InteractiveFly: GeneBrief

complexin: Biological Overview | References

SNARE-mediated synaptic exocytosis is orchestrated by facilitatory and inhibitory mechanisms. Genetic ablations of Complexins, a family of SNARE-complex-binding proteins, in mice and Drosophila cause apparently opposite effects on neurotransmitter release, leading to contradictory hypotheses of Complexin function. Reconstitution experiments with different fusion assays and Complexins also yield conflicting results. Cross-species rescue experiments were therefore performed to compare the functions of murine and Drosophila Complexins in both mouse and fly synapses. It was found that murine and Drosophila Complexins employ conserved mechanisms to regulate exocytosis despite their strikingly different overall effects on neurotransmitter release. Both mouse and fly Complexins contain distinct domains that facilitate or inhibit synaptic vesicle fusion, and the strength of each facilitatory or inhibitory function differs significantly between them. These results show that a relative shift in the balance of facilitatory and inhibitory functions results in differential regulation of neurotransmitter release by murine and Drosophila Complexins in vivo, reconciling previous incompatible findings (Xue, 2009).

SNARE (soluble N-ethylmaleimide-sensitive factor-attachment protein receptor)-mediated synaptic vesicle exocytosis is tightly controlled by a large number of regulatory proteins to ensure the exquisite temporal and spatial precision of neurotransmitter release at synapses. Complexins constitute a family of small and highly charged proteins that bind to the assembled SNARE complex (Ishizuka, 1995; McMahon, 1995; Reim, 2005; Takahashi, 1995). They generally contain a central α helix and an accessory α helix in the middle portion of the protein, and the N- and C-terminal sequences that are probably largely unstructured (Pabst, 2000). Complexins bind to the SNARE complex with high affinity (Bowen, 2005; Pabst, 2002). The central α helix of Complexins interacts with the SNARE motifs of Syntaxin-1 (see Drosophila Syntaxin) and Synaptobrevin-2 within the SNARE complex in an antiparallel fashion (Bracher, 2002; Chen, 2002). Complexins can also bind to the target-SNAREs' (Syntaxin-1 and SNAP-25) heterodimer with a lower affinity (Guan, 2008; Weninger, 2008; Yoon, 2008; Xue, 2009 and references therein).

Biophysical and physiological studies have indicated diverse functions for Complexins in vesicle fusion, some of which are incompatible (Brose, 2008). Complexins have been shown to inhibit SNARE-mediated cell fusion (Giraudo, 2006) and proteoliposome fusion in bulk ensemble assay (Schaub, 2006), and this inhibition is released by the Ca²⁺ sensor Synaptotagmin-1 (see Drosophila Synaptotagmin) and Ca²⁺. Biochemically, Synaptotagmin-1 competes with Complexins for the SNARE complex binding and displaces Complexins from the SNARE complex in a Ca²⁺-dependent manner (Tang, 2006). These studies suggest a fusion clamp model for Complexin function, in which Complexins inhibit the transfer of the force generated by the SNARE complex assembly onto the fusing membranes and arrest synaptic vesicle fusion before Ca²⁺ influx. Upon Ca²⁺ binding, Synaptotagmin-1 displaces Complexins from the SNARE complex to release this inhibition and triggers exocytosis (Giraudo, 2006; Maximov, 2009; Schaub, 2006; Tang, 2006). However, Complexins have also been shown to stimulate proteoliposome fusion in both single-vesicle fusion assay (Yoon, 2008) and bulk ensemble assay (Malsam, 2009), indicating a facilitatory role. These in vitro results are further confounded by in vivo genetic studies. Genetic knockout of Complexins in mice leads to a reduction in both evoked and spontaneous release at multiple glutamatergic and GABAergic synapses in cultures and in acute brain slices (Reim, 2001; Strenzke, 2009; Xue, 2008), and a decrease in Ca²⁺-triggered exocytosis in adrenal chromaffin cells (Cai, 2008), supporting a stimulatory function for Complexins. In contrast, genetic deletion of Complexin in fruit fly Drosophila melanogaster greatly enhances spontaneous release but decreases Ca²⁺-evoked release (Huntwork, 2007), favoring the fusion clamp model. Moreover, knockdown of Complexins by RNA interference in mass-cultured mouse cortical neurons decreases evoked release and increases spontaneous release at glutamatergic synapses (Maximov, 2009). To explain the discrepancy between this result and those obtained previously from Complexin knockout mice, the authors (Maximov, 2009) suggest that this is due to the different preparations used (autaptic cultures for knockout studies [Reim, 2001; Xue, 2008) versus mass cultures for knockdown study (Maximov, 2009), disavowing the fact that the knockout studies also employed mass cultures and acute brain slices (Xue, 2008), and similar results were found to those obtained from autaptic cultures. Hence, many studies seem at odds with each other and the precise in vivo role of Complexins in exocytosis is still unclear (Xue, 2009).

