Building lipid 'PIPelines' throughout the cell by ORP/Osh proteins

Abstract

In eukaryotic cells, a sterol gradient exists between the early and late regions of the secretory pathway. This gradient seems to rely on non-vesicular transport mechanisms mediated by specialized carriers. The oxysterol-binding protein-related protein (ORP)/oxysterol-binding homology (Osh) family has been assumed initially to exclusively include proteins acting as sterol sensors/transporters and many efforts have been made to determine their mode of action. Our recent studies have demonstrated that some ORP/Osh proteins are not mere sterol transporters, but sterol/phosphatidylinositol 4-phosphate [PI(4)P] exchangers. They exploit the PI(4)P gradient at the endoplasmic reticulum (ER)/Golgi interface, or at membrane-contact sites between these compartments, to actively create a sterol gradient. Other recent reports have suggested that all ORP/Osh proteins bind PI(4)P and recognize a second lipid that is not necessary sterol. We have thus proposed that ORP/Osh proteins use PI(4)P gradients between organelles to convey various lipid species.