Extensive research has yielded crucial insights into the mechanism of neurotransmitter release, and working models for the functions of key proteins involved in release. The SNAREs Syntaxin-1, ...Synaptobrevin, and SNAP-25 play a central role in membrane fusion, forming SNARE complexes that bridge the vesicle and plasma membranes and that are disassembled by NSF-SNAPs. Exocytosis likely starts with Syntaxin-1 folded into a self-inhibited closed conformation that binds to Munc18-1. Munc13s open Syntaxin-1, orchestrating SNARE complex assembly in an NSF-SNAP-resistant manner together with Munc18-1. In the resulting primed state, with partially assembled SNARE complexes, fusion is inhibited by Synaptotagmin-1 and Complexins, which also perform active functions in release. Upon influx of Ca(2+), Synaptotagmin-1 activates fast release, likely by relieving the inhibition caused by Complexins and cooperating with the SNAREs in bringing the membranes together. Although alternative models exist and fundamental questions remain unanswered, a definitive description of the basic release mechanism may be available soon.
Recent studies suggest revisions to the SNARE paradigm of membrane fusion. Membrane tethers and/or SNAREs recruit proteins of the Sec 1/Munc18 family to catalyze SNARE assembly into
-complexes. ...SNARE-domain zippering draws the bilayers into immediate apposition and provides a platform to position fusion triggers such as Sec 17/α-SNAP and/or synaptotagmin, which insert their apolar "wedge" domains into the bilayers, initiating the lipid rearrangements of fusion.
During the priming step that leaves synaptic vesicles ready for neurotransmitter release, the SNARE syntaxin-1 transitions from a closed conformation that binds Munc18-1 tightly to an open ...conformation within the highly stable SNARE complex. Control of this conformational transition is important for brain function, but the underlying mechanism is unknown. NMR and fluorescence experiments now show that the Munc13-1 MUN domain, which plays a central role in vesicle priming, markedly accelerates the transition from the syntaxin-1-Munc18-1 complex to the SNARE complex. This activity depends on weak interactions of the MUN domain with the syntaxin-1 SNARE motif, and probably with Munc18-1. Together with available physiological data, these results provide a defined molecular basis for synaptic vesicle priming, and they illustrate how weak protein-protein interactions can play crucial biological roles by promoting transitions between high-affinity macromolecular assemblies.
Major recent advances and previous data have led to a plausible model of how key proteins mediate neurotransmitter release. In this model, the soluble
-ethylmaleimide-sensitive factor (NSF) ...attachment protein (SNAP) receptor (SNARE) proteins syntaxin-1, SNAP-25, and synaptobrevin form tight complexes that bring the membranes together and are crucial for membrane fusion. NSF and SNAPs disassemble SNARE complexes and ensure that fusion occurs through an exquisitely regulated pathway that starts with Munc18-1 bound to a closed conformation of syntaxin-1. Munc18-1 also binds to synaptobrevin, forming a template to assemble the SNARE complex when Munc13-1 opens syntaxin-1 while bridging the vesicle and plasma membranes. Synaptotagmin-1 and complexin bind to partially assembled SNARE complexes, likely stabilizing them and preventing fusion until Ca
binding to synaptotagmin-1 causes dissociation from the SNARE complex and induces interactions with phospholipids that help trigger release. Although fundamental questions remain about the mechanism of membrane fusion, these advances provide a framework to investigate the mechanisms underlying presynaptic plasticity.
Research for three decades and major recent advances have provided crucial insights into how neurotransmitters are released by Ca2+‐triggered synaptic vesicle exocytosis, leading to reconstitution of ...basic steps that underlie Ca2+‐dependent membrane fusion and yielding a model that assigns defined functions for central components of the release machinery. The soluble N‐ethyl maleimide sensitive factor attachment protein receptors (SNAREs) syntaxin‐1, SNAP‐25, and synaptobrevin‐2 form a tight SNARE complex that brings the vesicle and plasma membranes together and is key for membrane fusion. N‐ethyl maleimide sensitive factor (NSF) and soluble NSF attachment proteins (SNAPs) disassemble the SNARE complex to recycle the SNAREs for another round of fusion. Munc18‐1 and Munc13‐1 orchestrate SNARE complex formation in an NSF‐SNAP‐resistant manner by a mechanism whereby Munc18‐1 binds to synaptobrevin and to a self‐inhibited “closed” conformation of syntaxin‐1, thus forming a template to assemble the SNARE complex, and Munc13‐1 facilitates assembly by bridging the vesicle and plasma membranes and catalyzing opening of syntaxin‐1. Synaptotagmin‐1 functions as the major Ca2+ sensor that triggers release by binding to membrane phospholipids and to the SNAREs, in a tight interplay with complexins that accelerates membrane fusion. Many of these proteins act as both inhibitors and activators of exocytosis, which is critical for the exquisite regulation of neurotransmitter release. It is still unclear how the actions of these various proteins and multiple other components that control release are integrated and, in particular, how they induce membrane fusion, but it can be expected that these fundamental questions can be answered in the near future, building on the extensive knowledge already available.
