Functions as a sodium-dependent neutral amino acid transporter. Exhibits preference for the branched-chain amino acids, particularly leucine, valine and isoleucine and methionine. Mediates the saturable, pH-sensitive and electrogenic cotransport of proline and sodium ions with a stoichiometry of 1:1. May have a role as transporter for neurotransmitter precursors into neurons. In contrast to other members of the neurotransmitter transporter family, does not appear to be chloride-dependent. Optimum pH is 7.5-8.5. Strongly inhibited at acidic PH. Almost exclusively expressed in the brain. Belongs to the sodium:neurotransmitter symporter (SNF) (TC 2.A.22) family. SLC6A15 subfamily.