Sterol-binding protein that mediates cholesterol transport from the endoplasmic reticulum to endosomes (By similarity). Creates contact site between the endoplasmic reticulum and late endosomes: localizes to late endosome membranes and contacts the endoplasmic reticulum via interaction with VAPA and VAPB (By similarity). Acts as a lipid transfer protein that redirects sterol to the endosome at the expense of the cell membrane and favors membrane formation inside endosomes (By similarity). May also mediate cholesterol transport between other membranes, such as mitochondria membrane or cell membrane (By similarity). However, such results need additional experimental evidences; probably mainly mediates cholesterol transport from the endoplasmic reticulum to endosomes (By similarity). Does not activate transcriptional cholesterol sensing (By similarity). Able to bind other lipids, such as lutein, a xanthophyll carotenoids that form the macular pigment of the retina (PubMed:21322544). Homodimer. Interacts (via the MENTAL domain) with STARD3NL. Interacts (via FFAT motif) with VAPA. Interacts (via FFAT motif) with VAPB. Interacts (via FFAT motif) with MOSPD2 (via MSP domain). Localizes to contact sites between the endoplasmic reticulum and late endosomes: associates with the endoplasmic reticulum membrane via interaction with VAPA, VAPB or MOSPD2. Present in retina. Localizes to all neurons of macular retina and especially cone inner segments and axons (at protein level). The FFAT motif mediates interaction with VAPA, VAPB and MOSPD2. The START domain mediates lipid-transfer between membranes. It contains a hydrophobic cavity able to accommodate one lipid molecule, thereby serving as a 'hydrophobic bridge' across the aqueous gap between donor and acceptor organelle membranes. The MENTAL domain anchors the protein in endosome membranes and exposes the START domain in the cytosol. It binds cholesterol and mediates homotypic as well as heterotypic interactions between STARD3 and STARD3NL. Belongs to the STARD3 family.