Functions as a sodium-dependent vesicular transporter selective for proline, glycine, leucine and alanine. In contrast to other members of this neurotransmitter transporter family, does not appear to be chloride-dependent. Optimum pH is 8.5. Localizes at synaptic junctions - at both pre- and post-synaptic sites - particularly in excitatory glutamatergic terminals. Found exclusively in the central nervous system and is more abundant in the cerebellum and the cerebral cortex. Expressed in PC-12 cell line. Belongs to the sodium:neurotransmitter symporter (SNF) (TC 2.A.22) family. SLC6A17 subfamily.