Characterization of the sterol and phosphatidylinositol 4-phosphate binding properties of Golgi-associated OSBP-related protein 9 (ORP9)

Key Mechanisms in Lysosome Stability, Degradation and Repair

Lysosomes are organelles that play pivotal roles in macromolecule digestion, signal transduction, autophagy, and cellular homeostasis. Lysosome instability, including the inhibition of lysosomal intracellular activity and the leakage of their contents, is associated with various pathologies, including cancer, neurodegenerative diseases, inflammatory diseases and infections. These lysosomal-related pathologies highlight the significance of factors contributing to lysosomal dysfunction. The vulnerability of the lysosomal membrane and its components to internal and external stimuli make lysosomes particularly susceptible to damage. Cells are equipped with mechanisms to repair or degrade damaged lysosomes to prevent cell death. Understanding the factors influencing lysosome stabilization and damage repair is essential for developing effective therapeutic interventions for diseases. This review explores the factors affecting lysosome acidification, membrane integrity, and functional homeostasis and examines the underlying mechanisms of lysosomal damage repair. In addition, we summarize how various risk factors impact lysosomal activity and cell fate.

Regulation of cellular cholesterol distribution via non-vesicular lipid transport at ER-Golgi contact sites

Abnormal distribution of cellular cholesterol is associated with numerous diseases, including cardiovascular and neurodegenerative diseases. Regulated transport of cholesterol is critical for maintaining its proper distribution in the cell, yet the underlying mechanisms remain unclear. Here, we show that lipid transfer proteins, namely ORP9, OSBP, and GRAMD1s/Asters (GRAMD1a/GRAMD1b/GRAMD1c), control non-vesicular cholesterol transport at points of contact between the ER and the trans-Golgi network (TGN), thereby maintaining cellular cholesterol distribution. ORP9 localizes to the TGN via interaction between its tandem α-helices and ORP10/ORP11. ORP9 extracts PI4P from the TGN to prevent its overaccumulation and suppresses OSBP-mediated PI4P-driven cholesterol transport to the Golgi. By contrast, GRAMD1s transport excess cholesterol from the Golgi to the ER, thereby preventing its build-up. Cells lacking ORP9 exhibit accumulation of cholesterol at the Golgi, which is further enhanced by additional depletion of GRAMD1s with major accumulation in the plasma membrane. This is accompanied by chronic activation of the SREBP-2 signalling pathway. Our findings reveal the importance of regulated lipid transport at ER-Golgi contacts for maintaining cellular cholesterol distribution and homeostasis.

Pathogen vacuole membrane contact sites - close encounters of the fifth kind

Vesicular trafficking and membrane fusion are well-characterized, versatile, and sophisticated means of 'long range' intracellular protein and lipid delivery. Membrane contact sites (MCS) have been studied in far less detail, but are crucial for 'short range' (10-30 nm) communication between organelles, as well as between pathogen vacuoles and organelles. MCS are specialized in the non-vesicular trafficking of small molecules such as calcium and lipids. Pivotal MCS components important for lipid transfer are the VAP receptor/tether protein, oxysterol binding proteins (OSBPs), the ceramide transport protein CERT, the phosphoinositide phosphatase Sac1, and the lipid phosphatidylinositol 4-phosphate (PtdIns(4)P). In this review, we discuss how these MCS components are subverted by bacterial pathogens and their secreted effector proteins to promote intracellular survival and replication.

ORP9 and ORP10 form a heterocomplex to transfer phosphatidylinositol 4-phosphate at ER-TGN contact sites

Oxysterol-binding protein (OSBP) and its related proteins (ORPs) are a family of lipid transfer proteins (LTPs) that mediate non-vesicular lipid transport. ORP9 and ORP10, members of the OSBP/ORPs family, are located at the endoplasmic reticulum (ER)-trans-Golgi network (TGN) membrane contact sites (MCSs). It remained unclear how they mediate lipid transport. In this work, we discovered that ORP9 and ORP10 form a binary complex through intermolecular coiled-coil (CC) domain-CC domain interaction. The PH domains of ORP9 and ORP10 specially interact with phosphatidylinositol 4-phosphate (PI4P), mediating the TGN targeting. The ORP9-ORP10 complex plays a critical role in regulating PI4P levels at the TGN. Using in vitro reconstitution assays, we observed that while full-length ORP9 efficiently transferred PI4P between two apposed membranes, the lipid transfer kinetics was further accelerated by ORP10. Interestingly, our data showed that the PH domains of ORP9 and ORP10 participate in membrane tethering simultaneously, whereas ORDs of both ORP9 and ORP10 are required for lipid transport. Furthermore, our data showed that the depletion of ORP9 and ORP10 led to increased vesicle transport to the plasma membrane (PM). These findings demonstrate that ORP9 and ORP10 form a binary complex through the CC domains, maintaining PI4P homeostasis at ER-TGN MCSs and regulating vesicle trafficking.

