The heterodimer with SLC3A2 functions as sodium-independent, high-affinity transporter that mediates uptake of large neutral amino acids such as phenylalanine, tyrosine, L-DOPA, leucine, histidine, methionine and tryptophan (PubMed:9751058, PubMed:10049700, PubMed:11557028, PubMed:10391915, PubMed:10574970, PubMed:11311135, PubMed:11564694, PubMed:12117417, PubMed:12225859, PubMed:25998567, PubMed:30867591). Functions as an amino acid exchanger (PubMed:11557028, PubMed:12117417, PubMed:12225859, PubMed:30867591). May play a role in the transport of L-DOPA across the blood-brain barrier (By similarity). May act as the major transporter of tyrosine in fibroblasts (Probable). May mediate blood-to-retina L-leucine transport across the inner blood-retinal barrier (By similarity). Can mediate the transport of thyroid hormones triiodothyronine (T3) and thyroxine (T4) across the cell membrane (PubMed:11564694, PubMed:12225859). When associated with LAPTM4B, the heterodimer formed by SLC3A2 and SLC7A5 is recruited to lysosomes to promote leucine uptake into these organelles, and thereby mediates mTORC1 activation (PubMed:25998567). Involved in the uptake of toxic methylmercury (MeHg) when administered as the L-cysteine or D,L-homocysteine complexes (PubMed:12117417). Involved in the cellular activity of small molecular weight nitrosothiols, via the stereoselective transport of L-nitrosocysteine (L-CNSO) across the membrane (PubMed:15769744). (Microbial infection) In case of hepatitis C virus/HCV infection, the complex formed by SLC3A2 and SLC7A5/LAT1 plays a role in HCV propagation by facilitating viral entry into host cell and increasing L-leucine uptake-mediated mTORC1 signaling activation, thereby contributing to HCV-mediated pathogenesis. Disulfide-linked heterodimer with the amino acid transport protein SLC3A2/4F2hc (PubMed:10049700, PubMed:10391915, PubMed:10574970, PubMed:11311135, PubMed:11389679, PubMed:11557028, PubMed:11564694, PubMed:12117417, PubMed:12225859, PubMed:15769744, PubMed:9751058, PubMed:25998567, PubMed:30867591). Interacts with LAPTM4B; this recruits the heterodimer formed by SLC3A2/4F2hc and SLC7A5 to lysosomes to promote leucine uptake into these organelles and is required for mTORC1 activation (PubMed:25998567). Located to the plasma membrane by SLC3A2/4F2hc (PubMed:9751058). Localized to the apical membrane of placental syncytiotrophoblastic cells (PubMed:11742812). Recruited to lysosomes by LAPTM4B (PubMed:25998567). Expressed in both luminal and abluminal membranes of brain capillary endothelial cells (By similarity). Detected in placenta, in the syncytiotrophoblast layer (at protein level) (PubMed:11389679). Expressed abundantly in adult lung, liver, brain, skeletal muscle, placenta, bone marrow, testis, resting lymphocytes and monocytes, and in fetal liver. Weaker expression in thymus, cornea, retina, peripheral leukocytes, spleen, kidney, colon and lymph node. During gestation, expression in the placenta was significantly stronger at full-term than at the mid-trimester stage. Also expressed in all human tumor cell lines tested and in the astrocytic process of primary astrocytic gliomas. Expressed in retinal endothelial cells and in the intestinal epithelial cell line Caco-2. (Microbial infection) Up-regulation of the complex formed by SLC3A2 and SLC7A5/LAT1 upon hepatitis C virus/HCV infection. Expression induced in quiescent peripheral blood lymphocytes after treatment with phorbol myristate acetate (PMA) and phytohemagglutinin (PHA). Expression and the uptake of leucine is stimulated in mononuclear, cytotrophoblast-like choriocarcinoma cells by combined treatment with PMA and calcium ionophore. The uptake of leucine, tyrosine and tryptophan is inhibited by the different iodothyronines, in particular T3. Leucine transport is also inhibited by small zwitterionic amino acids (i.e. glycine, alanine, serine, threonine and cysteine) and by glutamine and asparagine. The uptake of T3 is almost completely blocked by coincubation with leucine, tryptophan, tyrosine, and phenylalanine, or 2-amino-bicyclo-(2,2,1)-heptane-2-carboxylate (BCH). Methionine uptake was inhibited by the L-system substrates L-leucine, BCH, L-cysteine and by the MeHg-L-cysteine complex and structurally related S-ethyl-L-cysteine. MeHg-L-cysteine uptake is inhibited by L-methionine, L-leucine, BCH and S-ethyl-L-cysteine. L-leucine uptake was inhibited by L-CNSO. Tyrosine uptake in fibroblasts was inhibited by D-methionine, and methyl-aminoisobutyric acid (MeAIB). Belongs to the amino acid-polyamine-organocation (APC) superfamily. L-type amino acid transporter (LAT) (TC 2.A.3.8) family.