Lysosomal amino acid transporter involved in the activation of mTORC1 in response to amino acid levels. Probably acts as an amino acid sensor of the Rag GTPases and Ragulator complexes, 2 complexes involved in amino acid sensing and activation of mTORC1, a signaling complex promoting cell growth in response to growth factors, energy levels, and amino acids. Amino acid transport is sodium-dependent (By similarity). Transport of leucine, tyrosine and phenylalanine is increased by arginine binding (By similarity). Belongs to the amino acid/polyamine transporter 2 family. SLC38A9 subfamily.