The study on oxysterol-binding protein (OSBP) and related protein 9 (ORP9) ligands: sterols and phosphatidylinositol 4-phosphate

Abstract

The oxysterol-binding protein OSBP-gene family is composed of 12 members with a common C-terminal sterol-binding domain (SBD). OSBP and ORP9 are members of the family that are localized to the endoplasmic reticulum (ER) and Golgi apparatus by their FFAT (two phenylalanines in an acidic tract) motif and PH (pleckstrin homology) domain, respectively. These two proteins are implicated in sterol transfer between these organelles. Here we utilized Förster resonance energy transfer (FRET) between cholestatrienol (CTL), a fluorescent cholesterol analog, and dansyl-PE (DNS-PE) to demonstrate that both full-length ORP9L and truncated ORP9S rapidly extract sterols from liposomes. In vitro, OSBP, ORP9L and ORP9S specifically extracted phosphatidylinositol 4-phosphate (PI(4)P) from liposomes, suggesting PI(4)P is also a ligand for these proteins. Overexpression of ORP9L or ORP9S attenuated PI(4)P immunofluorescence detection in CHO cells. These results indicate that OSBP and ORP9 may transport sterol and PI(4)P in cells, possibly in opposite directions.