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). Optimum pH is 7.5-8.5. Significant expressed in brain, lung and kidney. regions, the cortex, the cerebellum and the brain stem. Detected throughout development, starting with the pre-implantation embryo. Mice are viable and fertile and fail to demonstrate any striking abnormality in motor and sensory functions. Other transporters could help to compensate for lost of SLC6A15. Belongs to the sodium:neurotransmitter symporter (SNF) (TC 2.A.22) family. SLC6A15 subfamily.