Kinetics of synaptotagmin responses to Ca2+ and assembly with the core SNARE complex onto membranes.
The synaptic vesicle protein synaptotagmin I binds Ca2+ and is required for efficient neurotransmitter release. Here, we measure the response time of the C2 domains of synaptotagmin to determine whether synaptotagmin is fast enough to function as a Ca2+ sensor for rapid exocytosis. We report that synaptotagmin is "tuned" to sense Ca2+ concentrations that trigger neuronal exocytosis. The speed of response is unique to synaptotagmin I and readily satisfies the kinetic constraints of synaptic vesicle membrane fusion. We further demonstrate that Ca2+ triggers penetration of synaptotagmin into membranes and simultaneously drives assembly of synaptotagmin onto the base of the ternary SNARE (soluble N-ethylmaleimide-sensitive fusion protein [NSF] attachment receptor) complex, near the transmembrane anchor of syntaxin. These data support a molecular model in which synaptotagmin triggers exocytosis through its interactions with membranes and the SNARE complex.
Davis
A F
AF
Department of Physiology, University of Wisconsin School of Medicine, Madison 53706, USA.
Bai
J
J
Fasshauer
D
D
Wolowick
M J
MJ
Lewis
J L
JL
Chapman
E R
ER
D000818:Animals; D002118:Calcium / Q000502:physiology*; D002135:Calcium-Binding Proteins; D007700:Kinetics; D008561:Membrane Fusion / Q000502:physiology; D008562:Membrane Glycoproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism*; D008566:Membranes / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D012091:Repetitive Sequences, Nucleic Acid; D050600:SNARE Proteins; D017385:Sequence Homology; D013572:Synaptic Vesicles / Q000378:metabolism*; D050863:Synaptotagmin I; D050857:Synaptotagmins; D033921:Vesicular Transport Proteins
D002135:Calcium-Binding Proteins; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D009419:Nerve Tissue Proteins; D050600:SNARE Proteins; D050863:Synaptotagmin I; C494312:Syt1 protein, rat; D033921:Vesicular Transport Proteins; D050857:Synaptotagmins; D002118:Calcium