Sodium-dependent neutral amino-acid transporter that mediates transport of alanine, serine, cysteine, proline, hydroxyproline and threonine. L-threonine(in) + Na(+)(in) = L-threonine(out) + Na(+)(out) L-serine(in) + Na(+)(in) = L-serine(out) + Na(+)(out) L-cysteine(in) + Na(+)(in) = L-cysteine(out) + Na(+)(out) L-alanine(in) + Na(+)(in) = L-alanine(out) + Na(+)(out) L-proline(in) + Na(+)(in) = L-proline(out) + Na(+)(out) 4-hydroxy-L-proline(in) + Na(+)(in) = 4-hydroxy-L-proline(out) + Na(+)(out) Identified by mass spectrometry in melanosome fractions from stage I to stage IV. Expressed mostly in brain, muscle, and pancreas but detected in all tissues examined. The disease is caused by variants affecting the gene represented in this entry. Belongs to the dicarboxylate/amino acid:cation symporter (DAACS) (TC 2.A.23) family. SLC1A4 subfamily.