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. Belongs to the dicarboxylate/amino acid:cation symporter (DAACS) (TC 2.A.23) family. SLC1A4 subfamily.