Roles of Phosphatidylinositol 4-Phosphorylation in Non-vesicular Cholesterol Trafficking

Cholesterol (Chol) is an essential component of all eukaryotic cell membranes that affects the function of numerous peripheral as well as integral membrane proteins. Chol is synthesized in the ER, but it is selectively enriched within the plasma membrane (PM) and other endomembranes, which requires Chol to cross the aqueous phase of the cytoplasm. In addition to the classical vesicular trafficking pathways that are known to facilitate the bulk transport of membrane intermediates, Chol is also transported via non-vesicular lipid transfer proteins that work primarily within specialized membrane contact sites. Some of these transport pathways work against established concentration gradients and hence require energy. Recent studies highlight the unique role of phosphoinositides (PPIns), and phosphatidylinositol 4-phosphate (PI4P) in particular, for the control of non-vesicular Chol transport. In this chapter, we will review the emerging connection between Chol, PPIns, and lipid transfer proteins that include the important family of oxysterol-binding protein related proteins, or ORPs.

A phosphoinositide signalling pathway mediates rapid lysosomal repair

Lysosomal dysfunction has been increasingly linked to disease and normal ageing1,2. Lysosomal membrane permeabilization (LMP), a hallmark of lysosome-related diseases, can be triggered by diverse cellular stressors3. Given the damaging contents of lysosomes, LMP must be rapidly resolved, although the underlying mechanisms are poorly understood. Here, using an unbiased proteomic approach, we show that LMP stimulates a phosphoinositide-initiated membrane tethering and lipid transport (PITT) pathway for rapid lysosomal repair. Upon LMP, phosphatidylinositol-4 kinase type 2α (PI4K2A) accumulates rapidly on damaged lysosomes, generating high levels of the lipid messenger phosphatidylinositol-4-phosphate. Lysosomal phosphatidylinositol-4-phosphate in turn recruits multiple oxysterol-binding protein (OSBP)-related protein (ORP) family members, including ORP9, ORP10, ORP11 and OSBP, to orchestrate extensive new membrane contact sites between damaged lysosomes and the endoplasmic reticulum. The ORPs subsequently catalyse robust endoplasmic reticulum-to-lysosome transfer of phosphatidylserine and cholesterol to support rapid lysosomal repair. Finally, the lipid transfer protein ATG2 is also recruited to damaged lysosomes where its activity is potently stimulated by phosphatidylserine. Independent of macroautophagy, ATG2 mediates rapid membrane repair through direct lysosomal lipid transfer. Together, our findings identify that the PITT pathway maintains lysosomal membrane integrity, with important implications for numerous age-related diseases characterized by impaired lysosomal function.

PI4P/PS countertransport by ORP10 at ER-endosome membrane contact sites regulates endosome fission

Membrane contact sites (MCSs) serve as a zone for nonvesicular lipid transport by oxysterol-binding protein (OSBP)-related proteins (ORPs). ORPs mediate lipid countertransport, in which two distinct lipids are transported counterdirectionally. How such lipid countertransport controls specific biological functions, however, remains elusive. We report that lipid countertransport by ORP10 at ER–endosome MCSs regulates retrograde membrane trafficking. ORP10, together with ORP9 and VAP, formed ER–endosome MCSs in a phosphatidylinositol 4-phosphate (PI4P)-dependent manner. ORP10 exhibited a lipid exchange activity toward its ligands, PI4P and phosphatidylserine (PS), between liposomes in vitro, and between the ER and endosomes in situ. Cell biological analysis demonstrated that ORP10 supplies a pool of PS from the ER, in exchange for PI4P, to endosomes where the PS-binding protein EHD1 is recruited to facilitate endosome fission. Our study highlights a novel lipid exchange at ER–endosome MCSs as a nonenzymatic PI4P-to-PS conversion mechanism that organizes membrane remodeling during retrograde membrane trafficking.

