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Science (New York, N.Y.) Jan 2009The two universally required components of the intracellular membrane fusion machinery, SNARE and SM (Sec1/Munc18-like) proteins, play complementary roles in fusion.... (Review)
Review
The two universally required components of the intracellular membrane fusion machinery, SNARE and SM (Sec1/Munc18-like) proteins, play complementary roles in fusion. Vesicular and target membrane-localized SNARE proteins zipper up into an alpha-helical bundle that pulls the two membranes tightly together to exert the force required for fusion. SM proteins, shaped like clasps, bind to trans-SNARE complexes to direct their fusogenic action. Individual fusion reactions are executed by distinct combinations of SNARE and SM proteins to ensure specificity, and are controlled by regulators that embed the SM-SNARE fusion machinery into a physiological context. This regulation is spectacularly apparent in the exquisite speed and precision of synaptic exocytosis, where synaptotagmin (the calcium-ion sensor for fusion) cooperates with complexin (the clamp activator) to control the precisely timed release of neurotransmitters that initiates synaptic transmission and underlies brain function.
Topics: Amino Acid Motifs; Animals; Membrane Fusion; Munc18 Proteins; Nerve Tissue Proteins; Protein Binding; Protein Conformation; Protein Structure, Quaternary; Protein Structure, Tertiary; Qa-SNARE Proteins; SNARE Proteins; Synapses; Synaptic Transmission; Synaptic Vesicles; Synaptotagmins; Vesicular Transport Proteins
PubMed: 19164740
DOI: 10.1126/science.1161748 -
Nature Jun 2023The endoplasmic reticulum and mitochondria are main hubs of eukaryotic membrane biogenesis that rely on lipid exchange via membrane contact sites, but the underpinning...
The endoplasmic reticulum and mitochondria are main hubs of eukaryotic membrane biogenesis that rely on lipid exchange via membrane contact sites, but the underpinning mechanisms remain poorly understood. In yeast, tethering and lipid transfer between the two organelles is mediated by the endoplasmic reticulum-mitochondria encounter structure (ERMES), a four-subunit complex of unresolved stoichiometry and architecture. Here we determined the molecular organization of ERMES within Saccharomyces cerevisiae cells using integrative structural biology by combining quantitative live imaging, cryo-correlative microscopy, subtomogram averaging and molecular modelling. We found that ERMES assembles into approximately 25 discrete bridge-like complexes distributed irregularly across a contact site. Each bridge consists of three synaptotagmin-like mitochondrial lipid binding protein domains oriented in a zig-zag arrangement. Our molecular model of ERMES reveals a pathway for lipids. These findings resolve the in situ supramolecular architecture of a major inter-organelle lipid transfer machinery and provide a basis for the mechanistic understanding of lipid fluxes in eukaryotic cells.
Topics: Endoplasmic Reticulum; Lipids; Mitochondria; Mitochondrial Membranes; Saccharomyces cerevisiae; Saccharomyces cerevisiae Proteins; Models, Molecular; Synaptotagmins
PubMed: 37165187
DOI: 10.1038/s41586-023-06050-3 -
Biochimica Et Biophysica Acta.... Sep 2017The extended-synaptotagmins (tricalbins in yeast) derive their name from their partial domain structure similarity to the synaptotagmins, which are characterized by an... (Review)
Review
The extended-synaptotagmins (tricalbins in yeast) derive their name from their partial domain structure similarity to the synaptotagmins, which are characterized by an N-terminal membrane anchor and cytosolically exposed C2 domains. However, they differ from the synaptotagmins in localization and function. The synaptotagmins tether secretory vesicles, including synaptic vesicles, to the plasma membrane (PM) via their C2 domains and regulate their Ca triggered exocytosis. In contrast, the extended-synaptotagmins are resident proteins of the endoplasmic reticulum (ER), which tether this organelle to the plasma membrane via their C2 domains, but not as a premise to fusion of the two membranes. They transport glycerolipids between the two bilayers via their lipid-harboring SMP domains and Ca regulates their membrane tethering and lipid transport function. Additionally, the extended-synaptotagmins are more widely expressed in different organisms, as they are present not only in animal cells, but also in fungi and plants, which do not express the synaptotagmins. Thus, they have a more general function than the synaptotagmins, whose appearance in animal species correlated with the occurrence of Ca triggered exocytosis. This article is part of a Special Issue entitled: Membrane Contact Sites edited by Christian Ungermann and Benoit Kornmann.
