Caught in the Act: Visualization of SNARE‐Mediated Fusion Events in Molecular Detail

Abstract

Neurotransmitter release at the synapse requires fusion of synaptic vesicles with the presynaptic plasma membrane. SNAREs are the core constituents of the protein machinery responsible for this membrane fusion, but the actual fusion mechanism remains unclear. Here, we have simulated neuronal SNARE‐mediated membrane fusion in molecular detail. In our simulations, membrane fusion progresses through an inverted micelle fusion intermediate before reaching the hemifused state. We show that at least one single SNARE complex is required for fusion, as has also been confirmed in a recent in vitro single‐molecule fluoresence study. Further, the transmembrane regions of the SNAREs were found to play a vital role in the initiation of fusion by causing distortions of the lipid packing of the outer membrane leaflets, and the C termini of the transmembrane regions are associated with the formation of the fusion pores. The inherent mechanical stress in the linker region of the SNARE complex was found to drive both the subsequent formation and expansion of fusion pores. Our simulations also revealed that the presence of homodimerizations between the transmembrane regions leads to the formation of unstable fusion intermediates that are under high curvature stress. We show that multiple SNARE complexes mediate membrane fusion in a cooperative and synchronized process. Finally, we show that after fusion, the zipping of the SNAREs extends into the membrane region, in agreement with the recently resolved X‐ray structure of the fully assembled state.

Stalk structures in lipid bilayer fusion studied by x-ray diffraction

“…In addition, it allows transient leakage due to hole formation, which is in agreement with several experimental reports [58]. A very recent simulation of SNARE-mediated vesicle fusion in molecular detail displayed fusion by this alternative pathway involving IMI formation [60] ( Fig. 1.6).…”

A continuum model of docking of synaptic vesicle to plasma membrane

“…As mentioned above, our numerical results indicate that the shortest gap between the two membranes lies near the SNARE location. This supports the experimental observation that the transmembrane segment of SNARE induces distortion in the lipid packing around it [46]. It is possible that this distortion will eventually lead to the lipid rearrangement in the membranes resulting in the formation of hemifusion diaphragm.…”

Entropic forces drive self-organization and membrane fusion by SNARE proteins

“…S1). We seek the collective behavior on millisecond-second fusion timescales, orders of magnitude beyond current simulations (27). Our highly coarse-grained description preserves key measured SNARE properties and represents membranes as continuous surfaces interacting via steric-hydration, electrostatic, and van der Waals forces (28,29).…”

“…accessible timescales (nanoseconds to microseconds) were much less than experimental fusion timescales (milliseconds to seconds), forcing the use of unphysical simulation conditions to achieve fusion (27).…”