Functions of Oxysterol-Binding Proteins at Membrane Contact Sites and Their Control by Phosphoinositide Metabolism

Lipids must be correctly transported within the cell to the right place at the right time in order to be fully functional. Non-vesicular lipid transport is mediated by so-called lipid transfer proteins (LTPs), which contain a hydrophobic cavity that sequesters lipid molecules. Oxysterol-binding protein (OSBP)-related proteins (ORPs) are a family of LTPs known to harbor lipid ligands, such as cholesterol and phospholipids. ORPs act as a sensor or transporter of those lipid ligands at membrane contact sites (MCSs) where two different cellular membranes are closely apposed. In particular, a characteristic functional property of ORPs is their role as a lipid exchanger. ORPs mediate counter-directional transport of two different lipid ligands at MCSs. Several, but not all, ORPs transport their lipid ligand from the endoplasmic reticulum (ER) in exchange for phosphatidylinositol 4-phosphate (PI4P), the other ligand, on apposed membranes. This ORP-mediated lipid “countertransport” is driven by the concentration gradient of PI4P between membranes, which is generated by its kinases and phosphatases. In this review, we will discuss how ORP function is tightly coupled to metabolism of phosphoinositides such as PI4P. Recent progress on the role of ORP-mediated lipid transport/countertransport at multiple MCSs in cellular functions will be also discussed.

YPIBP: A repository for phosphoinositide-binding proteins in yeast

Phosphoinositides (PIs) are a family of eight lipids consisting of phosphatidylinositol (PtdIns) and its seven phosphorylated forms. PIs have important regulatory functions in the cell including lipid signaling, protein transport, and membrane trafficking. Yeast has been recognized as a eukaryotic model system to study lipid-protein interactions. Hundreds of yeast PI-binding proteins have been identified, but this research knowledge remains scattered. Besides, the complete PI-binding spectrum and potential PI-binding domains have not been interlinked. No comprehensive databases are available to support the lipid-protein interaction research on phosphoinositides. Here we constructed the first knowledgebase of Yeast Phosphoinositide-Binding Proteins (YPIBP), a repository consisting of 679 PI-binding proteins collected from high-throughput proteome-array and lipid-array studies, QuickGO, and a rigorous literature mining. The YPIBP also contains protein domain information in categories of lipid-binding domains, lipid-related domains and other domains. The YPIBP provides search and browse modes along with two enrichment analyses (PI-binding enrichment analysis and domain enrichment analysis). An interactive visualization is given to summarize the PI-domain-protein interactome. Finally, three case studies were given to demonstrate the utility of YPIBP. The YPIBP knowledgebase consolidates the present knowledge and provides new insights of the PI-binding proteins by bringing comprehensive and in-depth interaction network of the PI-binding proteins. YPIBP is available at http://cosbi7.ee.ncku.edu.tw/YPIBP/.

Structural and Functional Specialization of OSBP-Related Proteins

Lipids are precisely distributed in the eukaryotic cell where they help to define organelle identity and function, in addition to their structural role. Once synthesized, many lipids must be delivered to other compartments by non-vesicular routes, a process that is undertaken by proteins called Lipid Transfer Proteins (LTPs). OSBP and the closely-related ORP and Osh proteins constitute a major, evolutionarily conserved family of LTPs in eukaryotes. Most of these target one or more subcellular regions, and membrane contact sites in particular, where two organelle membranes are in close proximity. It was initially thought that such proteins were strictly dedicated to sterol sensing or transport. However, over the last decade, numerous studies have revealed that these proteins have many more functions, and we have expanded our understanding of their mechanisms. In particular, many of them are lipid exchangers that exploit PI(4)P or possibly other phosphoinositide gradients to directionally transfer sterol or PS between two compartments. Importantly, these transfer activities are tightly coupled to processes such as lipid metabolism, cellular signalling and vesicular trafficking. This review describes the molecular architecture of OSBP/ORP/Osh proteins, showing how their specific structural features and internal configurations impart unique cellular functions.

