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Helical extension of the neuronal SNARE complex into the membrane.

Stein A, Weber G, Wahl MC, Jahn R - Nature (2009)

Bottom Line: Here, we report the X-ray structure of the neuronal SNARE complex, consisting of rat syntaxin 1A, SNAP-25 and synaptobrevin 2, with the carboxy-terminal linkers and transmembrane regions at 3.4 A resolution.The structure shows that assembly proceeds beyond the already known core SNARE complex, resulting in a continuous helical bundle that is further stabilized by side-chain interactions in the linker region.Our results suggest that the final phase of SNARE assembly is directly coupled to membrane merger.

View Article: PubMed Central - PubMed

Affiliation: Department of Neurobiology, Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany.

ABSTRACT
Neurotransmission relies on synaptic vesicles fusing with the membrane of nerve cells to release their neurotransmitter content into the synaptic cleft, a process requiring the assembly of several members of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) family. SNAREs represent an evolutionarily conserved protein family that mediates membrane fusion in the secretory and endocytic pathways of eukaryotic cells. On membrane contact, these proteins assemble in trans between the membranes as a bundle of four alpha-helices, with the energy released during assembly being thought to drive fusion. However, it is unclear how the energy is transferred to the membranes and whether assembly is conformationally linked to fusion. Here, we report the X-ray structure of the neuronal SNARE complex, consisting of rat syntaxin 1A, SNAP-25 and synaptobrevin 2, with the carboxy-terminal linkers and transmembrane regions at 3.4 A resolution. The structure shows that assembly proceeds beyond the already known core SNARE complex, resulting in a continuous helical bundle that is further stabilized by side-chain interactions in the linker region. Our results suggest that the final phase of SNARE assembly is directly coupled to membrane merger.

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Model of the synaptic SNARE complex inserted in a membrane. As a landmark, aromatic residues (black sticks) within the linker region (grey) are shown. The hydrophilic head groups of the phospholipids are shown as balls, their aliphatic chains as sticks. The position of the complex in the palmitoyl-oleoyl-phosphatidylethanolamine (POPE)-bilayer was estimated from a short (54.9 ns) molecular dynamics simulation, where the apolar parts of the TMRs were initially centered within the hydrophobic part of the bilayer. Since PE head groups are highly abundant in animal membranes and PO is a relatively short tail group, POPE lipids were chosen to mimic a simple membrane with a thickness of approx. 4.5–5.0 nm.
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Figure 4: Model of the synaptic SNARE complex inserted in a membrane. As a landmark, aromatic residues (black sticks) within the linker region (grey) are shown. The hydrophilic head groups of the phospholipids are shown as balls, their aliphatic chains as sticks. The position of the complex in the palmitoyl-oleoyl-phosphatidylethanolamine (POPE)-bilayer was estimated from a short (54.9 ns) molecular dynamics simulation, where the apolar parts of the TMRs were initially centered within the hydrophobic part of the bilayer. Since PE head groups are highly abundant in animal membranes and PO is a relatively short tail group, POPE lipids were chosen to mimic a simple membrane with a thickness of approx. 4.5–5.0 nm.

Mentions: Figure 4 shows a model in which the SNARE complex was placed into a lipid bilayer. It represents the final stage of SNARE-mediated membrane fusion in which both TMRs come to rest in the same bilayer (cis-complex). However, it raises interesting questions about the structural features of the complex before zippering is completed (trans-complex). While it is conceivable that in a partially zippered complex the non-assembled regions between the N-terminally assembled helical parts and the (presumably) helical transmembrane domains are unstructured, it is also possible that small helical regions exist that may provide a binding site for late regulatory proteins such as synaptotagmin and complexin. The model illustrates that the four-helix bundle of the SNARE motifs sits as a perpendicular rod directly on top of the phospholipid bilayer. With the linkers of synaptobrevin 2 and syntaxin 1A buried in the top layer of the membrane it is difficult to imagine how the four-helix bundle can be fully zippered before membrane merger is at least initiated but we cannot exclude that zippering is confined to SNARE complexes surrounding the neck of a fusion pore that may form only transiently (kiss-and-run mode). Accordingly, it is understandable that structural perturbations in the linkers are less disruptive for fusion and may be at least partially compensated by engaging more SNARE complexes in the fusion reaction. In summary, our data strongly support the view that SNAREs constitute a robust and simple fusion catalyst. SNAREs operate as nanomachines in which the energy barrier of membrane fusion is overcome by the energy provided during progressive N-C zippering of the SNARE proteins all the way into the membrane.


