Known pathways of vesicular sterol transport, and hypothetical models of nonvesicular sterol transport. Straight arrows, vesicular routes; curved arrows, nonvesicular pathways (brown curved arrows represent pathways for which there is no direct evidence, or evidence only by homology to mammalian systems). (Vesicular routes may comprise diffusion through cytoplasm as well as translocation across membranes in close apposition.) Pathways are as follows. Ergosterol is synthesized by a suite of enzymes (Erg) located in the endoplasmic reticulum (ER) and lipid particle (LP). Sterols in the ER may either by converted to steryl esters by acyl-CoA:sterol acyltransferases (Are) for storage in the LP; transported to the cell surface by a sterol carrier protein (Osh); or sent to the Golgi (TGN), where it associates with sphingolipid to form ‘rafts’ or detergent-resistant membranes, and is transported to the plasma membrane in secretory vesicles (SV). Steryl esters in the LP can be mobilized to free ergosterol by steryl ester hydrolases (Yeh1/2, Tgl1) at the ER/LP and PM. Extracellular sterol, or sterol within the PM, may be internalized by transporters within the membrane (Aus1/Pdr11), whereupon it may be transported to the ER by carrier proteins. PM sterol can also be internalized by endocytosis; early endosomes (EE) may send sterol to the late endosome/multivesicular body (MVB) and vacuole (Vac), whereupon the yeast homologues of the human Niemann-Pick C genes (Ncr1, Npc2) might facilitate sterol movement to other organelles. Alternately, recycling endosomes (RE) may take sterol from the EE back to the surface.

Proposed mechanism of sterol transport by Osh4p. Unliganded Osh4p possesses a relatively flexible lid domain, and binds to membranes most likely through highly conserved charged residues at the surface (yellow). This binding may be stabilized by the presence of PI(4,5)P₂ in the membrane (green headgroup). Sterol enters the interior of the ORD, inducing conformational changes. Upon dissociation from the membrane, hydrophobic residues in the lid domain (black) contact the hydrocarbon tail of sterol, stabilizing Osh4p in a ‘closed’ conformation. Sterol delivery to the acceptor membrane is likely a mirror of the extraction from the donor membrane.

Domain structure, subcellular localization (in yeast) and known lipid affinites of human oxysterol binding protein (OSBP) and the yeast OSBP homologues (Osh1p-Osh7p). The relative positioning of the ORDs and other domains is based on the ORD sequence alignments from the Supplementary Material in [78]. Black, oxysterol binding protein-related domain (ORD). Purple, pleckstrin homology (PH) domain. Green, ‘two phenylalanines in an acid tract’ (FFAT) motif. Red, ankyrin repeats. Yellow, Golgi dynamics (GOLD) domain.