SNAREpin assembly by Munc18-1 requires previous vesicle docking by synaptotagmin 1.
Regulated exocytosis requires the general membrane fusion machinery-soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) and Sec1/Munc18 (SM) proteins. Using reconstituted giant unilamellar vesicles containing preassembled t-SNARE proteins (syntaxin 1·SNAP-25), we determined how Munc18-1 controls the docking, priming, and fusion of small unilamellar vesicles containing the v-SNARE VAMP2 and the Ca(2+) sensor synaptotagmin 1. In vitro assays allowed us to position Munc18-1 in the center of a sequential reaction cascade; vesicle docking by synaptotagmin 1 is a prerequisite for Munc18-1 to accelerate trans-SNARE complex (SNAREpin) assembly and membrane fusion. Complexin II stalls SNAREpin zippering at a late stage and, hence, contributes to synchronize membrane fusion in a Ca(2+)- and synaptotagmin 1-dependent manner. Thus, at the neuronal synapse, the priming factor Munc18-1 may accelerate the conversion of docked synaptic vesicles into a readily releasable pool by activating SNAREs for efficient membrane fusion.
Heidelberg University Biochemistry Center, Im Neuenheimer Feld 328, 69120 Heidelberg, Germany.
Daniel
D
Parisotto
Jörg
J
Malsam
Andrea
A
Scheutzow
Jean Michel
JM
Krause
Thomas H
TH
Söllner
D000818:Animals; D002474:Cell-Free System; D008561:Membrane Fusion / Q000502:physiology*; D051379:Mice; D051938:Munc18 Proteins / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D050600:SNARE Proteins / Q000235:genetics / Q000378:metabolism*; D013570:Synaptic Membranes / Q000235:genetics / Q000378:metabolism*; D013572:Synaptic Vesicles / Q000235:genetics / Q000378:metabolism*; D050863:Synaptotagmin I / Q000235:genetics / Q000378:metabolism*
D051938:Munc18 Proteins; D050600:SNARE Proteins; C497938:Stxbp1 protein, mouse; C497939:Stxbp1 protein, rat; D050863:Synaptotagmin I; C494311:Syt1 protein, mouse; C494312:Syt1 protein, rat