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. Following activation by amino acids, the Ragulator and Rag GTPases function as a scaffold recruiting mTORC1 to lysosomes where it is in turn activated. SLC38A9 mediates transport of amino acids with low capacity and specificity with a slight preference for polar amino acids. Acts as an arginine sensor. Following activation by arginine binding, mediates transport of leucine, tyrosine and phenylalanine with high efficiency, and is required for the efficient utilization of these amino acids after lysosomal protein degradation. Amino acid transport is sodium-dependent (By similarity). Transport of leucine, tyrosine and phenylalanine is increased by arginine binding (By similarity). Associated component of the Ragulator complex (composed of LAMTOR1, LAMTOR2, LAMTOR3, LAMTOR4 and LAMTOR5) (By similarity). Associated component of the Rag GTPases heterodimers (composed of RRAGA, RRAGB, RRAGC and RRAGD) (By similarity). Interacts with TM4SF5 (By similarity). The cytosolic N-terminus part of the protein mediates interaction with the Ragulator complex and the Rag GTPases heterodimers. Glycosylated. Belongs to the amino acid/polyamine transporter 2 family. SLC38A9 subfamily. Truncated N-terminus.