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). Widely distributed in the central nervous system, including the olfactory bulb, the hypothalamus, the cerebral cortex, the hippocampus, and the cerebellum. In addition, intense expression is found in the motor nuclei including the oculomotor nucleus, abducens nucleus, trigeminal motor nucleus, facial nucleus, hypoglossal nucleus and ventral horn of spinal cord. Intense hybridization signals are also observed in the nuclei containing monoaminergic neurons, such as locus coeruleus, the substantia nigra pars compacta, the ventral tegmental area, the dorsal raphe nucleus and the median raphe nucleus. Belongs to the sodium:neurotransmitter symporter (SNF) (TC 2.A.22) family. SLC6A15 subfamily.