title: Exocytotic fusion pores are composed of both lipids and proteins.
abstract: During exocytosis, fusion pores form the first aqueous connection that allows escape of neurotransmitters and hormones from secretory vesicles. Although it is well established that SNARE proteins catalyze fusion, the structure and composition of fusion pores remain unknown. Here, we exploited the rigid framework and defined size of nanodiscs to interrogate the properties of reconstituted fusion pores, using the neurotransmitter glutamate as a content-mixing marker. Efficient Ca(2+)-stimulated bilayer fusion, and glutamate release, occurred with approximately two molecules of mouse synaptobrevin 2 reconstituted into ∼6-nm nanodiscs. The transmembrane domains of SNARE proteins assumed distinct roles in lipid mixing versus content release and were exposed to polar solvent during fusion. Additionally, tryptophan substitutions at specific positions in these transmembrane domains decreased glutamate flux. Together, these findings indicate that the fusion pore is a hybrid structure composed of both lipids and proteins.
authors.affiliation: Department of Neuroscience, University of Wisconsin-Madison, Madison, Wisconsin, USA.|Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, Wisconsin, USA.
authors.forename: Huan
authors.initials: H
authors.lastname: Bao
authors.affiliation: Department of Neuroscience, University of Wisconsin-Madison, Madison, Wisconsin, USA.
authors.forename: Marcel
authors.initials: M
authors.lastname: Goldschen-Ohm
authors.affiliation: Department of Pharmacology, University of Cambridge, Cambridge, UK.
authors.forename: Pia
authors.initials: P
authors.lastname: Jeggle
authors.affiliation: Department of Neuroscience, University of Wisconsin-Madison, Madison, Wisconsin, USA.|Department of Biomolecular Chemistry, University of Wisconsin-Madison, Madison, Wisconsin, USA.
authors.forename: Baron
authors.identifier: http://orcid.org/0000-0003-4954-7034
authors.initials: B
authors.lastname: Chanda
authors.affiliation: Department of Pharmacology, University of Cambridge, Cambridge, UK.
authors.forename: J Michael
authors.initials: JM
authors.lastname: Edwardson
authors.affiliation: Department of Neuroscience, University of Wisconsin-Madison, Madison, Wisconsin, USA.|Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, Wisconsin, USA.
authors.forename: Edwin R
authors.initials: ER
authors.lastname: Chapman
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D005089:Exocytosis; D018698:Glutamic Acid / Q000378:metabolism; D008051:Lipid Bilayers / Q000378:metabolism*; D008561:Membrane Fusion; D051379:Mice; D050600:SNARE Proteins / Q000378:metabolism*; D022142:Secretory Vesicles / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D008051:Lipid Bilayers; D050600:SNARE Proteins; D018698:Glutamic Acid; D002118:Calcium