Sodium-independent, high affinity transport of small neutral D- and L-amino acids. May play a role in the modulation of glutamatergic transmission through mobilization of D-serine at the glutamatergic synapse. Disulfide-linked heterodimer with the amino acid transport protein SLC3A2/4F2hc. Expressed in brain, heart, kidney, liver, lung, pancreas, placenta, and skeletal muscle. Belongs to the amino acid-polyamine-organocation (APC) superfamily.