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First published online August 29, 2005
doi: 10.1242/10.1242/jcs.02561

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Functions of SNAREs in intracellular membrane fusion and lipid bilayer mixing

¹ Biochemie Zentrum der Universität Heidelberg, Im Neuenheimer Feld 328, 69120 Heidelberg, Germany
² Lehrstuhl der Chemie der Biopolymere, Technische Universität München, Weihenstephaner Berg 3, 85354 Freising, Germany

View larger version (29K): [in a new window]   Fig. 1. SNARE function in trafficking and SNARE domain structure. (A) Vesicle trafficking. The stages of vesicle budding and fusion are shown. During SNARE activation, SNAREs become disassembled with the help of NSF, -SNAP and ATP. Budding includes uptake of the v-SNARE, Rab proteins and tethers, which assemble before or during tethering into a complete complex. Fusion requires SNARE complex assembly and lipid mixing. (B) Domain structure of selected SNAREs. Lipid anchors on Ykt6 and SNAP-25 are indicated. (C) The HABC N-terminal domain of syntaxin (PDB accession number 1BR0) (Fernandez et al., 1998). (D) The Ykt6 N-terminal longin domain (1H8M) (Tochio et al., 2001).

View larger version (38K): [in a new window]   Fig. 2. SNARE complex assembly and its control. (A) Interaction of syntaxin with nSec1, also known as Munc18 (1DN1) (Yang et al., 2000). nSec1 binds to the closed conformation of syntaxin, regulating its availability. (B) Binding of Sly1 to the N-terminal peptide of the SNARE Sed5 may function similarly (1MQS) (Bracher and Weissenhorn, 2002). (C) The open conformation of the syntaxin molecule (as shown in Fig. 1C). (D) The assembled SNARE complex modeled between two membranes (1SFC) (Sutton et al., 1998). (E) Interaction of complexin with the SNARE complex might regulate exocytosis (1KIL) (Chen et al., 2002).

View larger version (20K): [in a new window]   Fig. 3. Membrane fusion. (A) Putative model of SNARE-mediated membrane fusion. At stage I, Q- and R-SNAREs are separate from each other and the Q-SNARE syntaxin exists in its closed conformation. SNARE complex formation releases the Q-SNARE HABC N-terminal domain and is associated with vesicle docking (stage II). Several SNARE complexes may associate at the fusion site via their TMSs. Docking may result in hemifusion (III), which is followed by formation of lipidic (IV) or proteinaceous (V) fusion pores prior to full fusion (V). Note that proteins and membranes are not drawn to scale. (B) Lipid topology in hemifusion.