neurotransmitters glutamate and gamma-aminobutyric acid (GABA) in neurons and also functions as the major influx transporter in ganglion cells mediating the uptake of glutamine (By similarity). The transport activity is specific for L-glutamine, L-histidine and L-asparagine . The transport is electroneutral coupled to the cotransport of 1 Na(+) and the antiport of 1 H(+) (By similarity). The transport is pH dependent, saturable, Li(+) tolerant and functions in both direction depending on the concentration gradients of its substrates and cotransported ions . Also mediates an amino acid-gated H(+) conductance that is not stoichiometrically coupled to the amino acid transport but which influences the ionic gradients that drive the amino acid transport (By similarity). In addition, may play a role in nitrogen metabolism, amino acid homeostasis, glucose metabolism and renal ammoniagenesis (By similarity).
Subcellular locations: Cell membrane, Basolateral cell membrane
The localization appears to be basolateral in the plasma membrane of hepatocytes surrounding the central vein. Localized at the cerebrospinal fluid (CSF)-facing membrane of the choroid plexus epithelial cells. In astrocytes, the localization at cell membrane is decreased by ammonia through the PKC signaling. Expressed in both luminal and abluminal plasma membranes of larger microvessels and blood brain barrier (BBB) capillaries (By similarity). Restricted to the basolateral membranes of S3 segment cells of the proximal tubules (By similarity).