Article

Lipid-regulated sterol transfer between closely apposed membranes by oxysterol-binding protein homologues

Laboratory of Cell Biochemistry and Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
The Journal of Cell Biology (Impact Factor: 9.69). 12/2009; 187(6):889-903. DOI: 10.1083/jcb.200905007
Source: PubMed

ABSTRACT Sterols are transferred between cellular membranes by vesicular and poorly understood nonvesicular pathways. Oxysterol-binding protein-related proteins (ORPs) have been implicated in sterol sensing and nonvesicular transport. In this study, we show that yeast ORPs use a novel mechanism that allows regulated sterol transfer between closely apposed membranes, such as organelle contact sites. We find that the core lipid-binding domain found in all ORPs can simultaneously bind two membranes. Using Osh4p/Kes1p as a representative ORP, we show that ORPs have at least two membrane-binding surfaces; one near the mouth of the sterol-binding pocket and a distal site that can bind a second membrane. The distal site is required for the protein to function in cells and, remarkably, regulates the rate at which Osh4p extracts and delivers sterols in a phosphoinositide-dependent manner. Together, these findings suggest a new model of how ORPs could sense and regulate the lipid composition of adjacent membranes.

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    • "The other human ORPs thus far studied display different affinities for various oxysterols, with K d s in the nM– lM range [14] [15] [16]. Oxysterol binding in vitro by purified ORPs has been demonstrated for OSBP, OSBP2/ORP4, ORP1, and ORP2, and in vitro sterol transfer activity for OSBP, ORP9L, the ORD of ORP5 [17] [18], as well as several of the yeast S. cerevisiae Osh proteins [19] [20]. Of note, unlike OSBP, OSBP2/ORP4 and ORP1, ORP2 binds 25OHC with a low, micromolar affinity [21], and shows high affinity (K d 14 nM) for 22(R)OHC [15]. "
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    ABSTRACT: Oxysterol-binding protein (OSBP) and its homologues (ORPs) are lipid-binding/transfer proteins with affinity for oxysterols, cholesterol and glycerophospholipids. In addition to a ligand-binding domain, a majority of the ORPs carry a pleckstrin homology domain that targets organelle membranes via phosphoinositides, and a motif targeting the endoplasmic reticulum (ER) via VAMP-associated proteins (VAPs).
    Steroids 02/2015; 99. DOI:10.1016/j.steroids.2015.01.027 · 2.72 Impact Factor
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    • "Commun. (2013), http://dx.doi.org/10.1016/j.bbrc.2013.12.002 membrane (PM) to the ER [19] [20]. However, there are contradicting observations that rather suggest function of the Osh proteins as regulators of the lateral sterol organization at the PM [21], secretory vesicle transport [22], or nutrient signaling [22] [23]. "
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    ABSTRACT: Oxysterol-binding protein (OSBP) and OSBP-related (ORP) or OSBP-like (OSBPL) proteins constitute a family of lipid-binding/transfer proteins present in eukaryotes from yeast to man. The mechanisms of ORP function have remained incompletely understood. However, several ORPs are present at membrane contact sites and act as either lipid transporters or sensors that control lipid metabolism, cell signaling, and vesicle transport. Zebrafish, Danio rerio, has gained increasing popularity as a model organism in developmental biology, human disease, toxicology, and drug discovery. However, lipid binding/transfer proteins in the fish are thus far unexplored. In this article we report a series of bioinformatic analyses showing that the OSBPL gene family is highly conserved between the fish and human. The OSBPL subfamily structure is markedly similar between the two organisms, and all 12 human genes have orthologues, designated osbpl andlocated on 11 chromosomes in Danio rerio. Interestingly, osbpl2 and osbpl3 are present as two closely related homologues (a and b), due to gene duplication events in the teleost lineage. Moreover, the domain structures of the distinct ORP proteins are almost identical between zebrafish and man, and molecular modeling in the present study suggests that ORD liganding by phosphatidylinositol-4-phosphate (PI4P) is a feature conserved between yeast Osh3p, human ORP3, and zebrafish Osbpl3. The present analysis identifies Danio rerio as an attractive model to study the functions of ORPs in vertebrate development and metabolism.
    Biochemical and Biophysical Research Communications 12/2013; 446(3). DOI:10.1016/j.bbrc.2013.12.002 · 2.28 Impact Factor
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    • "One is tempted to speculate that both ORP1L and ORP5 could constitute parts of a molecular machinery controlling transport of small molecules between LE and ER – even though the proteins are capable of sterol binding and transfer in vitro, they do not necessarily execute a sterol carrier function, but the liganding could play a regulatory role in formation or function of the MCS, as shown for ORP1L in the regulation of LE motility [40]. Of the seven yeast OSBP homologue (Osh) proteins, four (Osh2p, Osh3p, Osh6p, Osh7p) were demonstrated by the group of Prinz to localize at the zones of ER associated with the plasma membrane (PM) [17]. Moreover, the authors presented evidence that the Osh proteins have the capacity to tether ER and PM vesicles to each other, thus potentially forming a MCS. "
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    ABSTRACT: Oxysterol-binding (OSBP)-related proteins (ORPs) constitute a family of sterol and phosphoinositide binding/transfer proteins in eukaryotes from yeast to man. While their functions have mainly been addressed in cellular lipid metabolism or sterol transport, increasing evidence points to more versatile regulatory roles in a spectrum of cellular regimes. In fact ORPs do not appear to be robust controllers of lipid homeostasis. Several ORPs localize at membrane contacts sites (MCS), where endoplasmic reticulum (ER) is apposed with other organelle limiting membranes. Apparently, ORPs have the capacity to control the formation of MCS or activity of enzymatic machineries at these sites. Thereby, ORPs most likely affect organelle membrane lipid compositions, with impacts on signaling and vesicle transport, but also cellular lipid metabolism. Moreover, an increasing number of protein interaction partners of ORPs have been identified, connecting these proteins with various aspects of cell regulation. Small molecular anti-proliferative compounds, ORPphilins, were recently found to target two members of the ORP family, OSBP and ORP4, revealing an essential function of ORPs in cancer cell proliferation and survival. Further functions assigned for ORPs include regulation of extracellular signal regulated kinase (ERK) activity (OSBP), control of ER-late endosome MCS and late endosome motility (ORP1L), regulation of β1-integrin activity (ORP3), modulation of hepatocyte insulin signaling and macrophage migration (ORP8), as well as post-Golgi vesicle transport, phosphatidylinositol-4-phosphate and target of rapamycin complex 1 signaling and nitrogen sensing (S.cerevisiae Osh4p). These and other recent observations shed light on the ORPs as integrators of lipid signals with an unforeseen variety of vital cellular processes.
    Biochemical pharmacology 02/2013; 86(1). DOI:10.1016/j.bcp.2013.02.016 · 4.65 Impact Factor
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