Journal
BIOCHEMICAL JOURNAL
Volume 443, Issue -, Pages 223-229Publisher
PORTLAND PRESS LTD
DOI: 10.1042/BJ20112044
Keywords
charge asymmetry; 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS); soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor (SNARE); synaptotagmin 1; vesicle fusion
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Funding
- National Institute of Health [R01GM51290, R01GM67629]
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Syt1 (synaptotagmin 1) is a major Ca2+ sensor for synaptic vesicle fusion. Although. Syt1 is known to bind, to SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor) complexes and to the membrane, the mechanism by which Syt1 regulates vesicle fusion is controversial. In the present study we used in vitro lipid-mixing assays to investigate the Ca2+-dependent Syt1 function in proteoliposome fusion. To study the role of acidic lipids, the concentration of negatively charged DOPS (1,2-dioleoyl-sn-glycero-3-phospho-L-serine) in the vesicle was varied. Syt1 stimulated lipid mixing by 3-10-fold without Ca2+. However, with Ca2+ there was an additional 2-5-fold enhancement. This Ca2+-dependent stimulation was observed only when there was excess PS (phosphatidylserine) on the t-SNARE (target SNARE) side. If there was equal or more PS on the v-SNARE (vesicule SNARE) side the Ca2+ dependent stimulation was not observed. We found that Ca2+ at a concentration between 10 and 50 mu M was sufficient to give rise to the maximal enhancement. The single-vesicle-fusion assay indicates that the Ca2+-dependent enhancement was mainly on docking, whereas its effect on lipid mixing was small. Thus for Syt1 to function as a Ca2+ sensor, a charge asymmetry appears to be important and this may play a role in steering Syt1 to productively trans bind to the plasma membrane.
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