Required for optimal lysosomal function. Blocks EGF-stimulated EGFR intraluminal sorting and degradation. Conversely by binding with the phosphatidylinositol 4,5-bisphosphate, regulates its PIP5K1C interaction, inhibits HGS ubiquitination and relieves LAPTM4B inhibition of EGFR degradation. Recruits SLC3A2 and SLC7A5 (the Leu transporter) to the lysosome, promoting entry of leucine and other essential amino acid (EAA) into the lysosome, stimulating activation of proton-transporting vacuolar (V)-ATPase protein pump (V-ATPase) and hence mTORC1 activation. Plays a role as negative regulator of TGFB1 production in regulatory T cells. Binds ceramide and facilitates its exit from late endosome in order to control cell death pathways. Homooligomer; upon reaching the lysosomes. Interacts with MCOLN1. Interacts with NEDD4; may play a role in the lysosomal sorting of LAPTM4B; enhances HGS association with NEDD4; mediates inhibition of EGFR degradation. Interacts with PIP5K1C; promotes SNX5 association with LAPTM4B; kinase activity of PIP5K1C is required; interaction is regulated by phosphatidylinositol 4,5-bisphosphate generated by PIP5K1C. Interacts with HGS; promotes HGS ubiquitination. Interacts with SNX5. Interacts with SLC3A2 AND SLC7A5; recruits SLC3A2 and SLC7A5 to lysosomes to promote leucine uptake into these organelles and is required for mTORC1 activation. Interacts with LRRC32; decreases TGFB1 production in regulatory T cells. Interacts with BECN1; competes with EGFR for LAPTM4B binding; regulates EGFR activity. Interacts with EGFR; positively correlates with EGFR activation. Strongly expressed in fetal ovary, testis, adrenal gland, liver and uterus, and weakly expressed in the spleen. Undergoes proteolytic cleavage following delivery to the lysosomes. Ubiquitinated by NEDD4. Belongs to the LAPTM4/LAPTM5 transporter family.