Lipid Exchangers: Cellular Functions and Mechanistic Links With Phosphoinositide Metabolism

Lipids are amphiphilic molecules that self-assemble to form biological membranes. Thousands of lipid species coexist in the cell and, once combined, define organelle identity. Due to recent progress in lipidomic analysis, we now know how lipid composition is finely tuned in different subcellular regions. Along with lipid synthesis, remodeling and flip-flop, lipid transfer is one of the active processes that regulates this intracellular lipid distribution. It is mediated by Lipid Transfer Proteins (LTPs) that precisely move certain lipid species across the cytosol and between the organelles. A particular subset of LTPs from three families (Sec14, PITP, OSBP/ORP/Osh) act as lipid exchangers. A striking feature of these exchangers is that they use phosphatidylinositol or phosphoinositides (PIPs) as a lipid ligand and thereby have specific links with PIP metabolism and are thus able to both control the lipid composition of cellular membranes and their signaling capacity. As a result, they play pivotal roles in cellular processes such as vesicular trafficking and signal transduction at the plasma membrane. Recent data have shown that some PIPs are used as energy by lipid exchangers to generate lipid gradients between organelles. Here we describe the importance of lipid counter-exchange in the cell, its structural basis, and presumed links with pathologies.

Cellular homeostasis in the Drosophila retina requires the lipid phosphatase Sac1

The complex functions of cellular membranes, and thus overall cell physiology, depend on the distribution of crucial lipid species. Sac1 is an essential, conserved, ER-localized phosphatase whose substrate, phosphatidylinositol 4-phosphate (PI4P), coordinates secretory trafficking and plasma membrane function. PI4P from multiple pools is delivered to Sac1 by oxysterol-binding protein and related proteins in exchange for other lipids and sterols, which places Sac1 at the intersection of multiple lipid distribution pathways. However, much remains unknown about the roles of Sac1 in subcellular homeostasis and organismal development. Using a temperature-sensitive allele (Sac1^ts), we show that Sac1 is required for structural integrity of the Drosophila retinal floor. The β_ps-integrin Myospheroid, which is necessary for basal cell adhesion, is mislocalized in Sac1^ts retinas. In addition, the adhesion proteins Roughest and Kirre, which coordinate apical retinal cell patterning at an earlier stage, accumulate within Sac1^ts retinal cells due to impaired endo-lysosomal degradation. Moreover, Sac1 is required for ER homeostasis in Drosophila retinal cells. Together, our data illustrate the importance of Sac1 in regulating multiple aspects of cellular homeostasis during tissue development.

Specialized ER membrane domains for lipid metabolism and transport

The endoplasmic reticulum (ER) is a highly organized organelle that performs vital functions including de novo membrane lipid synthesis and transport. Accordingly, numerous lipid biosynthesis enzymes are localized in the ER membrane. However, it is now evident that lipid metabolism is sub-compartmentalized within the ER and that lipid biosynthetic enzymes engage with lipid transfer proteins (LTPs) to rapidly shuttle newly synthesized lipids from the ER to other organelles. As such, intimate relationships between lipid metabolism and lipid transfer pathways exist within the ER network. Notably, certain LTPs enhance the activities of lipid metabolizing enzymes; likewise, lipid metabolism can ensure the specificity of LTP transfer/exchange reactions. Yet, our understanding of these mutual relationships is still emerging. Here, we highlight past and recent key findings on specialized ER membrane domains involved in efficient lipid metabolism and transport and consider unresolved issues in the field.

Polyphosphoinositide-Binding Domains: Insights from Peripheral Membrane and Lipid-Transfer Proteins