An in vivo structure-function analysis of murine Complexin I (CplxI) in Complexin I/II double knockout mouse neurons indicates that the SNARE complex binding is essential for CplxI function, and that the N terminus of CplxI facilitates release, whereas an accessory α helix between the N terminus and the central α helix inhibits release (Xue, 2007). A biophysical study reveals that CplxI inhibits SNARE complex formation, but strongly stimulates membrane fusion after the assembly of the SNARE complex in vitro (Yoon, 2008). These studies indicate that Complexins play both facilitatory and inhibitory roles in exocytosis, but they still do not explain why genetic deletions of Complexins in two model organisms, mouse and fly, have such different effects on neurotransmitter release. Furthermore, the amino acid sequence homology is low between murine and Drosophila Complexins except for the central α helix that is essential for the binding to the SNARE complex, and part of the N terminus. Thus, the dramatic difference in loss-of-function phenotypes of Complexin-deficient mice and flies leads to the conclusion that Complexin function must differ between mice and flies (Xue, 2009).

To test whether the functions of murine and Drosophila Complexins are conserved in synaptic vesicle exocytosis, and to gain insight into their functional and structural differences, it is essential to compare murine and Drosophila Complexins in the same experimental in vivo systems. A detailed structure-function analysis is also necessary because a complete removal of Complexins is unlikely to reveal all aspects of their function (Xue, 2007). Therefore a systematic cross-species rescue approach was undertaken to compare the functions of murine and Drosophila Complexins at both mouse and fly synapses. It was found that both murine and Drosophila Complexins contain distinct functional domains and play dual roles in neurotransmitter release. They facilitate and inhibit release via similar domains, but the facilitatory or inhibitory strength of a given domain varies between murine and Drosophila Complexins. Thus, both murine and Drosophila Complexins utilize conserved mechanisms in release process, but the integration of facilitation and inhibition differs substantially between them, leading to an apparently opposite overall effect on exocytosis. These results reveal conserved functions of Complexins between species and indicate that the interplay of dual functions orchestrates neurotransmitter release (Xue, 2009).

Synaptic exocytosis is exquisitely controlled by a set of facilitatory and inhibitory mechanisms, some of which are often executed by the very same protein (Rizo, 2008). As a key regulator of the release machinery, Complexins play both facilitatory and inhibitory roles in vesicle fusion through distinct mechanisms (Xue, 2007; Yoon, 2008). However, the remarkable phenotypic difference between mouse and fly Complexin null animals remained unexplained. This work compared the functions of murine and Drosophila Complexins in cross-species rescue experiments. The data establish that murine and Drosophila Complexins share a set of conserved mechanisms in synaptic vesicle fusion (Xue, 2009).

First, the SNARE complex binding mediated by the central α helix (residues 48-70 for CplxI and 54-76 for dmCplx) is essential for Complexin function. Mutations that diminish the interaction between the central α helix and the SNARE complex abolish the functions of both CplxI (Xue, 2007) and dmCplx, indicating that the actions of other domains all depend on this high-affinity interaction. The binding of the central α helix not only can stabilize the assembled SNARE complex (Chen, 2002), but perhaps more importantly, can strategically position the accessory α helix and the N terminus for their actions (Xue, 2007; Xue, 2009 and references therein).

Second, the accessory α helix (approximately residues 29-47 for CplxI and 33-53 for dmCplx) located between the N terminus and the central α helix inhibits vesicle fusion. It was proposed that the inhibitory action of the accessory α helix might arise from its interference with the binding of Synaptobrevin-2 to Syntaxin-1 and SNAP-25 heterodimer, which would consequently prevent the complete zippering of the SNARE complex (Xue, 2007). This model has recently been supported by the findings that Complexins can bind to Syntaxin-1 and SNAP-25 heterodimer in vitro (Guan, 2008; Weninger, 2008; Yoon, 2008) and may form an alternative four-helix bundle with target-SNAREs to inhibit fusion in a reconstituted fusion system (Giraudo, 2009; Xue, 2009 and references therein).

