Functional role of the Mso1p-Sec1p complex in membrane fusion regulation

Sec1/Munc18 (SM) protein family members are evolutionary conserved proteins. They perform an essential, albeit poorly understood function in SNARE complex formation in membrane fusion. In addition to the SNARE complex components, only a few SM protein binding proteins are known. Typically, their binding modes to SM proteins and their contribution to the membrane fusion regulation is poorly characterised. We identified Mso1p as a novel Sec1p interacting partner. It was shown that Mso1p and Sec1p interact at sites of polarised secretion and that this localisation is dependent on the Rab GTPase Sec4p and its GEF Sec2p. Using targeted mutagenesis and N- and C-terminal deletants, it was discovered that the interaction between an N-terminal peptide of Mso1p and the putative Syntaxin N-peptide binding area in Sec1p domain 1 is important for membrane fusion regulation. The yeast Syntaxin homologues Sso1p and Sso2p lack the N-terminal peptide. Our results show that in addition to binding to the putative N-peptide binding area in Sec1p, Mso1p can interact with Sso1p and Sso2p. This result suggests that Mso1p can mimic the N-peptide binding to facilitate membrane fusion. In addition to Mso1p, a novel role in membrane fusion regulation was revealed for the Sec1p C-terminal tail, which is missing in its mammalian homologues. Deletion of the Sec1p-tail results in temperature sensitive growth and reduced sporulation. Using in vivo and in vitro experiments, it was shown that the Sec1p-tail mediates SNARE complex binding and assembly. These results propose a regulatory role for the Sec1p-tail in SNARE complex formation. Furthermore, two novel interaction partners for Mso1p, the Rab GTPase Sec4p and plasma membrane phospholipids, were identified. The Sec4p link was identified using Bimolecular Fluorescence Complementation assays with Mso1p and the non-SNARE binding Sec1p(1-657). The assay revealed that Mso1p can target Sec1p(1-657) to sites of secretion. This effect is mediated via the Mso1p C-terminus, which previously has been genetically linked to Sec4p. These results and in vitro binding experiments suggest that Mso1p acts in cooperation with the GTP-bound form of Sec4p on vesicle-like structures prior to membrane fusion. Mso1p shares homology with the PIP2 binding domain of the mammalian Munc18 binding Mint proteins. It was shown both in vivo and in vitro that Mso1p is a phospholipid inserting protein and that this insertion is mediated by the conserved Mso1p amino terminus. In vivo, the Mso1p phospholipid binding is needed for sporulation and Mso1p-Sec1p localisation at the sites of secretion at the plasma membrane. The results reveal a novel layer of membrane fusion regulation in exocytosis and propose a coordinating role for Mso1p in connection with membrane lipids, Sec1p, Sec4p and SNARE complexes in this process.Sec1/Munc18 (SM) perheen jäsenet ovat hyvin evoluutiossa säilyneitä, mutta toiminnallisesti huonosti tunnettuja solunsisäisille kalvofuusiotapahtumille välttämättömiä proteiineja. SM proteiinit säätelevät kalvofuusiossa SNARE proteiinikompleksien muodostumista toistaiseksi tuntemattomalla tavalla. SM proteiinien vuorovaikutuksia muiden kuin SNARE komponenttien kanssa on tutkittu vain vähän eikä jo tunnettujen komponenttien sitoutumistapoja ja merkitystä membraanifuusiossa ole yksityiskohtaisesti selvitetty. Työssä selvitettiin Saccharomyces cerevisiae malliorganismissa Sec1 proteiinin ja sitä sitovan Mso1 proteiinin toiminta kalvofuusiossa. Käyttäen uutta Bimolecular fluorescence complementation (BiFC) tekniikkaa, työssä osoitettiin, että Mso1 ja Sec1 muodostavat proteiiniparin solumembraanilla kohdassa missä polaarinen solujen kasvu ja proteiinieritys tapahtuu. Tämä vuorovaikutus oli riippuvainen Rab GTPaasi Sec4:stä ja sen säätelijästä Sec2 proteiinista. Mso1 ja Sec4 proteiinien osoitettiin interaktoivan suoraan toistensa kanssa. Tämä interaktio oli riippuvainen GTP:tä sitovan Sec4 proteiinin aktivaatioasteesta. Työssä selvitettiin mutageneesia käyttäen ne Mso1 ja Sec1 proteiinien alueet, jotka ovat välttämättömät membraanifuusiolle. Mso1:ssä tämä alue on N-terminaalinen peptidi ja Sec1:ssä domaini 1:n alue, jonka tiedetään nisäkässoluissa sitovan SNARE kompleksien Syntaksiinien N-terminaalista peptidiä. S. cerevisiaeen syntaksiineilta puuttuu N-terminaalinen peptidi. Tulokset osoittavat, että Mso1 korvaa toiminnallisesti syntaksiinien N-terminaalisen peptidin ja näin ollen edesauttaa membraanifuusiota. Tämän uudenlaisen kalvofuusiota säätelevän mekanismin lisäksi työssä karakterisoitiin Sec1 proteiinin hiivaspesifisen C-terminaalisen hännän merkitys membraanifuusiossa. Sec1 C-terminaalisen hännän poistaminen heikensi solujen kasvua ja niiden kykyä erittää proteiineja. In vivo ja in vitro lähetymistapoja käyttäen osoitettiin, että Sec1:n C-terminaalinen häntä myötävaikuttaa SNARE kompleksien muodostumiseen ja kykenee siten toimimaan SNARE kompleksin muodostumista säätelevänä tekijänä. Lisäksi osoitettiin että Mso1p kykenee suoraan, fyysiseen vuorovaikutukseen solukalvojen kanssa ja että tämä ominaisuus on välttämätön Mso1 proteiinin toiminnalle kalvofuusiossa. Mso1 proteiinin nisäkäsvastine Mint proteiini sitoo Alzheimerin taudin syntyyn vaikuttavaa amyloid precursor proteiinia (APP). Nyt saadut tuloksen paljastavat uudenlaisia ominaisuuksia tämän proteiiniperheen toiminnasta ja voivat tulavaisudessa edesauttaa APP:n muodostumista säätelevien molekulaaristen mekanismien selvitystyöta

