Journal
JOURNAL OF BIOLOGICAL CHEMISTRY
Volume 284, Issue 19, Pages 13143-13152Publisher
AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC
DOI: 10.1074/jbc.M900703200
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- Deutsche Forschungsgemeinschaft [SFB523]
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The three key players in the exocytotic release of neurotransmitters from synaptic vesicles are the SNARE (soluble N-ethylmaleimidesensitive factor attachment protein receptor) proteins synaptobrevin 2, syntaxin 1a, and SNAP-25. Their assembly into a tight four-helix bundle complex is thought to pull the two membranes into close proximity. It is debated, however, whether the energy generated suffices for membrane fusion. Here, we have determined the thermodynamic properties of the individual SNARE assembly steps by isothermal titration calorimetry. We found extremely large favorable enthalpy changes counterbalanced by positive entropy changes, reflecting the major conformational changes upon assembly. To circumvent the fact that ternary complex formation is essentially irreversible, we used a stabilized syntaxin-SNAP-25 heterodimer to study synaptobrevin binding. This strategy revealed that the N-terminal synaptobrevin coil binds reversibly with nanomolar affinity. This suggests that individual, membrane-bridging SNARE complexes can provide much less pulling force than previously claimed.
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