Within eukaryotic cells, biochemical reactions need to be organized on the surface of membrane compartments that use distinct lipid constituents to dynamically modulate the functions of integral proteins or influence the selective recruitment of peripheral membrane effectors. As a result of these complex interactions, a variety of human pathologies can be traced back to improper communication between proteins and membrane surfaces; either due to mutations that directly alter protein structure or as a result of changes in membrane lipid composition. Among the known structural lipids found in cellular membranes, phosphatidylinositol (PtdIns) is unique in that it also serves as the membrane-anchored precursor of low-abundance regulatory lipids, the polyphosphoinositides (PPIn), which have restricted distributions within specific subcellular compartments. The ability of PPIn lipids to function as signaling platforms relies on both non-specific electrostatic interactions and the selective stereospecific recognition of PPIn headgroups by specialized protein folds. In this chapter, we will attempt to summarize the structural diversity of modular PPIn-interacting domains that facilitate the reversible recruitment and conformational regulation of peripheral membrane proteins. Outside of protein folds capable of capturing PPIn headgroups at the membrane interface, recent studies detailing the selective binding and bilayer extraction of PPIn species by unique functional domains within specific families of lipid-transfer proteins will also be highlighted. Overall, this overview will help to outline the fundamental physiochemical mechanisms that facilitate localized interactions between PPIn lipids and the wide-variety of PPIn-binding proteins that are essential for the coordinate regulation of cellular metabolism and membrane dynamics.

Bridging the molecular and biological functions of the oxysterol-binding protein family

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) constitute a large eukaryotic gene family that transports and regulates the metabolism of sterols and phospholipids. The original classification of the family based on oxysterol-binding activity belies the complex dual lipid-binding specificity of the conserved OSBP homology domain (OHD). Additional protein- and membrane-interacting modules mediate the targeting of select OSBP/ORPs to membrane contact sites between organelles, thus positioning the OHD between opposing membranes for lipid transfer and metabolic regulation. This unique subcellular location, coupled with diverse ligand preferences and tissue distribution, has identified OSBP/ORPs as key arbiters of membrane composition and function. Here, we will review how molecular models of OSBP/ORP-mediated intracellular lipid transport and regulation at membrane contact sites relate to their emerging roles in cellular and organismal functions.

Oxysterol-binding protein-related protein (ORP) 6 localizes to the ER and ER-plasma membrane contact sites and is involved in the turnover of PI4P in cerebellar granule neurons

Oxysterol-binding protein (OSBP)-related proteins (ORPs) are conserved lipid binding proteins found in organisms ranging from yeast to mammals. Recent findings have indicated that these proteins mainly localize to contact sites of 2 different membranous organelles. ORP6, a member of the ORP subfamily III, is one of the least studied ORPs. Using approaches in molecular cell biology, we attempted to study the characteristics of ORP6 and found that ORP6 is abundantly expressed in mouse cultured neurons. Deconvolution microscopy of cultured cerebellar granular cells revealed that ORP6 is localized to the endoplasmic reticulum (ER) and ER-plasma membrane (PM) contact sites, where it co-localized with extended synaptotagmin2 (E-Syt2), a well-known ER-PM contact site marker. E-Syt2 also co-localized with ORP3, another subfamily III member, and ORP5, a subfamily IV member. However, ORP5 does not distribute to the same ER-PM contact sites as subfamily III members. Also, the co-expression of ORP3 but not ORP5 altered the distribution of ORP6 into the processes of cerebellar neurons. Immunoprecipitation demonstrated binding between the intermediate region of ORP6 and ORP3 or ORP6 itself. Additionally, the localization of ORP6 in the PM decreased when co-expressed with the intermediate region of ORP6, in which the pleckstrin homology (PH) domain and OSBP-related ligand binding domain (ORD) are deleted. Over-expression of this intermediate region shifted the location of a phophtidylinositol-4-phosphate (PI4P) marker from the Golgi to the PM. Knockdown of ORP6 resulted in the same shift of the PI4P marker. Collectively, our data suggests that the recruitment of ORP6 to ER-PM contact sites is involved in the turnover of PI4P in cerebellar granular neurons.

The Rab11-binding protein RELCH/KIAA1468 controls intracellular cholesterol distribution

Cholesterol, which is endocytosed to the late endosome (LE)/lysosome, is delivered to other organelles through vesicular and nonvesicular transport mechanisms. In this study, we discuss a novel mechanism of cholesterol transport from recycling endosomes (REs) to the trans-Golgi network (TGN) through RELCH/KIAA1468, which is newly identified in this study as a Rab11-GTP- and OSBP-binding protein. After treating cells with 25-hydroxycholesterol to induce OSBP relocation from the cytoplasm to the TGN, REs accumulated around the TGN area, but this accumulation was diminished in RELCH- or OSBP-depleted cells. Cholesterol content in the TGN was decreased in Rab11-, RELCH-, and OSBP-depleted cells and increased in the LE/lysosome. According to in vitro reconstitution experiments, RELCH tethers Rab11-bound RE-like and OSBP-bound TGN-like liposomes and promotes OSBP-dependent cholesterol transfer from RE-like to TGN-like liposomes. These data suggest that RELCH promotes nonvesicular cholesterol transport from REs to the TGN through membrane tethering.