The lipid transporter ORP2 regulates synaptic neurotransmitter release via two distinct mechanisms

Funding Information: We thank Marisa Brockmann and Gülcin Vardar for initial help with SynGCamp6f imaging and electrophysiology, respectively. We thank Katja Pötschke, Bettina Brokowski, Heike Lerch, Nadine Albrecht-Koepke, and Berit Söhl-Kielczynski for expert technical assistance and the Viral Core Facility of the Charité – Universitätsmedizin Berlin for lentivirus and AAV production. We thank the Core Facility for Electron Microscopy of the Charité for their support with the electron microscope. This study was supported by the Charité Universitätsmedizin Berlin (M.W.-B., J.K., T.T., C.R.), the German Research Council via a Reinhart Koselleck project (C.R.), the Lydia Rabinowitsch-Förderung (M.W.-B.), the Academy of Finland (grant 3222647 to V.M.O.), and the Sigrid Jusélius Foundation (V.M.O.). Publisher Copyright: © 2022 The AuthorsCholesterol is crucial for neuronal synaptic transmission, assisting in the molecular and structural organization of lipid rafts, ion channels, and exocytic proteins. Although cholesterol absence was shown to result in impaired neurotransmission, how cholesterol locally traffics and its route of action are still under debate. Here, we characterized the lipid transfer protein ORP2 in murine hippocampal neurons. We show that ORP2 preferentially localizes to the presynapse. Loss of ORP2 reduces presynaptic cholesterol levels by 50%, coinciding with a profoundly reduced release probability, enhanced facilitation, and impaired presynaptic calcium influx. In addition, ORP2 plays a cholesterol-transport-independent role in regulating vesicle priming and spontaneous release, likely by competing with Munc18-1 in syntaxin1A binding. To conclude, we identified a dual function of ORP2 as a physiological modulator of the synaptic cholesterol content and a regulator of neuronal exocytosis.Peer reviewe

