title: Calcium-triggered membrane fusion proceeds independently of specific presynaptic proteins.
abstract: Complexes of specific presynaptic proteins have been hypothesized to drive or catalyze the membrane fusion steps of exocytosis. Here we use a stage-specific preparation to test the roles of SNAREs, synaptotagmin, and SNARE-binding proteins in the mechanism of Ca2+-triggered membrane fusion. Excess exogenous proteins, sufficient to block SNARE interactions, did not inhibit either the Ca2+ sensitivity, extent, or kinetics of fusion. In contrast, despite a limited effect on SNARE and synaptotagmin densities, treatments with high doses of chymotrypsin markedly inhibited fusion. Conversely, low doses of chymotrypsin had no effect on the Ca2+ sensitivity or extent of fusion but did alter the kinetic profile, indicating a more direct involvement of other proteins in the triggered fusion pathway. SNAREs, synaptotagmin, and their immediate binding partners are critical to exocytosis at a stage other than membrane fusion, although they may still influence the triggered steps.
authors.affiliation: Department of Physiology and Biophysics, Cellular and Molecular Neurobiology Research Group, University of Calgary, Health Sciences Centre, Faculty of Medicine, Alberta T2N 4N1, Canada.
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Szule
authors.forename: Scott E
authors.initials: SE
authors.lastname: Jarvis
authors.forename: Julie E
authors.initials: JE
authors.lastname: Hibbert
authors.forename: J David
authors.initials: JD
authors.lastname: Spafford
authors.forename: Janice E A
authors.initials: JE
authors.lastname: Braun
authors.forename: Gerald W
authors.initials: GW
authors.lastname: Zamponi
authors.forename: Gary M
authors.initials: GM
authors.lastname: Wessel
authors.forename: Jens R
authors.initials: JR
authors.lastname: Coorssen
mesh_terms: D000818:Animals; D002118:Calcium / Q000502:physiology*; D002135:Calcium-Binding Proteins; D005089:Exocytosis / Q000502:physiology; D005260:Female; D008561:Membrane Fusion / Q000502:physiology*; D008562:Membrane Glycoproteins / Q000502:physiology*; D008565:Membrane Proteins / Q000502:physiology*; D009419:Nerve Tissue Proteins / Q000502:physiology*; D011485:Protein Binding; D050600:SNARE Proteins; D012617:Sea Urchins; D050857:Synaptotagmins; D033921:Vesicular Transport Proteins
chemical_list: D002135:Calcium-Binding Proteins; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D009419:Nerve Tissue Proteins; D050600:SNARE Proteins; D033921:Vesicular Transport Proteins; D050857:Synaptotagmins; D002118:Calcium