title: Munc18-bound syntaxin readily forms SNARE complexes with synaptobrevin in native plasma membranes.
abstract: Munc18-1, a protein essential for regulated exocytosis in neurons and neuroendocrine cells, belongs to the family of Sec1/Munc18-like (SM) proteins. In vitro, Munc18-1 forms a tight complex with the SNARE syntaxin 1, in which syntaxin is stabilized in a closed conformation. Since closed syntaxin is unable to interact with its partner SNAREs SNAP-25 and synaptobrevin as required for membrane fusion, it has hitherto not been possible to reconcile binding of Munc18-1 to syntaxin 1 with its biological function. We now show that in intact and exocytosis-competent lawns of plasma membrane, Munc18-1 forms a complex with syntaxin that allows formation of SNARE complexes. Munc18-1 associated with membrane-bound syntaxin 1 can be effectively displaced by adding recombinant synaptobrevin but not syntaxin 1 or SNAP-25. Displacement requires the presence of endogenous SNAP-25 since no displacement is observed when chromaffin cell membranes from SNAP-25-deficient mice are used. We conclude that Munc18-1 allows for the formation of a complex between syntaxin and SNAP-25 that serves as an acceptor for vesicle-bound synaptobrevin and that thus represents an intermediate in the pathway towards exocytosis.
authors.affiliation: Department of Neurobiology, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany.
authors.forename: Felipe E
authors.initials: FE
authors.lastname: Zilly
authors.forename: Jakob B
authors.initials: JB
authors.lastname: Sørensen
authors.forename: Reinhard
authors.initials: R
authors.lastname: Jahn
authors.forename: Thorsten
authors.initials: T
authors.lastname: Lang
mesh_terms: D000818:Animals; D001665:Binding Sites; D002462:Cell Membrane / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D008954:Models, Biological; D051938:Munc18 Proteins / Q000502:physiology*; D016716:PC12 Cells; D010957:Plasmids / Q000378:metabolism; D011485:Protein Binding; D050765:Qa-SNARE Proteins / Q000378:metabolism*; D050683:R-SNARE Proteins / Q000378:metabolism*; D051381:Rats; D011994:Recombinant Proteins / Q000378:metabolism; D050600:SNARE Proteins / Q000378:metabolism*; D050988:Vesicle-Associated Membrane Protein 2 / Q000378:metabolism
chemical_list: D051938:Munc18 Proteins; D050765:Qa-SNARE Proteins; D050683:R-SNARE Proteins; D011994:Recombinant Proteins; D050600:SNARE Proteins; D050988:Vesicle-Associated Membrane Protein 2