ORP/Osh mediate cross-talk between ER-plasma membrane contact site components and plasma membrane SNAREs

OSBP-homologous proteins (ORPs, Oshp) are lipid binding/transfer proteins. Several ORP/Oshp localize to membrane contacts between the endoplasmic reticulum (ER) and the plasma membrane, where they mediate lipid transfer or regulate lipid-modifying enzymes. A common way in which they target contacts is by binding to the ER proteins, VAP/Scs2p, while the second membrane is targeted by other interactions with lipids or proteins. We have studied the cross-talk of secretory SNARE proteins and their regulators with ORP/Oshp and VAPA/Scs2p at ER-plasma membrane contact sites in yeast and murine primary neurons. We show that Oshp-Scs2p interactions depend on intact secretory SNARE proteins, especially Sec9p. SNAP-25/Sec9p directly interact with ORP/Osh proteins and their disruption destabilized the ORP/Osh proteins, associated with dysfunction of VAPA/Scs2p. DeletingOSH1-3in yeast or knocking down ORP2 in primary neurons reduced the oligomerization of VAPA/Scs2p and affected their multiple interactions with SNAREs. These observations reveal a novel cross-talk between the machineries of ER-plasma membrane contact sites and those driving exocytosis.Peer reviewe

ORP/Osh mediate cross-talk between ER-plasma membrane contact site components and plasma membrane SNAREs

OSBP-homologous proteins (ORPs, Oshp) are lipid binding/transfer proteins. Several ORP/Oshp localize to membrane contacts between the endoplasmic reticulum (ER) and the plasma membrane, where they mediate lipid transfer or regulate lipid-modifying enzymes. A common way in which they target contacts is by binding to the ER proteins, VAP/Scs2p, while the second membrane is targeted by other interactions with lipids or proteins. We have studied the cross-talk of secretory SNARE proteins and their regulators with ORP/Oshp and VAPA/Scs2p at ER-plasma membrane contact sites in yeast and murine primary neurons. We show that Oshp-Scs2p interactions depend on intact secretory SNARE proteins, especially Sec9p. SNAP-25/Sec9p directly interact with ORP/Osh proteins and their disruption destabilized the ORP/Osh proteins, associated with dysfunction of VAPA/Scs2p. Deleting OSH1-3 in yeast or knocking down ORP2 in primary neurons reduced the oligomerization of VAPA/Scs2p and affected their multiple interactions with SNAREs. These observations reveal a novel cross-talk between the machineries of ER-plasma membrane contact sites and those driving exocytosis

OSBP-related protein 3 (ORP3) coupling with VAMP-associated protein A regulates R-Ras activity

ORP3 is an R-Ras interacting oxysterol-binding protein homolog that regulates cell adhesion and is overexpressed in several cancers. We investigated here a novel function of ORP3 dependent on its targeting to both the endoplasmic reticulum (ER) and the plasma membrane (PM). Using biochemical and cell imaging techniques we demonstrate the mechanistic requirements for the subcellular targeting and function of ORP3 in control of R-Ras activity. We show that hyperphosphorylated ORP3 (ORP3-P) selectively interacts with the ER membrane protein VAPA, and ORP3–VAPA complexes are targeted to PM sites via the ORP3 pleckstrin homology (PH) domain. A novel FFAT (two phenylalanines in an acidic tract)-like motif was identified in ORP3; only disruption of both the FFAT-like and canonical FFAT motif abolished the phorbol-12-myristate-13-acetate (PMA) stimulated interaction of ORP3-P with VAPA. Co-expression of ORP3 and VAPA induced R-Ras activation, dependent on the interactions of ORP3 with VAPA and the PM. Consistently, downstream AktS473 phosphorylation and β1-integrin activity were enhanced by ORP3–VAPA. To conclude, phosphorylation of ORP3 controls its association with VAPA. Furthermore, we present evidence that ORP3–VAPA complexes stimulate R-Ras signaling

