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The Munc18-1 domain 3a hinge-loop controls syntaxin-1A nanodomain assembly and engagement with the SNARE complex during secretory vesicle priming

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ABSTRACT

Kasula et al. use single-molecule imaging to reveal the diffusional signature for the SNARE proteins Munc18-1 and syntaxin-1A during secretory vesicle priming. The authors show that a conformational change in the Munc18-1 domain 3a hinge-loop regulates engagement of syntaxin-1A in the SNARE complex.

No MeSH data available.


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Schematic of the fate of the Munc18-1. (A) Syntaxin-1A interacts with Munc18-1 in a closed conformation during trafficking and before stimulation of exocytosis. (B) A conformational change in the Munc18-1 domain 3a loop (triggered by unknown mechanisms) leads to the opening of syntaxin-1A. The closed conformation in the first panel has been observed in a crystal structure of a complex of Munc18-1 and the soluble portion of syntaxin-1A (Burkhardt et al., 2008). The extended conformation of Munc18-1 domain 3a depicted here is observed in Sec1/Munc18 proteins crystallized in the absence of SNARE partners (Hu et al., 2011). (C and D) The domain 3a loop provides a platform for interacting with the v-SNARE VAMP2, allowing assembly of syntaxin-1A with VAMP2 and SNAP-25. This model is based on the structure of the yeast SM protein Vps33 and the cognate v-SNARE Nyv1 reported by Baker et al. (2015).
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fig7: Schematic of the fate of the Munc18-1. (A) Syntaxin-1A interacts with Munc18-1 in a closed conformation during trafficking and before stimulation of exocytosis. (B) A conformational change in the Munc18-1 domain 3a loop (triggered by unknown mechanisms) leads to the opening of syntaxin-1A. The closed conformation in the first panel has been observed in a crystal structure of a complex of Munc18-1 and the soluble portion of syntaxin-1A (Burkhardt et al., 2008). The extended conformation of Munc18-1 domain 3a depicted here is observed in Sec1/Munc18 proteins crystallized in the absence of SNARE partners (Hu et al., 2011). (C and D) The domain 3a loop provides a platform for interacting with the v-SNARE VAMP2, allowing assembly of syntaxin-1A with VAMP2 and SNAP-25. This model is based on the structure of the yeast SM protein Vps33 and the cognate v-SNARE Nyv1 reported by Baker et al. (2015).

Mentions: The Munc18-1 domain 3a hinge-loop therefore acts as a dual trigger that promotes both syntaxin-1A incorporation into the trans-SNARE complex and the concomitant release of Munc18-1 (Fig. 7). The opening of syntaxin-1A and the disassembly of Munc18-1 (Gerber et al., 2008) are therefore likely to enable its subsequent interaction with SNAP25. It has been proposed that the Munc18-1 hinge-loop can undergo a significant conformational change, adopting an extended structure that precludes binding to “closed” syntaxin-1A (Hu et al., 2011; Fig. 7, A and B). The extended hinge-loop has also been suggested to bind directly to VAMP2 and promote SNARE assembly and vesicle fusion (Fig. 7, C and D; Parisotto et al., 2014). Our data showing the opposing changes in Munc18-1 and syntaxin-1A mobility suggest that Munc18-1 engages syntaxin-1A into the trans-SNARE complex before being disassembled during priming, which is suggestive of a lock-and-go mechanism. Although our data suggest that Munc18-1 is released from syntaxin-1A, it is still unclear whether this release occurs before or after syntaxin-1A engagement into the SNARE complex. The latter hypothesis would imply that Munc18-1 undergoes a transient reduction in mobility during syntaxin-1A engagement before its release. We did not detect such a transient confinement in our experiments, suggesting this is not the case, although this could possibly be caused by the limited temporal resolution of our data acquisition. Alternatively, Munc18-1 release could occur before syntaxin-1A engagement and simply control the level of opened syntaxin-1A available in nanodomains of the plasma membrane. Future experiments will be key to distinguishing between these two possibilities.


