Tag team action at the synapse

Abstract

Communication between neurons relies on chemical synapses and the release of neurotransmitters into the synaptic cleft. Neurotransmitter release is an exquisitely regulated membrane fusion event that requires the linking of an electrical nerve stimulus to Ca(2+) influx, which leads to the fusion of neurotransmitter-filled vesicles with the cell membrane. The timing of neurotransmitter release is controlled through the regulation of the soluble N-ethylmaleimide sensitive factor attachment receptor (SNARE) proteins-the core of the membrane fusion machinery. Assembly of the fusion-competent SNARE complex is regulated by several neuronal proteins, including complexin and the Ca(2+)-sensor synaptotagmin. Both complexin and synaptotagmin bind directly to SNAREs, but their mechanism of action has so far remained unclear. Recent studies revealed that synaptotagmin-Ca(2+) and complexin collaborate to regulate membrane fusion. These compelling new results provide a molecular mechanistic insight into the functions of both proteins: complexin 'clamps' the SNARE complex in a pre-fusion intermediate, which is then released by the action of Ca(2+)-bound synaptotagmin to trigger rapid fusion.

Figure 1

Structures of proteins required for the fast, Ca²⁺-stimulated release of neurotransmitters. (A) The SNARE complex (Protein Data Bank (PDB) ID , Sutton et al, 1998) and the C2A and C2B domains of synaptotagmin I (PDB IDs , Shao et al, 1998; and , Fernandez et al, 2001, respectively) are required at sites of synaptic vesicle fusion on the presynaptic plasma membrane. (B) Complexin bound to the SNARE complex (PDB ID , Chen et al, 2002). The transmembrane domains and unstructured regions are modelled in the figure. Amino- and carboxy-terminal ends of each protein are indicated. SNAP-25, synaptosome-associated protein of 25kDa; VAMP, vesicle-associated membrane protein.

Figure 2

Schematic for the clamp–release model of complexin–synaptotagmin function. (A) Clamp: SNARE complexes assemble when synaptic vesicles dock at the presynaptic membrane. Complexin clamps SNARE complexes and prevents membrane fusion. (B) Intermediate state: synaptotagmin recognizes and binds to the complexin–SNARE complex in the absence of Ca²⁺. (C) Release: after Ca²⁺ influx (yellow spheres), synaptotagmin binds to both Ca²⁺ and membranes. This conformational change releases complexin, and the shape of the membrane-inserted, Ca²⁺–synaptotagmin–SNARE complex might buckle the membranes outwards to complete fusion (Martens et al, 2007). The clamped state has been proposed to be metastable or ‘superprimed' (Tang et al, 2006), perhaps by hemifusion of the membranes (Schaub et al, 2006). The molecular details of the interactions between synaptotagmin, complexin and the SNARE complex, and the conformational changes that occur on Ca²⁺ binding have yet to be determined.

Chavela M. Carr

Mary Munson