How SNARE molecules mediate membrane fusion: Recent insights from molecular simulations

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SNARE molecules are the core constituents of the protein machinery that facilitate fusion of synaptic vesicles with the presynaptic plasma membrane, resulting in the release of neurotransmitter. On a molecular level, SNARE complexes seem to play a quite versatile and involved role during all stages of fusion. In addition to merely triggering fusion by forcing the opposing membranes into close proximity, SNARE complexes are now seen to also overcome subsequent fusion barriers and to actively guide the fusion reaction up to the expansion of the fusion pore. Here, we review recent advances in the understanding of SNARE-mediated membrane fusion by molecular simulations.

Highlights

► Molecular simulations have revisited the original stalk-pore hypothesis. ► Initial membrane merger is determined by the free energy of leaflet approach. ► SNARE complexes reduce the free energy of leaflet approach and are actively involved in the lipidic transition states. ► SNARE complexes release mechanical stress to actively overcome the barrier against stalk expansion and enforce opening of the fusion pore. ► SNARE complexes can prevent the formation/expansion of a metastable hemifusion diaphragm.

Section snippets

Mechanical coupling between the SNARE complex and the bilayer

Atomistic simulations have revealed that the partly assembled coiled-coil complex forms a considerably stiff platform that allows force transduction between the SNARE complex and the membrane via the trans membrane domains (TMDs) of the SNARE molecules [10•]. During SNARE zipping a considerable fraction of the released energy is expected to dissipate, and the remaining fraction is stored as molecular bending stress in the individual SNARE molecules. This mechanical stress plays an important

Stalk formation

When the membranes are brought into sufficiently close proximity, a stalk can be formed [1•]. Here, continuum descriptions tend to imply a transient stalk of infinitesimal radius before it expands into the well characterized hour-glass-shaped stalk structure — in contrast to the molecular nature of the lipid membrane. In fact, there is a growing body of evidence from recent molecular simulations that stalk formation is neither the initial nor the rate limiting step in membrane fusion [3•]. These

Fusion pathways: the good, the bad, the ugly

After stalk formation, simulations have suggested mainly three pathways through which SNARE-mediated fusion can proceed 2•, 3•. As an example, Figure 3 shows these different pathways in the SNARE-mediated fusion between a lipid bilayer and 20 nm-sized vesicle, representing a synaptic vesicle (30–50 nm sized [39•]):

  • (i)

    Stalk elongation (Figure 3b,e). Stalk elongation relates to the inverted hexagonal phase transition (HII-phase) 30••, 40••, 41•, 42•, where a stacked bilayer system spontaneously

Formation of the fusion pore

Fusion pore formation seems related to both the presence of mechanical stress in the SNARE complex as well as the TMR ends. On one hand, the binding affinity between SNARE molecules and the nature of the linkers have been linked to fusion pore formation 52•, 53; on the other hand, deletions of TMR end residues and addition of polar amino acids to the Syb2 C-terminus have been shown to arrest fusion pore formation 54, 55•. Further, in agreement with current models 4, 56•, 57, conductance

Conclusions

In the last decennia molecular simulations have revisited the original stalk-pore hypothesis from different angles. Initially, and mainly motivated by continuum elastic models, it was the stalk that was believed to be the relevant fusion barrier. Accordingly, the main focus was on predicting the free energy of the stalk structure. The discovery of the rhombohedral phase (stalk phase) by X-ray experiments in 2003 proved that the stalk can be a stable structure, that is, a free energy minimum [27•

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgments

We thank Reinhard Jahn, Marcus Müller, Tim Salditt, Sebastian Aeffner and Yuliya Smirnova for stimulating discussions and constructive comments. Financial support has been provided by the DFG under grant SFB 803/B2.

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