Sterol-binding protein that mediates cholesterol transport from the endoplasmic reticulum to endosomes. Creates contact site between the endoplasmic reticulum and late endosomes: localizes to late endosome membranes and contacts the endoplasmic reticulum. 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. Homodimer. Localizes to contact sites between the endoplasmic reticulum and late endosomes: associates with the endoplasmic reticulum membrane via interaction with VAPA and VAPB. 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. Belongs to the STARD3 family.