Topics: Animals; Cell Membrane; Endoplasmic Reticulum; Humans; Membrane Lipids; Synaptotagmins
PubMed: 28363589
DOI: 10.1016/j.bbamcr.2017.03.013 -
Cold Spring Harbor Perspectives in... Dec 2011Presynaptic nerve terminals release neurotransmitters by synaptic vesicle exocytosis. Membrane fusion mediating synaptic exocytosis and other intracellular membrane... (Review)
Review
Presynaptic nerve terminals release neurotransmitters by synaptic vesicle exocytosis. Membrane fusion mediating synaptic exocytosis and other intracellular membrane traffic is affected by a universal machinery that includes SNARE (for "soluble NSF-attachment protein receptor") and SM (for "Sec1/Munc18-like") proteins. During fusion, vesicular and target SNARE proteins assemble into an α-helical trans-SNARE complex that forces the two membranes tightly together, and SM proteins likely wrap around assembling trans-SNARE complexes to catalyze membrane fusion. After fusion, SNARE complexes are dissociated by the ATPase NSF (for "N-ethylmaleimide sensitive factor"). Fusion-competent conformations of SNARE proteins are maintained by chaperone complexes composed of CSPα, Hsc70, and SGT, and by nonenzymatically acting synuclein chaperones; dysfunction of these chaperones results in neurodegeneration. The synaptic membrane-fusion machinery is controlled by synaptotagmin, and additionally regulated by a presynaptic protein matrix (the "active zone") that includes Munc13 and RIM proteins as central components.
Topics: Animals; Exocytosis; GTP-Binding Proteins; Humans; Membrane Fusion; Mice; Models, Biological; Munc18 Proteins; Nerve Tissue Proteins; Neurotransmitter Agents; Protein Folding; SNARE Proteins; Synaptic Vesicles; Synaptotagmins
PubMed: 22026965
DOI: 10.1101/cshperspect.a005637 -
Nature Communications Aug 2022During pancreas development endocrine cells leave the ductal epithelium to form the islets of Langerhans, but the morphogenetic mechanisms are incompletely understood....
During pancreas development endocrine cells leave the ductal epithelium to form the islets of Langerhans, but the morphogenetic mechanisms are incompletely understood. Here, we identify the Ca-independent atypical Synaptotagmin-13 (Syt13) as a key regulator of endocrine cell egression and islet formation. We detect specific upregulation of the Syt13 gene and encoded protein in endocrine precursors and the respective lineage during islet formation. The Syt13 protein is localized to the apical membrane of endocrine precursors and to the front domain of egressing endocrine cells, marking a previously unidentified apical-basal to front-rear repolarization during endocrine precursor cell egression. Knockout of Syt13 impairs endocrine cell egression and skews the α-to-β-cell ratio. Mechanistically, Syt13 is a vesicle trafficking protein, transported via the microtubule cytoskeleton, and interacts with phosphatidylinositol phospholipids for polarized localization. By internalizing a subset of plasma membrane proteins at the front domain, including α6β4 integrins, Syt13 modulates cell-matrix adhesion and allows efficient endocrine cell egression. Altogether, these findings uncover an unexpected role for Syt13 as a morphogenetic driver of endocrinogenesis and islet formation.