Third, the N termini (residues 1-16) of both CplxI and dmCplx promote release. It has been speculated that the CplxI N terminus may interact with lipid membranes (Maximov, 2009; Xue, 2007), but so far, there are no supporting biochemical data. Instead, this facilitatory effect is likely mediated by a direct interaction of the Complexin N terminus with the SNARE complex C terminus. Mutations of methionine 5 and lysine 6 of CplxI disrupt the binding of the CplxI N terminus to the SNARE complex C terminus and abolish the facilitatory activity of the N terminus. Interestingly, methionine 5 is not conserved in dmCplx and an alanine residue is at position 6 (corresponding to residue 5 of CplxI). It is possible that a methionine is not absolutely required for dmCplx and other residues may compensate for the interaction with the SNARE complex C terminus (Xue, 2009).

Furthermore, at fly neuromuscular junctions, both murine and Drosophila Complexins promote Ca²⁺-triggered release and suppress spontaneous release, but to very different degrees. Neuronal expression of murine or Drosophila Complexins rescues the lethality and sterility of Complexin null mutant flies, showing again that murine and Drosophila Complexins share conserved functions (Xue, 2009).

Therefore, these cross-species rescue experiments show that murine and Drosophila Complexins have both facilitatory and inhibitory functions associated with similar protein domains in synaptic vesicle exocytosis. It is proposed that the Complexin central α helix binds to the middle portion of the SNARE complex, stabilizing the SNARE complex and positioning the accessory α helix and the N terminus (Chen, 2002; Xue, 2007). The accessory α helix replaces the C terminus of the Synaptobrevin-2 SNARE motif in the four-helix bundle, preventing the full assembly of the SNARE complex to suppress fusion (Giraudo, 2009; Xue, 2007). The N terminus directly interacts with the C-terminal portion of the SNARE complex, likely stabilizing this unstable region of the SNARE complex to promote membrane fusion. However, the relative strengths of these functions are remarkably different between murine and Drosophila Complexins. It is proposed that the integration of facilitation and inhibition, which are associated with distinct domains, determines the overall effect of murine and Drosophila Complexins on neurotransmitter release in a given synapse. The overall action of murine and Drosophila Complexins is unlikely to be a linearly additive effect of all facilitatory and inhibitory actions. However, it is clear that the facilitatory function is preponderant in murine Complexins, whereas the inhibitory functions of the accessory α helix and the C terminus predominate in Drosophila Complexin. Thus, a relative shift in the balance of facilitatory and inhibitory functions results in differential roles of murine and Drosophila Complexins in neurotransmitter release, and leads to apparently very different loss-of-function phenotypes in flies and mice. These results emphasize the functional similarities and differences between murine and Drosophila Complexins, and reconcile previous contradictory hypotheses of Complexin in vivo function (Cai, 2008; Huntwork, 2007; Reim, 2001; Xue, 2008). Moreover, the data illustrate the complexity of Complexin function and strongly support the notion that Complexins play dual roles in vesicle fusion (Xue, 2009).

This model is clearly different from the previous models of Complexins based on the fusion clamp hypothesis (Giraudo, 2006; Maximov, 2009; Schaub, 2006; Tang, 2006). These models propose that Complexins arrest primed synaptic vesicles at a hemifused and metastable state, which provides the substrate for Ca²⁺-bound Synaptotagmin-1 to release the clamping function of Complexins, allowing the fast and synchronous fusion. The lack of Complexins and therefore the lack of metastable vesicles for Synaptotagmin-1 action causes excessive spontaneous release and deficient Ca²⁺-triggered fast release. However, the current in vivo results speak against this model because murine Complexins do not completely clamp the excessive spontaneous release in Drosophila Complexin null mutants, yet they actually enhance Ca²⁺-evoked fast release even better than Drosophila Complexin. This observation indicates that the decreased Ca²⁺-evoked fast release in Drosophila Complexin null mutants is not functionally coupled to the increased spontaneous release frequency. Could it be that the reduced evoked release in Drosophila Complexin null mutants is due to a partial depletion of readily releasable vesicles by the high-frequency spontaneous release? This is unlikely because the vesicle recruitment rate is usually at least 100-fold higher than the spontaneous release rate at resting intracellular Ca²⁺ level, and therefore a 20- to 30-fold increase in spontaneous release rate should not significantly change the vesicle pool size in Drosophila Complexin null mutants. In addition, murine-Complexin-rescued Drosophila Complexin null synapses still exhibit strongly increased spontaneous release, yet the evoked release is even larger than that of WT synapses, arguing that high-frequency spontaneous release in null mutants is unlikely to exhaust vesicles, causing a decreased evoked release (Xue, 2009).

