Functions as a sodium-dependent neutral amino acid transporter. Exhibits preference for methionine and for the branched-chain amino acids, particularly leucine, valine and isoleucine. 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 (By similarity). Belongs to the sodium:neurotransmitter symporter (SNF) (TC 2.A.22) family. SLC6A15 subfamily. Extended N-terminus.