Topics: Endocrine Cells; Integrins; Islets of Langerhans; Morphogenesis; Pancreas; Synaptotagmins
PubMed: 35927244
DOI: 10.1038/s41467-022-31862-8 -
Cellular and Molecular Life Sciences :... Nov 2015Despite intensive research, it is still unclear how an immediate and profound acceleration of exocytosis is triggered by appropriate Ca(2+)-stimuli in presynaptic... (Review)
Review
Despite intensive research, it is still unclear how an immediate and profound acceleration of exocytosis is triggered by appropriate Ca(2+)-stimuli in presynaptic terminals. This is due to the fact that the molecular mechanisms of "docking" and "priming" reactions, which set up secretory vesicles to fuse at millisecond time scale, are extremely hard to study. Yet, driven by a fruitful combination of in vitro and in vivo analyses, our mechanistic understanding of Ca(2+)-triggered vesicle fusion has certainly advanced in the past few years. In this review, we aim to highlight recent progress and emerging views on the molecular mechanisms, by which constitutively forming SNAREpins are organized in functional, tightly regulated units for synchronized release. In particular, we will focus on the role of the small regulatory factor complexin whose function in Ca(2+)-dependent exocytosis has been controversially discussed for more than a decade. Special emphasis will also be laid on the functional relationship of complexin and synaptotagmin, as both proteins possibly act as allies and/or antagonists to govern SNARE-mediated exocytosis.
Topics: Adaptor Proteins, Vesicular Transport; Calcium; Exocytosis; Humans; Membrane Fusion; Models, Biological; Nerve Tissue Proteins; Protein Binding; SNARE Proteins; Synaptic Vesicles; Synaptotagmins
PubMed: 26245303
DOI: 10.1007/s00018-015-1998-8 -
Scientific Reports Dec 2022Synaptotagmin-1 is a vesicular protein and Ca sensor for Ca-dependent exocytosis. Ca induces synaptotagmin-1 binding to its own vesicle membrane, called the...
Synaptotagmin-1 is a vesicular protein and Ca sensor for Ca-dependent exocytosis. Ca induces synaptotagmin-1 binding to its own vesicle membrane, called the cis-interaction, thus preventing the trans-interaction of synaptotagmin-1 to the plasma membrane. However, the electrostatic regulation of the cis- and trans-membrane interaction of synaptotagmin-1 was poorly understood in different Ca-buffering conditions. Here we provide an assay to monitor the cis- and trans-membrane interactions of synaptotagmin-1 by using native purified vesicles and the plasma membrane-mimicking liposomes (PM-liposomes). Both ATP and EGTA similarly reverse the cis-membrane interaction of synaptotagmin-1 in free [Ca] of 10-100 μM. High PIP concentrations in the PM-liposomes reduce the Hill coefficient of vesicle fusion and synaptotagmin-1 membrane binding; this observation suggests that local PIP concentrations control the Ca-cooperativity of synaptotagmin-1. Our data provide evidence that Ca chelators, including EGTA and polyphosphate anions such as ATP, ADP, and AMP, electrostatically reverse the cis-interaction of synaptotagmin-1.
Topics: Liposomes; Static Electricity; Egtazic Acid; Synaptotagmin I; Cell Membrane; Membrane Fusion; Exocytosis; Adenosine Triphosphate; Calcium; Synaptotagmins; SNARE Proteins
PubMed: 36575295
DOI: 10.1038/s41598-022-26723-9 -
Journal of Molecular Biology Jan 2023Synaptic neurotransmitter release is mediated by an orchestra of presynaptic proteins that precisely control and trigger fusion between synaptic vesicles and the neuron... (Review)
Review
Synaptic neurotransmitter release is mediated by an orchestra of presynaptic proteins that precisely control and trigger fusion between synaptic vesicles and the neuron terminal at the active zone upon the arrival of an action potential. Critical to this process are the neuronal SNAREs (Soluble N-ethylmaleimide sensitive factor Attachment protein REceptor), the Ca-sensor synaptotagmin, the activator/regulator complexin, and other factors. Here, we review the interactions between the SNARE complex and synaptotagmin, with focus on the so-called primary interface between synaptotagmin and the SNARE complex that has been validated in terms of its physiological relevance. We discuss several other but less validated interfaces as well, including the so-called tripartite interface, and we discuss the pros and cons for these possible alternative interfaces. We also present new molecular dynamics simulations of the tripartite interface and new data of an inhibitor of the primary interface in a reconstituted system of synaptic vesicle fusion.