A recent fusion clamp model proposes that Complexins control the force transfer from the SNARE complex to the membranes and assist the SNAREs in exerting force on the membranes (Maximov, 2009). This model assumes that Complexins are released from the SNARE complex by Synaptotagmin-1 and Ca²⁺, but it is physically unclear how Complexins can help SNAREs exert force on the membranes if they are dissociated upon Ca²⁺ influx. In contrast, the current model requires Complexins to remain bound to the SNARE complex upon Ca²⁺ influx and is consistent with the notion that Complexins could function independently from Synaptotagmin-1 (Xue, 2009).

Drosophila Complexin in Cplx-TKO neurons abolishes both evoked and spontaneous release without altering the number of fusion-competent vesicles measured by hypertonic sucrose solution. This effect is intriguing, because very few molecular manipulations specifically block the synaptic vesicle cycle at the final fusion step. Drosophila Complexin does not change the number of primed vesicles, indicating that the initial formation of the SNARE complex is not affected by Drosophila Complexin. The inhibitory effect of Drosophila Complexin requires its binding to the SNARE complex. Hence, it is hypothesized that when the Drosophila Complexin central α helix binds to the partially assembled SNARE complex, the accessory α helix together with the C terminus prevents the further assembly of the SNARE complex C terminus, thereby arresting vesicles at the primed state. It is currently unknown how, mechanistically, the C terminus of Drosophila Complexin inhibits release. One possibility is that the C terminus may fold back toward the N-terminal direction and cooperate with the accessory α helix to inhibit vesicle fusion (Xue, 2009).

The phenotypic differences between fly and mouse knockouts seem dramatic, but it is worth noting that an increase of just 1.4 kcal/mol in the strength of a protein-protein interaction, which can arise simply from the formation of one hydrogen bond or salt bridge, leads to a 10-fold increase in affinity according to the Boltzmann equation. Hence, subtle changes in the molecular interactions of murine and Drosophila Complexins can suffice to tip the balance between facilitatory and inhibitory strengths. For example, protein sequence alignments show that the lengths and the amino acid compositions of the accessory α helices differ among different Complexins, which may cause different interactions of the accessory α helix with Syntaxin-1 and SNAP-25 heterodimer, thus changing its inhibitory strength (Xue, 2009).

The effects of murine Complexins in murine and fly synapses are not identical, as murine Complexins promote evoked release and inhibit spontaneous release in fly neuromuscular junctions, and promotes both types of release in mouse central synapses. Likewise, the effects of Drosophila Complexin in murine and fly synapses are not identical either, as it strongly inhibits spontaneous release and mildly promotes evoked release in fly neuromuscular junctions, and strongly inhibits both types of release in mouse synapses. These observations indicate that in addition to the Complexin-intrinsic properties, the molecular differences between species or synapses could differentially affect the facilitatory and inhibitory functions of murine and Drosophila Complexins, thereby tilting the facilitation and inhibition balance and contributing to the phenotypic differences (Xue, 2009).

Complexins represent a family of proteins that maintain a highly conserved core of sequences and at the same time display great diversity across paralogs and orthologs (Huntwork, 2007; Reim, 2005). This is likely reflected in their functions, namely conserved facilitatory and inhibitory mechanisms with varying strengths in neurotransmitter release. It will be interesting to test Complexin function in some other model organisms along the phylogenetic tree, such as worm and fish, to determine if and how the balance between facilitatory and inhibitory functions of Complexins has changed during evolution. At different synapses, the strengths of facilitation and inhibition of Complexins may be differentially regulated in a paralog- and ortholog-dependent fashion, thereby regulating release in a synapse-specific manner, and contributing to synaptic diversity and specificity. Furthermore, the ability of Drosophila Complexin to inhibit neurotransmitter release in mammalian neurons potentially provides a powerful tool to manipulate synaptic function to study neural circuits, as one should be able to express Drosophila Complexin to inhibit or even abolish synaptic transmission in a spatially and temporally specific manner (Xue, 2009).

