Concurrent binding of complexin and synaptotagmin to liposome-embedded SNARE complexes.
Synaptotagmin and complexin regulate SNARE-mediated synaptic vesicle exocytosis. It has been proposed that complexin clamps membrane fusion and that Ca(2+)-synaptotagmin displaces complexin from SNARE complexes to relieve this clamping activity. Using a reconstituted system, we demonstrate that complexin and synaptotagmin simultaneously bind to neuronal SNARE complexes and that both apo-synaptotagmin and complexin inhibit SNARE-mediated membrane fusion. Moreover, the clamping ability of apo-synaptotagmin occluded the clamping activity of complexin until the arrival of a Ca(2+) trigger, at which point synaptotagmin accelerated fusion while high concentrations of complexin inhibited fusion. Thus, the inhibitory patterns of synaptotagmin and complexin are different, suggesting that SNAREs assemble into distinct states along the fusion pathway. These data also suggest that during synaptotagmin-regulated vesicle-vesicle fusion, complexin does not function as a fusion clamp that is relieved by Ca(2+)-synaptotagmin.
Chicka
Michael C
MC
Department of Physiology and Programs in Cellular and Molecular Biology, University of Wisconsin, 1300 University Avenue, SMI 129, Madison, Wisconsin 53706, USA.
Chapman
Edwin R
ER
D033942:Adaptor Proteins, Vesicular Transport; D000818:Animals; D006801:Humans; D008081:Liposomes / Q000737:chemistry / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding / Q000502:physiology; D050600:SNARE Proteins / Q000737:chemistry / Q000378:metabolism*; D050857:Synaptotagmins / Q000737:chemistry / Q000378:metabolism*
D033942:Adaptor Proteins, Vesicular Transport; D008081:Liposomes; D009419:Nerve Tissue Proteins; D050600:SNARE Proteins; C095992:complexin I; C095993:complexin II; D050857:Synaptotagmins