The actions of SNAREs and Munc18 in neuronal vesicle fusion

Abstract

SNAREs are believed to constitute the core engine of intracellular membrane fusion. Fusion is initiated when the SNARE motifs of the vesicle‐rooted SNARE (v‐SNARE) and the target membrane‐associated SNAREs (t‐SNAREs) zipper into a four‐helix coiled‐coil bundle (trans‐SNARE complex) between two apposed bilayers. N‐ to C‐terminal zippering of the SNARE core bundle brings two membranes into close proximity to fuse. In neuronal vesicle fusion (neurotransmitter release), mutations of individual SNARE layer residues can strongly inhibit vesicle fusion. It is generally thought that the fusion inhibition is caused by defects in the SNARE bundle assembly. However, our recent biochemical and biophysical data showed that SNARE layer mutations have little effects on the SNARE bundle formation or its fusogenicity. Unexpectedly, we found that mutations of SNARE layer residues selectively block the stimulatory activity of Munc18‐1, a soluble factor belonging to the SM protein family. We will present biochemical, biophysical and genetic evidence to support a model whereby Munc18‐1 acts upon a partially‐zippered SNARE bundle in the fusion reaction. Intriguingly, our data also suggest that Munc18‐1 binding is fully compatible with the association of complexin with the SNAREs, implying that SNAREs, Munc18‐1 and complexin may form a pre‐fusion supracomplex.