Research Article: An Electrostatically Preferred Lateral Orientation of SNARE Complex Suggests Novel Mechanisms for Driving Membrane Fusion

Date Published: January 26, 2010

Publisher: Public Library of Science

Author(s): Ting Guo, Lin-Chen Gong, Sen-Fang Sui, Peter Laggner.

Abstract: Biological membrane fusion is a basic cellular process catalyzed by SNARE proteins and additional auxiliary factors. Yet, the critical mechanistic details of SNARE-catalyzed membrane fusion are poorly understood, especially during rapid synaptic transmission. Here, we systematically assessed the electrostatic forces between SNARE complex, auxiliary proteins and fusing membranes by the nonlinear Poisson-Boltzmann equation using explicit models of membranes and proteins. We found that a previously unrecognized, structurally preferred and energetically highly favorable lateral orientation exists for the SNARE complex between fusing membranes. This preferred orientation immediately suggests a novel and simple synaptotagmin-dependent mechanistic trigger of membrane fusion. Moreover, electrostatic interactions between membranes, SNARE complex, and auxiliary proteins appear to orchestrate a series of membrane curvature events that set the stage for rapid synaptic vesicle fusion. Together, our electrostatic analyses of SNAREs and their regulatory factors suggest unexpected and potentially novel mechanisms for eukaryotic membrane fusion proteins.

Partial Text: Biological membrane fusion is a basic cellular process necessary for exocytosis, endocytosis and exchange of vesicular contents in eukaryotic cells. Fusion of membranes is highly energetically demanding since tremendous electrostatic repulsion between negatively charged lipid bilayers has to be overcome.[1]In vivo, fusion is catalyzed by three families of conserved proteins collectively termed the SNARE proteins (soluble N-ethyl-maleimide-sensitive factor attachment receptor).[1], [2], [3], [4] During membrane fusion, SNARE proteins, anchored on the two fusing membranes, combine to form SNARE complex, a highly stable coiled coil structure consisting of four α-helices. Formation of the SNARE complex is thought to supply the energy needed to drive close apposition of fusing membranes and perhaps initiate the fusion process.[5] In addition, at least two auxiliary proteins, complexin and synaptotagmin, also participate in membrane fusion and are particularly important for rapid and tight control of fusion at synapses in response to neuronal activity.[6], [7]

Using molecular mechanics simulations, we performed a systematic theoretical analysis of electrostatic interactions in SNARE-mediated membrane fusion. Our findings suggest that surface charges of SNARE complex are conserved and strategically placed so that a highly energetically favorable and structurally preferred orientation exists for SNARE complexes in between fusing membranes. Such a preferred orientation prompted us to propose a “propeller” mechanism for SNAREs, in which synaptotagmin binding and membrane insertion drive SNARE complex to laterally turn over by 180°, causing SNARE TMDs to disrupt lipid bilayer structures. We further predicted that as SNARE complex associates and dissociates with complexin and synaptotagmin, a series of membrane bending events may occur which sequentially set the stage for rapid and tightly controlled release of neurotransmitters at synapses.



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