FRET detection of synaptophysin I and VAMP2 interactions during exocytosis from single live synapses

To investigate the molecular interactions of synaptophysin I and VAMP2/synaptobrevin II during exocytosis, we have used time-lapse videomicroscopy to measure fluorescence resonance energy transfer in live neurons. For this purpose, fluorescent protein variants fused to synaptophysin I or VAMP2 were expressed in rat hippocampal neurons. We show that synaptophysin I and VAMP2 form both homoand hetero-oligomers on the synaptic vesicle membrane. When exocytosis is stimulated with α-latrotoxin, VAMP2 dissociates from synaptophysin I even in the absence of appreciable exocytosis, whereas synaptophysin I oligomers disassemble only upon incorporation of the vesicle with the plasma membrane. We propose that synaptophysin I has multiple roles in neurotransmitter release, regulating VAMP2 availability for the SNARE complex and possibly participating in the late steps of exocytosis.