In vitro study of the distinct stages in SNARE-mediated vesicle fusion
In the neuron, membrane fusion is the fundamental process in which neurotransmitters are released to the synapse. It is believed that the SNARE complex is the minimal fusion machinery that drives synaptic membrane fusion. This is a multi-step process that is regulated by a host of accessory proteins, however it is unclear how they affect the distinct stages of membrane fusion. In this work, we have identified regions on the SNARE complex that are integral for progression to vesicle fusion, while also investigating the function of the presynaptic protein, α-synuclein. Through investigation of Botulinum toxin A/E cleavage products, we have found that truncation of the C- terminal of SNAP-25 causes dynamic destabilization leading to impaired SNARE complex assembly resulting in loss of vesicle docking and pore formation. In addition, we have found that monomeric α-synuclein continues to enhance vesicle docking at overexpressed conditions. The removal of the higher ordered oligomers eliminated any inhibition of lipid merger. Furthermore, α-synuclein was shown to directly stimulate fusion pore expansion by stabilizing the large pore state through its membrane binding ability. Thus, α-synuclein biases the membrane fusion pathway towards full exocytosis in vesicle recycling. Altogether, the work presented here suggests a model where the SNARE complex, assisted by α-synuclein, is capable of progression through several intermediate stages prior full vesicle fusion.