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Membrane Fusion Intermediates via Directional and Full Assembly of the SNARE Complex

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Science  22 Jun 2012:
Vol. 336, Issue 6088, pp. 1581-1584
DOI: 10.1126/science.1221976

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No More Fusion Confusion

Biophysical models explain membrane fusion as a sequence of steps—including membrane contact, formation of a fusion stalk (merger of proximal monolayers), development of contact between distal monolayers that may or may not expand (hemifusion), and, finally, rupture of this diaphragm resulting in the opening of a fusion pore. Biological membrane fusion reactions are often driven by so-called SNARE proteins. By using a reconstituted membrane fusion system, Hernandez et al. (p. 1581, published online 31 May) have now been able to correlate precisely the states of SNARE zippering with intermediate structures along the fusion pathway. The results suggest that a tightly docked state, with a membrane distance so close that no proteins fit in between them, represents a critical fusion intermediate as a consequence of SNARE zippering. This intermediate is incompatible with a SNARE-driven stalk or with a ringlike arrangement of SNAREs depicted in most current models of membrane fusion.

Abstract

Cellular membrane fusion is thought to proceed through intermediates including docking of apposed lipid bilayers, merging of proximal leaflets to form a hemifusion diaphragm, and fusion pore opening. A membrane-bridging four-helix complex of soluble N-ethylmaleimide–sensitive factor attachment protein receptors (SNAREs) mediates fusion. However, how assembly of the SNARE complex generates docking and other fusion intermediates is unknown. Using a cell-free reaction, we identified intermediates visually and then arrested the SNARE fusion machinery when fusion was about to begin. Partial and directional assembly of SNAREs tightly docked bilayers, but efficient fusion and an extended form of hemifusion required assembly beyond the core complex to the membrane-connecting linkers. We propose that straining of lipids at the edges of an extended docking zone initiates fusion.

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