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 (By similarity). Localizes at synaptic junctions - at both pre- and post-synaptic sites - particularly in excitatory glutamatergic terminals. Belongs to the sodium:neurotransmitter symporter (SNF) (TC 2.A.22) family. SLC6A17 subfamily.