Neurotransmitter release depends critically on Munc18-1, Munc13, the Ca 2+ sensor synaptotagmin-1, and the soluble N-ethylmaleimide—sensitive factor (NSF) attachment protein (SNAP) receptors (SNAREs) ...syntaxin-1, synaptobrevin, and SNAP-25. In vitro reconstitutions have shown that syntaxin-1—SNAP-25 liposomes fuse efficiently with synaptobrevin liposomes in the presence of synaptotagmin-1—Ca 2+ , but neurotransmitter release also requires Munc18-1 and Munc13 in vivo. We found that Munc18-1 could displace SNAP-25 from syntaxin-1 and that fusion of syntaxin-1—Munc18-1 liposomes with synaptobrevin liposomes required Munc13, in addition to SNAP-25 and synaptotagmin-1-Ca 2+ . Moreover, when starting with syntaxin-1—SNAP-25 liposomes, NSF—α-SNAP disassembled the syntaxin-1—SNAP-25 heterodimers and abrogated fusion, which then required Munc18-1 and Munc13. We propose that fusion does not proceed through syntaxin-1—SNAP-25 heterodimers but starts with the syntaxin-1—Munc18-1 complex; Munc18-1 and Munc13 then orchestrate membrane fusion together with the SNAREs and synaptotagmin-1-Ca 2+ in an NSF- and SNAP-resistant manner.
MLKL is crucial for necroptosis, permeabilizing membranes through its N-terminal region upon phosphorylation of its kinase-like domain by RIP3. However, the mechanism underlying membrane ...permeabilization is unknown. The solution structure of the MLKL N-terminal region determined by nuclear magnetic resonance spectroscopy reveals a four-helix bundle with an additional helix at the top that is likely key for MLKL function, and a sixth, C-terminal helix that interacts with the top helix and with a poorly packed interface within the four-helix bundle. Fluorescence spectroscopy measurements indicate that much of the four-helix bundle inserts into membranes, but not the C-terminal helix. Moreover, we find that the four-helix bundle is sufficient to induce liposome leakage and that the C-terminal helix inhibits this activity. These results suggest that the four-helix bundle mediates membrane breakdown during necroptosis and that the sixth helix acts as a plug that prevents opening of the bundle and is released upon RIP3 phosphorylation.
Display omitted
•The solution structure of the MLKL N-terminal domain has been elucidated•It contains a four-helix bundle, a helix at the top and a sixth C-terminal helix•The four-helix bundle is responsible for membrane permeation by MLKL•The sixth helix likely acts as a plug that inhibits the four-helix bundle
Membrane permeabilization by MLKL is critical for necropotosis. Su et al. show that the MLKL N-terminal region forms an autonomously folded domain with a four-helix bundle, which mediates membrane permeabilization, and a C-terminal helix that inhibits this activity and provides a means to regulate necropoptosis.
Computed structures of core eukaryotic protein complexes Humphreys, Ian R; Pei, Jimin; Baek, Minkyung ...
Science (American Association for the Advancement of Science),
2021-Dec-10, 2021-12-10, 20211210, Volume:
374, Issue:
6573
Journal Article
Peer reviewed
Open access
Protein-protein interactions play critical roles in biology, but the structures of many eukaryotic protein complexes are unknown, and there are likely many interactions not yet identified. We take ...advantage of advances in proteome-wide amino acid coevolution analysis and deep-learning–based structure modeling to systematically identify and build accurate models of core eukaryotic protein complexes within the
proteome. We use a combination of RoseTTAFold and AlphaFold to screen through paired multiple sequence alignments for 8.3 million pairs of yeast proteins, identify 1505 likely to interact, and build structure models for 106 previously unidentified assemblies and 806 that have not been structurally characterized. These complexes, which have as many as five subunits, play roles in almost all key processes in eukaryotic cells and provide broad insights into biological function.
Synaptic vesicles are primed into a state that is ready for fast neurotransmitter release upon Ca
-binding to Synaptotagmin-1. This state likely includes trans-SNARE complexes between the vesicle and ...plasma membranes that are bound to Synaptotagmin-1 and complexins. However, the nature of this state and the steps leading to membrane fusion are unclear, in part because of the difficulty of studying this dynamic process experimentally. To shed light into these questions, we performed all-atom molecular dynamics simulations of systems containing trans-SNARE complexes between two flat bilayers or a vesicle and a flat bilayer with or without fragments of Synaptotagmin-1 and/or complexin-1. Our results need to be interpreted with caution because of the limited simulation times and the absence of key components, but suggest mechanistic features that may control release and help visualize potential states of the primed Synaptotagmin-1-SNARE-complexin-1 complex. The simulations suggest that SNAREs alone induce formation of extended membrane-membrane contact interfaces that may fuse slowly, and that the primed state contains macromolecular assemblies of trans-SNARE complexes bound to the Synaptotagmin-1 C
B domain and complexin-1 in a spring-loaded configuration that prevents premature membrane merger and formation of extended interfaces, but keeps the system ready for fast fusion upon Ca
influx.
Munc13-1 plays a crucial role in neurotransmitter release. We recently proposed that the C-terminal region encompassing the C
, C
B, MUN and C
C domains of Munc13-1 (C
C
BMUNC
C) bridges the synaptic ...vesicle and plasma membranes through interactions involving the C
C domain and the C
-C
B region. However, the physiological relevance of this model has not been demonstrated. Here we show that C
C
BMUNC
C bridges membranes through opposite ends of its elongated structure. Mutations in putative membrane-binding sites of the C
C domain disrupt the ability of C
C
BMUNC
C to bridge liposomes and to mediate liposome fusion in vitro. These mutations lead to corresponding disruptive effects on synaptic vesicle docking, priming, and Ca
-triggered neurotransmitter release in mouse neurons. Remarkably, these effects include an almost complete abrogation of release by a single residue substitution in this 200 kDa protein. These results show that bridging the synaptic vesicle and plasma membranes is a central function of Munc13-1.
PDF