The Oxysterol-Binding Protein Cycle: Burning Off PI(4)P to Transport Cholesterol

To maintain an asymmetric distribution of ions across membranes, protein pumps displace ions against their concentration gradient by using chemical energy. Here, we describe a functionally analogous but topologically opposite process that applies to the lipid transfer protein (LTP) oxysterol-binding protein (OSBP). This multidomain protein exchanges cholesterol for the phosphoinositide phosphatidylinositol 4-phosphate [PI(4)P] between two apposed membranes. Because of the subsequent hydrolysis of PI(4)P, this counterexchange is irreversible and contributes to the establishment of a cholesterol gradient along organelles of the secretory pathway. The facts that some natural anti-cancer molecules block OSBP and that many viruses hijack the OSBP cycle for the formation of intracellular replication organelles highlight the importance and potency of OSBP-mediated lipid exchange. The architecture of some LTPs is similar to that of OSBP, suggesting that the principles of the OSBP cycle-burning PI(4)P for the vectorial transfer of another lipid-might be general.

Intracellular cholesterol transport proteins: roles in health and disease

Effective cholesterol homoeostasis is essential in maintaining cellular function, and this is achieved by a network of lipid-responsive nuclear transcription factors, and enzymes, receptors and transporters subject to post-transcriptional and post-translational regulation, whereas loss of these elegant, tightly regulated homoeostatic responses is integral to disease pathologies. Recent data suggest that sterol-binding sensors, exchangers and transporters contribute to regulation of cellular cholesterol homoeostasis and that genetic overexpression or deletion, or mutations, in a number of these proteins are linked with diseases, including atherosclerosis, dyslipidaemia, diabetes, congenital lipoid adrenal hyperplasia, cancer, autosomal dominant hearing loss and male infertility. This review focuses on current evidence exploring the function of members of the ‘START’ (steroidogenic acute regulatory protein-related lipid transfer) and ‘ORP’ (oxysterol-binding protein-related proteins) families of sterol-binding proteins in sterol homoeostasis in eukaryotic cells, and the evidence that they represent valid therapeutic targets to alleviate human disease.

Oxysterol-binding Protein Activation at Endoplasmic Reticulum-Golgi Contact Sites Reorganizes Phosphatidylinositol 4-Phosphate Pools

Oxysterol-binding protein (OSBP) exchanges cholesterol and phosphatidylinositol 4-phosphate (PI-4P) at contact sites between the endoplasmic reticulum (ER) and the trans-Golgi/trans-Golgi network. 25-Hydroxycholesterol (25OH) competitively inhibits this exchange reaction in vitro and causes the constitutive localization of OSBP at the ER/Golgi interface and PI-4P-dependent recruitment of ceramide transfer protein (CERT) for sphingomyelin synthesis. We used PI-4P probes and mass analysis to determine how OSBP controls the availability of PI-4P for this metabolic pathway. Treatment of fibroblasts or Chinese hamster ovary (CHO) cells with 25OH caused a 50-70% reduction in Golgi-associated immunoreactive PI-4P that correlated with Golgi localization of OSBP. In contrast, 25OH caused an OSBP-dependent enrichment in Golgi PI-4P that was detected with a pleckstrin homology domain probe. The cellular mass of phosphatidylinositol monophosphates and Golgi PI-4P measured with an unbiased PI-4P probe (P4M) was unaffected by 25OH and OSBP silencing, indicating that OSBP shifts the distribution of PI-4P upon localization to ER-Golgi contact sites. The PI-4P and sterol binding activities of OSBP were both required for 25OH activation of sphingomyelin synthesis, suggesting that 25OH must be exchanged for PI-4P to be concentrated at contact sites. We propose a model wherein 25OH activation of OSBP promotes the binding and retention of PI-4P at ER-Golgi contact sites. This pool of PI-4P specifically recruits pleckstrin homology domain-containing proteins involved in lipid transfer and metabolism, such as CERT.

