Required for optimal lysosomal function (PubMed:21224396). 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 (PubMed:25588945). 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 (PubMed:25998567). Plays a role as negative regulator of TGFB1 production in regulatory T cells (PubMed:26126825). Binds ceramide and facilitates its exit from late endosome in order to control cell death pathways (PubMed:26280656). Homooligomer; upon reaching the lysosomes (PubMed:21224396). Interacts with MCOLN1 (PubMed:21224396). Interacts with NEDD4; may play a role in the lysosomal sorting of LAPTM4B; enhances HGS association with NEDD4; mediates inhibition of EGFR degradation (PubMed:22096579, PubMed:25588945). 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 (PubMed:25588945). Interacts with HGS; promotes HGS ubiquitination (PubMed:25588945). Interacts with SNX5 (PubMed:25588945). Interacts with SLC3A2 AND SLC7A5; recruits SLC3A2 and SLC7A5 to lysosomes to promote leucine uptake into these organelles and is required for mTORC1 activation (PubMed:25998567). Interacts with LRRC32; decreases TGFB1 production in regulatory T cells (PubMed:26126825). Interacts with BECN1; competes with EGFR for LAPTM4B binding; regulates EGFR activity (PubMed:28479384). Interacts with EGFR; positively correlates with EGFR activation (PubMed:28479384). Undergoes proteolytic cleavage following delivery to the lysosomes. Ubiquitinated by NEDD4. Belongs to the LAPTM4/LAPTM5 transporter family. Probable cloning artifact.