A complexin fusion clamp regulates spontaneous neurotransmitter release and synaptic growth

Neuronal signaling occurs through both action potential-triggered synaptic vesicle fusion and spontaneous release, although the fusion clamp machinery that prevents premature exocytosis of synaptic vesicles in the absence of calcium is unknown. This study demonstrates that spontaneous release at synapses is regulated by complexin, a SNARE complex-binding protein. Analysis of Drosophila complexin null mutants showed a marked increase in spontaneous fusion and a profound overgrowth of synapses, suggesting that complexin functions as the fusion clamp in vivo and may modulate structural remodeling of neuronal connections by controlling the rate of spontaneous release (Huntwork, 2007).

To determine the function of complexin in vivo, a complete knockout of complexin was generated. Drosophila has a single gene encoding the complexin protein (Tokumaru, 2001). Antisera was generated to recombinant Drosophila complexin protein that recognized a 16-kDa protein in brain extracts that is enriched in both CNS and peripheral synapses. Costaining for complexin and Discs large (Dlg) or the active-zone protein Bruchpilot demonstrated that complexin is expressed diffusely in presynaptic terminals. A 17-kb intragenic deletion within complexin (cpx^SH1) that removed most of the coding region was generated by imprecise excision of a P element. A precise excision lacking any deletion was also generated, and served as a control for genetic background in all experiments. Western analysis and immunocytochemistry confirmed the loss of expression of complexin in cpx^SH1. Null mutants lacking complexin are semilethal, with most animals dying before adult eclosion. Escaper adults are infertile, show severe motor defects and lack the on- and off-transients that represent normal synaptic transmission in the visual system. Pan-neuronal expression of a UAS-complexin transgene with the elav-GAL4 neuronal driver rescues the reduced viability and abnormal synaptic transmission of cpx^SH1 mutants (Huntwork, 2007).

To analyze the role of complexin in neurotransmitter release, electrophysiological recordings were performed at Drosophila third-instar larval abdominal muscle 6 synapses. cpx^SH1 mutants had profound defects in spontaneous vesicle fusion. The frequency of miniature excitatory junctional potentials (minis) was greatly enhanced at cpx^SH1 mutant synapses, demonstrating continuous exocytosis of synaptic vesicles in the absence of any stimulation. Neuronal expression of a UAS-complexin transgene rescued the enhanced frequency of spontaneous release. In high extracellular [Ca²⁺], evoked responses in cpx^SH1 mutants after nerve stimulation were significantly reduced compared with controls, whereas in low extracellular [Ca²⁺] no difference was observed. As described below, complexin mutant synapses showed a 64% increase in active zone number, indicating that neuromuscular junction (NMJ) synapses lacking complexin had a reduction in evoked fusion events per active zone that was exacerbated at high calcium levels, where large numbers of synchronous synaptic vesicle fusion events are required (Huntwork, 2007).

To confirm that the increase in miniature postsynaptic potentials resulted from increased spontaneous vesicle fusion, as opposed to nonvesicular dumping of glutamate at the synapse, a temperature-sensitive allele of dynamin (shibire^TS1) was used that blocks endocytosis at 32°C. High-frequency stimulation at 32°C depletes the synaptic vesicle pool in shibire mutants and would be predicted to eliminate the enhanced spontaneous vesicle fusion in double mutants. After a 5-min 10-Hz stimulation train at 32°C in shibire^TS1; cpx^SH1 double mutants, which abolished evoked release, the elevated mini frequency observed at permissive temperatures was eliminated. The frequency of spontaneous release in cpx^SH1 mutants remained strongly elevated in 0 mM extracellular calcium, ruling out an aberrant influx of calcium into the presynaptic nerve terminal as the cause for the enhanced spontaneous release. These observations suggest that complexin functions as the synaptic vesicle fusion clamp in vivo, and with its loss, synaptic vesicles continuously fuse in the absence of stimulation (Huntwork, 2007).