Topics: Calcium; Membrane Fusion; Neurons; SNARE Proteins; Synaptic Transmission; Synaptic Vesicles; Synaptotagmins
PubMed: 36243149
DOI: 10.1016/j.jmb.2022.167853 -
Acta Physiologica (Oxford, England) Sep 2022SYT11 and SYT13, two calcium-insensitive synaptotagmins, are downregulated in islets from type 2 diabetic donors, but their function in insulin secretion is unknown. To...
AIM
SYT11 and SYT13, two calcium-insensitive synaptotagmins, are downregulated in islets from type 2 diabetic donors, but their function in insulin secretion is unknown. To address this, we investigated the physiological role of these two synaptotagmins in insulin-secreting cells.
METHODS
Correlations between gene expression levels were performed using previously described RNA-seq data on islets from 188 human donors. SiRNA knockdown was performed in EndoC-βH1 and INS-1 832/13 cells. Insulin secretion was measured with ELISA. Patch-clamp was used for single-cell electrophysiology. Confocal microscopy was used to determine intracellular localization.
RESULTS
Human islet expression of the transcription factor PDX1 was positively correlated with SYT11 (p = 2.4e ) and SYT13 (p < 2.2e ). Syt11 and Syt13 both co-localized with insulin, indicating their localization in insulin granules. Downregulation of Syt11 in INS-1 832/13 cells (siSYT11) resulted in increased basal and glucose-induced insulin secretion. Downregulation of Syt13 (siSYT13) decreased insulin secretion induced by glucose and K . Interestingly, the cAMP-raising agent forskolin was unable to enhance insulin secretion in siSYT13 cells. There was no difference in insulin content, exocytosis, or voltage-gated Ca currents in the two models. Double knockdown of Syt11 and Syt13 (DKD) resembled the results in siSYT13 cells.
CONCLUSION
SYT11 and SYT13 have similar localization and transcriptional regulation, but they regulate insulin secretion differentially. While downregulation of SYT11 might be a compensatory mechanism in type-2 diabetes, downregulation of SYT13 reduces the insulin secretory response and overrules the compensatory regulation of SYT11 in a way that could aggravate the disease.
Topics: Calcium; Glucose; Humans; Insulin; Insulin Secretion; Insulin-Secreting Cells; Synaptotagmins
PubMed: 35753051
DOI: 10.1111/apha.13857 -
Journal of Neurochemistry Apr 2021The revolution in genetic technology has ushered in a new age for our understanding of the underlying causes of neurodevelopmental, neuromuscular and neurodegenerative... (Review)
Review
The revolution in genetic technology has ushered in a new age for our understanding of the underlying causes of neurodevelopmental, neuromuscular and neurodegenerative disorders, revealing that the presynaptic machinery governing synaptic vesicle fusion is compromised in many of these neurological disorders. This builds upon decades of research showing that disturbance to neurotransmitter release via toxins can cause acute neurological dysfunction. In this review, we focus on disorders of synaptic vesicle fusion caused either by toxic insult to the presynapse or alterations to genes encoding the key proteins that control and regulate fusion: the SNARE proteins (synaptobrevin, syntaxin-1 and SNAP-25), Munc18, Munc13, synaptotagmin, complexin, CSPα, α-synuclein, PRRT2 and tomosyn. We discuss the roles of these proteins and the cellular and molecular mechanisms underpinning neurological deficits in these disorders.
Topics: Animals; Exocytosis; Humans; Membrane Fusion; Neurons; Synaptic Transmission; Synaptic Vesicles; Synaptotagmins
PubMed: 32916768
DOI: 10.1111/jnc.15181