Helical extension of the neuronal SNARE complex into the membrane.

Stein A, Weber G, Wahl MC, Jahn R - Nature (2009)

Model of the synaptic SNARE complex inserted in a membrane. As a landmark, aromatic residues (black sticks) within the linker region (grey) are shown. The hydrophilic head groups of the phospholipids are shown as balls, their aliphatic chains as sticks. The position of the complex in the palmitoyl-oleoyl-phosphatidylethanolamine (POPE)-bilayer was estimated from a short (54.9 ns) molecular dynamics simulation, where the apolar parts of the TMRs were initially centered within the hydrophobic part of the bilayer. Since PE head groups are highly abundant in animal membranes and PO is a relatively short tail group, POPE lipids were chosen to mimic a simple membrane with a thickness of approx. 4.5–5.0 nm.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3108252&req=5

Figure 4: Model of the synaptic SNARE complex inserted in a membrane. As a landmark, aromatic residues (black sticks) within the linker region (grey) are shown. The hydrophilic head groups of the phospholipids are shown as balls, their aliphatic chains as sticks. The position of the complex in the palmitoyl-oleoyl-phosphatidylethanolamine (POPE)-bilayer was estimated from a short (54.9 ns) molecular dynamics simulation, where the apolar parts of the TMRs were initially centered within the hydrophobic part of the bilayer. Since PE head groups are highly abundant in animal membranes and PO is a relatively short tail group, POPE lipids were chosen to mimic a simple membrane with a thickness of approx. 4.5–5.0 nm.
Mentions: Figure 4 shows a model in which the SNARE complex was placed into a lipid bilayer. It represents the final stage of SNARE-mediated membrane fusion in which both TMRs come to rest in the same bilayer (cis-complex). However, it raises interesting questions about the structural features of the complex before zippering is completed (trans-complex). While it is conceivable that in a partially zippered complex the non-assembled regions between the N-terminally assembled helical parts and the (presumably) helical transmembrane domains are unstructured, it is also possible that small helical regions exist that may provide a binding site for late regulatory proteins such as synaptotagmin and complexin. The model illustrates that the four-helix bundle of the SNARE motifs sits as a perpendicular rod directly on top of the phospholipid bilayer. With the linkers of synaptobrevin 2 and syntaxin 1A buried in the top layer of the membrane it is difficult to imagine how the four-helix bundle can be fully zippered before membrane merger is at least initiated but we cannot exclude that zippering is confined to SNARE complexes surrounding the neck of a fusion pore that may form only transiently (kiss-and-run mode). Accordingly, it is understandable that structural perturbations in the linkers are less disruptive for fusion and may be at least partially compensated by engaging more SNARE complexes in the fusion reaction. In summary, our data strongly support the view that SNAREs constitute a robust and simple fusion catalyst. SNAREs operate as nanomachines in which the energy barrier of membrane fusion is overcome by the energy provided during progressive N-C zippering of the SNARE proteins all the way into the membrane.

Bottom Line: Here, we report the X-ray structure of the neuronal SNARE complex, consisting of rat syntaxin 1A, SNAP-25 and synaptobrevin 2, with the carboxy-terminal linkers and transmembrane regions at 3.4 A resolution.The structure shows that assembly proceeds beyond the already known core SNARE complex, resulting in a continuous helical bundle that is further stabilized by side-chain interactions in the linker region.Our results suggest that the final phase of SNARE assembly is directly coupled to membrane merger.

View Article: PubMed Central - PubMed

Affiliation: Department of Neurobiology, Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany.

ABSTRACT
Neurotransmission relies on synaptic vesicles fusing with the membrane of nerve cells to release their neurotransmitter content into the synaptic cleft, a process requiring the assembly of several members of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) family. SNAREs represent an evolutionarily conserved protein family that mediates membrane fusion in the secretory and endocytic pathways of eukaryotic cells. On membrane contact, these proteins assemble in trans between the membranes as a bundle of four alpha-helices, with the energy released during assembly being thought to drive fusion. However, it is unclear how the energy is transferred to the membranes and whether assembly is conformationally linked to fusion. Here, we report the X-ray structure of the neuronal SNARE complex, consisting of rat syntaxin 1A, SNAP-25 and synaptobrevin 2, with the carboxy-terminal linkers and transmembrane regions at 3.4 A resolution. The structure shows that assembly proceeds beyond the already known core SNARE complex, resulting in a continuous helical bundle that is further stabilized by side-chain interactions in the linker region. Our results suggest that the final phase of SNARE assembly is directly coupled to membrane merger.

Show MeSH
Related in: MedlinePlus