The large increase in spontaneous fusion observed at complexin mutant NMJs provided an opportunity to determine the consequences of altered complexin function and increased mini frequency on synaptic growth. To analyze synaptic structure, synaptic varicosities were counted in age-matched control and cpx^SH1 third-instar NMJs. cpx^SH1 mutants showed a two-fold overproliferation of boutons at each muscle examined, as well as 64% more active zones. Neuronal expression of a complexin transgene reverted the synaptic overgrowth phenotype. These data eliminate any structural considerations that could account for the enhanced minis, as the 64% increase in the number of active zones is insufficient by far to explain the >20-fold increase in spontaneous release (Huntwork, 2007).

This analysis demonstrates that the interactions of complexin with SNAREs provide a molecular fusion clamp to prevent spontaneous release at synapses in the absence of stimulation. These in vivo observations match well with the predicted fusion clamp model based on the in vitro reduction of SNARE-mediated fusion by complexin. Although analysis of cultured hippocampal autaptic neurons lacking two of the four mouse complexin genes showed reduced neurotransmitter release, which matches the observations in Drosophila, mini frequency in the mouse system was reported to be unchanged. Although it is unclear what underlies the discrepancy in mini frequency between models, one potential explanation is that in mouse either complexin 3 or 4 or other unknown components of the fusion clamp machinery compensate for the loss of complexin (Huntwork, 2007).

It has long been known that minis represent single vesicle fusion events, but their underlying function has remained unclear. The current results unexpectedly uncovered a marked effect on synaptic growth at the Drosophila NMJ in complexin mutants. Although previous studies in hyperexcitable Drosophila mutants have demonstrated that increased neuronal activity promotes synaptic growth, the contributions of spontaneous versus evoked signaling are unknown. Activity-dependent retrograde signaling by the postsynaptic calcium sensor Synaptotagmin 4 has also been shown to transiently increase mini frequency 100-fold and trigger enhanced synaptic growth at Drosophila embryonic NMJs. It is tempting to speculate that regulation of complexin during retrograde signaling underlies activity-dependent enhancement of spontaneous fusion and synaptic growth. There is at present no evidence that the increased minis cause the morphological overgrowth, and further studies will be required to dissect the mechanism by which complexin mutants enhance synaptic growth. Recent work in mammals suggests that spontaneous release can regulate dendritic spine morphogenesis and dendritic protein synthesis. Complexin dysfunction has also been implicated in human diseases, including schizophrenia, indicating that abnormal spontaneous fusion may contribute to certain neurological diseases. Given that complexin and the synaptic vesicle calcium sensor Synaptotagmin 1 may compete for binding to SNARE complexes, an attractive model for synaptic vesicle exocytosis is that complexin stabilizes a hemifused intermediate that can complete full fusion upon calcium activation of Synaptotagmin 1. Together with Synaptotagmin 1, complexin provides a key neuronal modulator of SNARE function that adapts the ubiquitous membrane trafficking machinery for synaptic vesicle fusion (Huntwork, 2007).

Search PubMed for articles about Drosophila Complexin

Bowen, M. E., et al. (2005). Single-molecule studies of synaptotagmin and complexin binding to the SNARE complex. Biophys. J. 89: 690-702. PubMed Citation: 15821166

Bracher, A., et al. (2002). X-ray structure of a neuronal complexin-SNARE complex from squid. J. Biol. Chem. 277: 26517-26523. PubMed Citation: 12004067

Brose, N. (2008). For better or for worse: complexins regulate SNARE function and vesicle fusion. Traffic 9: 1403-1413. PubMed Citation: 18445121

Cai, H., et al. (2008). Complexin II plays a positive role in Ca2+-triggered exocytosis by facilitating vesicle priming. Proc. Natl. Acad. Sci. 105: 19538-19543. PubMed Citation: 19033464

Chen, X., et al. (2002). Three-dimensional structure of the complexin/SNARE complex. Neuron 33: 397-409. PubMed Citation: 11832227

Giraudo, C. G., et al. (2006). A clamping mechanism involved in SNARE-dependent exocytosis, Science 313: 676-680. PubMed Citation: 16794037

