Submitted on July 14, 2008
Accepted on August 5, 2008

Conformational Switch of Syntaxin-1 Controls Synaptic Vesicle Fusion

Stefan H. Gerber ¹, Jong-Cheol Rah ², Sang-Won Min ³, Xinran Liu ⁴, Heidi de Wit ⁵, Irina Dulubova ⁶, Alexander C. Meyer ⁷, Josep Rizo ⁸, Marife Arancillo ⁹, Robert E. Hammer ⁸, Matthijs Verhage ⁵, Christian Rosenmund ¹⁰, Thomas C. Südhof ^11*

¹ Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX 75390-9111 USA.; Present address: Abt. Innere Medizin III, Universität Heidelberg, Germany.
² Departments of Molecular and Human Genetics and Neuroscience, Baylor College of Medicine, Houston TX 77030, USA.; Department of Membrane Biophysics, Max-Planck-Institute for Biophysical Chemistry, 37077 Göttingen, Germany.; Present address: Developmental Synaptic Plasticity Unit, NINDS, Bethesda, MD 20892, USA.
³ Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX 75390-9111 USA.; Present address: UCSF Mission Bay Campus.
⁴ Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX 75390-9111 USA.; Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390-9111 USA.
⁵ Department of Functional Genomics, Vrije Universiteit, Amsterdam, The Netherlands.
⁶ Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, TX 75390-9111 USA.
⁷ Department of Membrane Biophysics, Max-Planck-Institute for Biophysical Chemistry, 37077 Göttingen, Germany.
⁸ Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, TX 75390-9111 USA.; Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, TX 75390-9111 USA.
⁹ Departments of Molecular and Human Genetics and Neuroscience, Baylor College of Medicine, Houston TX 77030, USA.
¹⁰ Departments of Molecular and Human Genetics and Neuroscience, Baylor College of Medicine, Houston TX 77030, USA.; Department of Membrane Biophysics, Max-Planck-Institute for Biophysical Chemistry, 37077 Göttingen, Germany.
¹¹ Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX 75390-9111 USA.; Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390-9111 USA.; Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390-9111 USA.; Present address: Department of Molecular and Cellular Physiology and Neuroscience Institute, Stanford University, USA.

^* To whom correspondence should be addressed.
Thomas C. Südhof , E-mail: tcs1{at}stanford.edu

These authors contributed equally to this work.

During synaptic vesicle fusion, the SNARE-protein syntaxin-1 exhibits two conformations that both bind to Munc18-1: a ‘closed’ conformation outside the SNARE-complex, and an 'open' conformation in the SNARE-complex. Whereas SNARE-complexes containing 'open' syntaxin-1 and Munc18-1 are essential for exocytosis, the significance of 'closed' syntaxin-1 is unknown. Here, we generated knockin/knockout mice that expressed only ‘open’ syntaxin-1B. Syntaxin-1B^Open mice were viable, but succumbed to generalized seizures at 2 to 3 months of age. Binding of Munc18-1 to syntaxin-1 was impaired in syntaxin-1B^Open synapses, and the size of the readily releasable vesicle pool was decreased, whereas the rate of synaptic vesicle fusion was dramatically enhanced. Thus, the closed conformation of syntaxin-1 gates the initiation of the synaptic vesicle fusion reaction, which is then mediated by SNARE-complex/Munc18-1 assemblies.