Munc-18 (an acronym for mammalian uncoordinated-18) proteins are the mammalian homologue of unc-18 proteins (which can be found in organisms such as the C. elegans) and are a member of the Sec1/Munc18-like (SM) protein family. Munc-18 proteins have been identified as essential components of the synaptic vesicle fusion protein complex and are crucial for the regulated exocytosis of neurons and neuroendocrine cells.
Munc-18 binds syntaxin and forms a syntaxin/munc-18 complex which is thought to precede and/or regulate the formation of vesicle priming, a process mediated by VAMP, SNAP-25 and syntaxin. Munc18-1, a member of the SM family, has multiple roles in exocytosis. It directly promotes syntaxin stability and either controls the spatially correct assembly of core complexes for SNARE-dependent fusion, or acts as a direct component of the fusion machinery through the interaction with SNARE core. Munc18a, which binds specifically to the N-terminal of syntaxin, causes a conformation change, activating syntaxin, which in turn connects to the ternary-SNARE complex. Deletion of munc18-1 leads to a defect in secretory vesicle docking. Furthermore, the munc18-1 deficient mouse is the first mouse model wherein neurotransmitter secretion is completely absent. This mouse model is appropriately titled the "silent mouse."
This outline below presents a broad modeling of how Munc-18 is thought to play a role in vesicle docking and fusion, allowing for intentional exocytosis. As it is a combined preliminary modeling, more research is necessary to fully understand the role of Munc-18 in this process.
- Munc18-1 binds to a closed form of syntaxin-1, blocking SNARE complex formation. This is thought to affect vesicle docking
- Munc13 opens syntaxin-1, Munc18-1 is translocated to the SNARE complex, which releases the inhibitory affect, allowing assembly (specifically of the alpha helix 4 part bundle)
- It is thought that Munc18-1 stabilizes the formed trans-SNARE complex, preventing its dissociation
- The SNARE complex, potentially with the assistance of Munc-18 brings the membranes together and causes fusion
The following is a list of human munc-18 proteins:
|MUNC18-1||STXBP1||syntaxin binding protein 1|
|MUNC18-2||STXBP2||syntaxin binding protein 2|
|MUNC18-3||STXBP3||syntaxin binding protein 3|
|MUNC18-4||STXBP4||syntaxin binding protein 4|
|MUNC18-5||STXBP5||syntaxin binding protein 5|
|MUNC18-6||STXBP6||syntaxin binding protein 6|
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- Pevsner J, Hsu SC, Braun JE, Calakos N, Ting AE, Bennett MK, Scheller RH (August 1994). "Specificity and regulation of a synaptic vesicle docking complex". Neuron 13 (2): 353–61. doi:10.1016/0896-6273(94)90352-2. PMID 8060616.
- Burgoyne RD, Barclay JW, Ciufo LF, Graham ME, Handley MT, Morgan A. (2009). "The functions of Munc18-1 in regulated exocytosis.". Ann N Y Acad Sci 1152: 76–86. doi:10.1111/j.1749-6632.2008.03987.x. PMID 19161378.
- Diao J, Su Z, Lu X, Yoon TY, Shin YK, Ha T (March 2010). "Single-Vesicle Fusion Assay Reveals Munc18-1 Binding to the SNARE Core Is Sufficient for Stimulating Membrane Fusion.". ACS Chem Neurosci 1 (3): 168–174. doi:10.1021/cn900034p. PMID 20300453.
- Kasai, H.; Takahashi, N.; Tokumaru, H. (2012). "Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis". Physiological Reviews 92 (4): 1915–1964. doi:10.1152/physrev.00007.2012. PMID 23073634.
- Toonen RF, de Vries KJ, Zalm R, Südhof TC, Verhage M (June 2005). "Munc18-1 stabilizes syntaxin 1, but is not essential for syntaxin 1 targeting and SNARE complex formation". J. Neurochem. 93 (6): 1393–400. doi:10.1111/j.1471-4159.2005.03128.x. PMID 15935055.
- Rizo, J.; Südhof, T. C. (2012). "The Membrane Fusion Enigma: SNAREs, Sec1/Munc18 Proteins, and Their Accomplices—Guilty as Charged?". Annual Review of Cell and Developmental Biology 28: 279–308. doi:10.1146/annurev-cellbio-101011-155818. PMID 23057743.