OSBP-Related Protein Family: Mediators of Lipid Transport and Signaling at Membrane Contact Sites

Oxysterol-binding protein (OSBP) and its related protein homologs, ORPs, constitute a conserved family of lipid-binding/transfer proteins (LTPs) expressed ubiquitously in eukaryotes. The ligand-binding domain of ORPs accommodates cholesterol and oxysterols, but also glycerophospholipids, particularly phosphatidylinositol-4-phosphate (PI4P). ORPs have been implicated as intracellular lipid sensors or transporters. Most ORPs carry targeting determinants for the endoplasmic reticulum (ER) and non-ER organelle membrane. ORPs are located and function at membrane contact sites (MCSs), at which ER is closely apposed with other organelle limiting membranes. Such sites have roles in lipid transport and metabolism, control of Ca(2+) fluxes, and signaling events. ORPs are postulated either to transport lipids over MCSs to maintain the distinct lipid compositions of organelle membranes, or to control the activity of enzymes/protein complexes with functions in signaling and lipid metabolism. ORPs may transfer PI4P and another lipid class bidirectionally. Transport of PI4P followed by its hydrolysis would in this model provide the energy for transfer of the other lipid against its concentration gradient. Control of organelle lipid compositions by OSBP/ORPs is important for the life cycles of several pathogenic viruses. Targeting ORPs with small-molecular antagonists is proposed as a new strategy to combat viral infections. Several ORPs are reported to modulate vesicle transport along the secretory or endocytic pathways. Moreover, antagonists of certain ORPs inhibit cancer cell proliferation. Thus, ORPs are LTPs, which mediate interorganelle lipid transport and coordinate lipid signals with a variety of cellular regimes.

OSBP-Related Protein Family in Lipid Transport Over Membrane Contact Sites

Increasing evidence suggests that oxysterol-binding protein-related proteins (ORPs) localize at membrane contact sites, which are high-capacity platforms for inter-organelle exchange of small molecules and information. ORPs can simultaneously associate with the two apposed membranes and transfer lipids across the interbilayer gap. Oxysterol-binding protein moves cholesterol from the endoplasmic reticulum to trans-Golgi, driven by the retrograde transport of phosphatidylinositol-4-phosphate (PI4P). Analogously, yeast Osh6p mediates the transport of phosphatidylserine from the endoplasmic reticulum to the plasma membrane in exchange for PI4P, and ORP5 and -8 are suggested to execute similar functions in mammalian cells. ORPs may share the capacity to bind PI4P within their ligand-binding domain, prompting the hypothesis that bidirectional transport of a phosphoinositide and another lipid may be a common theme among the protein family. This model, however, needs more experimental support and does not exclude a function of ORPs in lipid signaling.

Ligand-dependent localization and function of ORP-VAP complexes at membrane contact sites

Oxysterol-binding protein/OSBP-related proteins (ORPs) constitute a conserved family of sterol/phospholipid-binding proteins with lipid transporter or sensor functions. We investigated the spatial occurrence and regulation of the interactions of human OSBP/ORPs or the S. cerevisiae orthologs, the Osh (OSBP homolog) proteins, with their endoplasmic reticulum (ER) anchors, the VAMP-associated proteins (VAPs), by employing bimolecular fluorescence complementation and pull-down set-ups. The ORP-VAP interactions localize frequently at distinct subcellular sites, shown in several cases to represent membrane contact sites (MCSs). Using established ORP ligand-binding domain mutants and pull-down assays with recombinant proteins, we show that ORP liganding regulates the ORP-VAP association, alters the subcellular targeting of ORP-VAP complexes, or modifies organelle morphology. There is distinct protein specificity in the effects of the mutants on subcellular targeting of ORP-VAP complexes. We provide evidence that complexes of human ORP2 and VAPs at ER-lipid droplet interfaces regulate the hydrolysis of triglycerides and lipid droplet turnover. The data suggest evolutionarily conserved, complex ligand-dependent functions of ORP-VAP complexes at MCSs, with implications for cellular lipid homeostasis and signaling.