A loss-of-function variant in OSBPL1A predisposes to low plasma HDL cholesterol levels and impaired cholesterol efflux capacity

Background and aims: Among subjects with high-density-lipoprotein cholesterol (HDL-C) below the 1st percentile in the general population, we identified a heterozygous variant OSBPL1A p.C39X encoding a short truncated protein fragment that co-segregated with low plasma HDL-C. Methods: We investigated the composition and function of HDL from the carriers and non-carriers and studied the properties of the mutant protein in cultured hepatocytes. Results: Plasma HDL-C and apolipoprotein (apo) A-I were lower in carriers versus non-carriers, whereas the other analyzed plasma components or HDL parameters did not differ. Sera of the carriers displayed a reduced capacity to act as cholesterol efflux acceptors (p <0.01), whereas the cholesterol acceptor capacity of their isolated HDL was normal. Fibroblasts from a p.C39X carrier showed reduced cholesterol efflux to lipid-free apoA-I but not to mature HDL particles, suggesting a specific defect in ABCA1-mediated efflux pathway. In hepatic cells, GFP-OSBPL1A partially co-localized in endosomes containing fluorescent apoA-I, suggesting that OSBPL1A may regulate the intracellular handling of apoA-I. The GFP-OSBPL1A-39X mutant protein remained in the cytosol and failed to interact with Rab7, which normally recruits OSBPL1A to late endosomes/lysosomes, suggesting that this mutation represents a loss-of-function. Conclusions: The present work represents the first characterization of a human OSBPL1A mutation. Our observations provide evidence that a familial loss-of-function mutation in OSBPL1A affects the first step of the reverse cholesterol transport process and associates with a low HDL-C phenotype. This suggests that rare mutations in OSBPL genes may contribute to dyslipidemias. (C) 2016 Elsevier Ireland Ltd. All rights reserved.Peer reviewe

Oculocerebrorenal syndrome of Lowe (OCRL) controls leukemic T-cell survival by preventing excessive PI(4,5)P2 hydrolysis in the plasma membrane

T-cell acute lymphoblastic leukemia (T-ALL) is one of the deadliest and most aggressive hematological malignancies, but its pathological mechanism in controlling cell survival is not fully understood. Oculocerebrorenal syndrome of Lowe is a rare X-linked recessive disorder characterized by cataracts, intellectual disability, and proteinuria. This disease has been shown to be caused by mutation of oculocerebrorenal syndrome of Lowe 1 (OCRL1; OCRL), encoding a phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] 5-phosphatase involved in regulating membrane trafficking; however, its function in cancer cells is unclear. Here, we uncovered that OCRL1 is overexpressed in T-ALL cells, and knockdown of OCRL1 results in cell death, indicating the essential role of OCRL in controlling T-ALL cell survival. We show OCRL is primarily localized in the Golgi and can translocate to plasma membrane (PM) upon ligand stimulation. We found OCRL interacts with oxysterol-binding protein–related protein 4L, which facilitates OCRL translocation from the Golgi to the PM upon cluster of differentiation 3 stimulation. Thus, OCRL represses the activity of oxysterol-binding protein–related protein 4L to prevent excessive PI(4,5)P2 hydrolysis by phosphoinositide phospholipase C β3 and uncontrolled Ca2+ release from the endoplasmic reticulum. We propose OCRL1 deletion leads to accumulation of PI(4,5)P2 in the PM, disrupting the normal Ca2+ oscillation pattern in the cytosol and leading to mitochondrial Ca2+ overloading, ultimately causing T-ALL cell mitochondrial dysfunction and cell death. These results highlight a critical role for OCRL in maintaining moderate PI(4,5)P2 availability in T-ALL cells. Our findings also raise the possibility of targeting OCRL1 to treat T-ALL disease.Peer reviewe