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. Expressed in the brain. The strongest expression levels in embryonic, postnatal, and adult stages are found in both cortical and hippocampal tissues. Expressed during embryonic brain development and the highest levels are observed postnatally. Belongs to the sodium:neurotransmitter symporter (SNF) (TC 2.A.22) family. SLC6A17 subfamily.