The Munc18-1 domain 3a hinge-loop controls syntaxin-1A nanodomain assembly and engagement with the SNARE complex during secretory vesicle priming
Schematic of the fate of the Munc18-1. (A) Syntaxin-1A interacts with Munc18-1 in a closed conformation during trafficking and before stimulation of exocytosis. (B) A conformational change in the Munc18-1 domain 3a loop (triggered by unknown mechanisms) leads to the opening of syntaxin-1A. The closed conformation in the first panel has been observed in a crystal structure of a complex of Munc18-1 and the soluble portion of syntaxin-1A (Burkhardt et al., 2008). The extended conformation of Munc18-1 domain 3a depicted here is observed in Sec1/Munc18 proteins crystallized in the absence of SNARE partners (Hu et al., 2011). (C and D) The domain 3a loop provides a platform for interacting with the v-SNARE VAMP2, allowing assembly of syntaxin-1A with VAMP2 and SNAP-25. This model is based on the structure of the yeast SM protein Vps33 and the cognate v-SNARE Nyv1 reported by Baker et al. (2015).
© Copyright Policy - openaccess
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5037406&req=5

fig7: Schematic of the fate of the Munc18-1. (A) Syntaxin-1A interacts with Munc18-1 in a closed conformation during trafficking and before stimulation of exocytosis. (B) A conformational change in the Munc18-1 domain 3a loop (triggered by unknown mechanisms) leads to the opening of syntaxin-1A. The closed conformation in the first panel has been observed in a crystal structure of a complex of Munc18-1 and the soluble portion of syntaxin-1A (Burkhardt et al., 2008). The extended conformation of Munc18-1 domain 3a depicted here is observed in Sec1/Munc18 proteins crystallized in the absence of SNARE partners (Hu et al., 2011). (C and D) The domain 3a loop provides a platform for interacting with the v-SNARE VAMP2, allowing assembly of syntaxin-1A with VAMP2 and SNAP-25. This model is based on the structure of the yeast SM protein Vps33 and the cognate v-SNARE Nyv1 reported by Baker et al. (2015).
Mentions: The Munc18-1 domain 3a hinge-loop therefore acts as a dual trigger that promotes both syntaxin-1A incorporation into the trans-SNARE complex and the concomitant release of Munc18-1 (Fig. 7). The opening of syntaxin-1A and the disassembly of Munc18-1 (Gerber et al., 2008) are therefore likely to enable its subsequent interaction with SNAP25. It has been proposed that the Munc18-1 hinge-loop can undergo a significant conformational change, adopting an extended structure that precludes binding to “closed” syntaxin-1A (Hu et al., 2011; Fig. 7, A and B). The extended hinge-loop has also been suggested to bind directly to VAMP2 and promote SNARE assembly and vesicle fusion (Fig. 7, C and D; Parisotto et al., 2014). Our data showing the opposing changes in Munc18-1 and syntaxin-1A mobility suggest that Munc18-1 engages syntaxin-1A into the trans-SNARE complex before being disassembled during priming, which is suggestive of a lock-and-go mechanism. Although our data suggest that Munc18-1 is released from syntaxin-1A, it is still unclear whether this release occurs before or after syntaxin-1A engagement into the SNARE complex. The latter hypothesis would imply that Munc18-1 undergoes a transient reduction in mobility during syntaxin-1A engagement before its release. We did not detect such a transient confinement in our experiments, suggesting this is not the case, although this could possibly be caused by the limited temporal resolution of our data acquisition. Alternatively, Munc18-1 release could occur before syntaxin-1A engagement and simply control the level of opened syntaxin-1A available in nanodomains of the plasma membrane. Future experiments will be key to distinguishing between these two possibilities.

View Article: PubMed Central - HTML - PubMed

ABSTRACT

Kasula et al. use single-molecule imaging to reveal the diffusional signature for the SNARE proteins Munc18-1 and syntaxin-1A during secretory vesicle priming. The authors show that a conformational change in the Munc18-1 domain 3a hinge-loop regulates engagement of syntaxin-1A in the SNARE complex.

No MeSH data available.


Related in: MedlinePlus