Interaction of the Complexin Accessory Helix with Synaptobrevin Regulates Spontaneous Fusion.
Neuronal transmitters are released from nerve terminals via the fusion of synaptic vesicles with the plasma membrane. Vesicles attach to membranes via a specialized protein machinery composed of membrane-attached (t-SNARE) and vesicle-attached (v-SNARE) proteins that zipper together to form a coiled-coil SNARE bundle that brings the two fusing membranes into close proximity. Neurotransmitter release may occur either in response to an action potential or through spontaneous fusion. A cytosolic protein, Complexin (Cpx), binds the SNARE complex and restricts spontaneous exocytosis by acting as a fusion clamp. We previously proposed a model in which the interaction between Cpx and the v-SNARE serves as a spring to prevent premature zippering of the SNARE complex, thereby reducing the likelihood of fusion. To test this model, we combined molecular-dynamics (MD) simulations and site-directed mutagenesis of Cpx and SNAREs in Drosophila. MD simulations of the Drosophila Cpx-SNARE complex demonstrated that Cpx's interaction with the v-SNARE promotes unraveling of the v-SNARE off the core SNARE bundle. We investigated clamping properties in the syx3-69paralytic mutant, which has a single-point mutation in the t-SNARE and displays enhanced spontaneous release. MD simulations demonstrated an altered interaction of Cpx with the SNARE bundle that hindered v-SNARE unraveling by Cpx, thus compromising clamping. We used our model to predict mutations that should enhance the ability of Cpx to prevent full assembly of the SNARE complex. MD simulations predicted that a weakened interaction between the Cpx accessory helix and the v-SNARE would enhance Cpx flexibility and thus promote separation of SNAREs, reducing spontaneous fusion. We generated transgenic Drosophila with mutations in Cpx and the v-SNARE that disrupted a salt bridge between these two proteins. As predicted, both lines demonstrated a selective inhibition in spontaneous release, suggesting that Cpx acts as a fusion clamp that restricts full SNARE zippering.
Department of Neurology, Wayne State University, Detroit, Michigan.
Alexander
A
Vasin
Massachusetts Institute of Technology, Cambridge, Massachusetts.
Dina
D
Volfson
Massachusetts Institute of Technology, Cambridge, Massachusetts.
J Troy
JT
Littleton
Department of Neurology, Wayne State University, Detroit, Michigan; Department of Anatomy and Cell Biology, Wayne State University, Detroit, Michigan. Electronic address: mbykhovs@med.wayne.edu.
Maria
M
Bykhovskaia
D033942:Adaptor Proteins, Vesicular Transport / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D029721:Drosophila Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D004331:Drosophila melanogaster; D006860:Hydrogen Bonding; D008561:Membrane Fusion; D056004:Molecular Dynamics Simulation; D009154:Mutation; D009419:Nerve Tissue Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D017354:Point Mutation; D011485:Protein Binding; D000072756:Protein Conformation, alpha-Helical; D050765:Qa-SNARE Proteins / Q000235:genetics / Q000378:metabolism; D050683:R-SNARE Proteins / Q000235:genetics / Q000378:metabolism*; D055672:Static Electricity; D013572:Synaptic Vesicles / Q000378:metabolism
D033942:Adaptor Proteins, Vesicular Transport; D029721:Drosophila Proteins; D009419:Nerve Tissue Proteins; D050765:Qa-SNARE Proteins; D050683:R-SNARE Proteins; C000605369:cpx protein, Drosophila