OSBP (oxysterol binding protein) is a lipid transporter that mediates cholesterol homeostasis through coordinated lipid exchange at membrane contact sites. Primary function: OSBP exchanges cholesterol and oxysterols with phosphatidylinositol 4-phosphate (PI4P) between the endoplasmic reticulum and Golgi complex, delivering sterol to the Golgi while retrieving PI4P for subsequent degradation 12. Mechanism: OSBP binds cholesterol and 25-hydroxycholesterol 34, and functions at ER-Golgi membrane contact sites through interactions with VAP-A and VAP-B anchor proteins 56. Cholesterol binding promotes formation of a PP2A-tyrosine phosphatase complex that dephosphorylates ERK1/2, while 25-hydroxycholesterol causes complex disassembly 3. OSBP also regulates cholesterol efflux by destabilizing the ABCA1 protein 7. Disease relevance: OSBP-mediated cholesterol delivery to lysosomes is critical for mTORC1 activation and growth signaling; loss of this function contributes to Niemann-Pick type C pathology, where OSBP inhibition restores autophagic function 8. Clinical significance: OSBP represents a potential therapeutic target for lipid metabolism disorders and certain viral infections that exploit host lipid homeostasis mechanisms 6.