Calcium-independent stimulation of membrane fusion and SNAREpin formation by synaptotagmin I |
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Authors: | Mahal Lara K Sequeira Sonia M Gureasko Jodi M Söllner Thomas H |
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Institution: | Cellular Biochemistry and Biophysics Program, Memorial Sloan Kettering Cancer Center, 1275 York Avenue, New York, NY 10021, USA. |
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Abstract: | Neurotransmitter release requires the direct coupling of the calcium sensor with the machinery for membrane fusion. SNARE proteins comprise the minimal fusion machinery, and synaptotagmin I, a synaptic vesicle protein, is the primary candidate for the main neuronal calcium sensor. To test the effect of synaptotagmin I on membrane fusion, we incorporated it into a SNARE-mediated liposome fusion assay. Synaptotagmin I dramatically stimulated membrane fusion by facilitating SNAREpin zippering. This stimulatory effect was topologically restricted to v-SNARE vesicles (containing VAMP 2) and only occurred in trans to t-SNARE vesicles (containing syntaxin 1A and SNAP-25). Interestingly, calcium did not affect the overall fusion reaction. These results indicate that synaptotagmin I can directly accelerate SNARE-mediated membrane fusion and raise the possibility that additional components might be required to ensure tight calcium coupling. |
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