The proper intracellular distribution of sterols such as cholesterol is critical for numerous cellular functions, including signal transduction and protein trafficking. Sterols are moved among cellular compartments by both vesicular and poorly understood nonvesicular pathways. We have previously shown that a non-vesicular pathway moves sterols from the plasma membrane (PM) to the endoplasmic reticulum in the yeast S. cerevisiae. We found that this transport requires oxysterol-binding protein (OSBP)-related proteins (ORPs), a large family of lipid-binding proteins that is conserved from yeast to humans. In addition, we have been able to show that some of these yeast ORPs transport sterols and other lipids in vitro. Thus, lipid transport is one of the functions of this important class of proteins. We also found that sterol transport by OSBPs is regulated by particular phosphoinositides (PIPs). Because different PIP-species are enriched in various cellular compartments, PIP-stimulation of ORPs likely serves to regulate the movement of sterols (and possibility other lipids) to particular organelles by ORPs, perhaps in response to cell signaling events. This work is described in a paper in the Journal of Cell Biology. We have continued characteriztion of the yeast ORPs. We have found that all have more than one membrane binding surface and we are in the process of determining how these proteins bind membranes and extract lipids. This work will be described in a manuscript that is being prepared.? ? In a second project, we have established a system to study phospholipids transport between the ER and peroxisomes. We find that it is non-vesicular and likely requires close contact between these two organelles. This work is described in a manuscript that is being prepared.? ? In a third project, we have developed a photoactivatable cholesterol analog that we hope to use to identify novel chlesterol-binding proteins. We have demonstrated that this compound will work with a known cholesterol-binding protein and are in the process of working to identify novel proteins. This work is described in a paper that is being prepared for publication.
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