Giraudo, C. G., et al. (2009). Alternative zippering as an on-off switch for SNARE-mediated fusion. Science 323: 512-516. PubMed Citation: 19164750

Guan, R., Dai, H. and Rizo, J. (2008). Binding of the Munc13-1 MUN domain to membrane-anchored SNARE complexes. Biochemistry 47: 1474-1481. PubMed Citation: 18201107

Huntwork, S. and Littleton, J. T. (2007). A complexin fusion clamp regulates spontaneous neurotransmitter release and synaptic growth. Nat. Neurosci. 10: 1235-1237. PubMed Citation: 17873870

Ishizuka, T., et al. (1995). Synaphin: a protein associated with the docking/fusion complex in presynaptic terminals. Biochem. Biophys. Res. Commun. 213: 1107-1114. PubMed Citation: 7654227

Malsam, J., et al. (2009). The carboxy-terminal domain of complexin I stimulates liposome fusion. Proc. Natl. Acad. Sci. 106: 2001-2006. PubMed Citation: 19179400

Maximov, A., et al. (2009). Complexin controls the force transfer from SNARE complexes to membranes in fusion. Science 323: 516-521. PubMed Citation: 19164751

McMahon, H. T., et al. (1995). Complexins: cytosolic proteins that regulate SNAP receptor function, Cell 83: 111-119. PubMed Citation: 7553862

Pabst, S., et al. (2000). Selective interaction of complexin with the neuronal SNARE complex. Determination of the binding regions. J. Biol. Chem. 275: 19808-19818. PubMed Citation: 10777504

Pabst, S., et al. (2002). Rapid and selective binding to the synaptic SNARE complex suggests a modulatory role of complexins in neuroexocytosis. J. Biol. Chem. 277: 7838-7848. PubMed Citation: 11751907

Reim, K., et al. (2005). Structurally and functionally unique complexins at retinal ribbon synapses, J. Cell Biol. 169: 669-680. PubMed Citation: 15911881

Rizo, J. and Rosenmund, C. (2008). Synaptic vesicle fusion. Nat. Struct. Mol. Biol. 15: 665-674. PubMed Citation: 18618940

Schaub, J. R., et al. (2006). Hemifusion arrest by complexin is relieved by Ca(2+)-synaptotagmin I. Nat. Struct. Mol. Biol. 13: 748-750. PubMed Citation: 16845390

Strenzke, N., et al. (2009). Complexin-I is required for high-fidelity transmission at the endbulb of held auditory synapse. J. Neurosci. 29: 7991-8004. PubMed Citation: 19553439

Takahashi, S., et al. (1995). Identification of two highly homologous presynaptic proteins distinctly localized at the dendritic and somatic synapses. FEBS Lett. 368: 455-460. PubMed Citation: 7635198

Tang, J., et al. (2006). A complexin/synaptotagmin 1 switch controls fast synaptic vesicle exocytosis. Cell 126: 1175-1187. PubMed Citation: 16990140

Tokumaru, H. et al. (2001). SNARE complex oligomerization by synaphin/complexin is essential for synaptic vesicle exocytosis. Cell 104: 421-432. PubMed Citation: 11239399

Weninger, K., et al. (2008). Accessory proteins stabilize the acceptor complex for synaptobrevin, the 1:1 Syntaxin/SNAP-25 complex. Structure 16: 308-320. PubMed Citation: 18275821

Xue, M., et al. (2007). Distinct domains of complexin I differentially regulate neurotransmitter release. Nat. Struct. Mol. Biol. 14: 949-958. PubMed Citation: 17828276

Xue, M., et al. (2008). Complexins facilitate neurotransmitter release at excitatory and inhibitory synapses in mammalian central nervous system. Proc. Natl. Acad. Sci. 105: 7875-7880. PubMed Citation: 18505837

Xue, M., et al. (2009). Tilting the balance between facilitatory and inhibitory functions of mammalian and Drosophila Complexins orchestrates synaptic vesicle exocytosis. Neuron 64: 367-380. PubMed Citation: 19914185

Yoon, T. Y., et al. (2008). Complexin and Ca2+ stimulate SNARE-mediated membrane fusion. Nat. Struct. Mol. Biol. 15: 707-713. PubMed Citation: 18552825

date revised: 2 July 2010

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