Munc18-1 stabilizes syntaxin 1, but is not essential for syntaxin 1 targeting and SNARE complex formation.
Munc18-1, a member of the Sec1/Munc18 (SM) protein family, is essential for synaptic vesicle exocytosis. Munc18-1 binds tightly to the SNARE protein syntaxin 1, but the physiological significance and functional role of this interaction remain unclear. Here we show that syntaxin 1 levels are reduced by 70% in munc18-1 knockout mice. Pulse-chase analysis in transfected HEK293 cells revealed that Munc18-1 directly promotes the stability of syntaxin 1, consistent with a chaperone function. However, the residual syntaxin 1 in munc18-1 knockout mice is still correctly targeted to synapses and efficiently forms SDS-resistant SNARE complexes, demonstrating that Munc18-1 is not required for syntaxin 1 function as such. These data demonstrate that the Munc18-1 interaction with syntaxin 1 is physiologically important, but does not represent a classical chaperone-substrate relationship. Instead, the presence of SNARE complexes in the absence of membrane fusion in munc18-1 knockout mice indicates that Munc18-1 either controls the spatially correct assembly of core complexes for SNARE-dependent fusion, or acts as a direct component of the fusion machinery itself.
Toonen
Ruud F G
RF
Department of Functional Genomics, Center for Neurogenomics and Cognitive Research, Vrije Universiteit Amsterdam, Amsterdam, Netherlands.
de Vries
Klaas Jan
KJ
Zalm
Robbert
R
Südhof
Thomas C
TC
Verhage
Matthijs
M
D000818:Animals; D000954:Antigens, Surface / Q000235:genetics / Q000378:metabolism*; D001921:Brain / Q000378:metabolism; D002460:Cell Line; D005089:Exocytosis / Q000502:physiology*; D005260:Female; D006801:Humans; D046911:Macromolecular Substances / Q000378:metabolism; D008297:Male; D008561:Membrane Fusion / Q000502:physiology*; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D018832:Molecular Chaperones / Q000378:metabolism; D051938:Munc18 Proteins; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D017729:Presynaptic Terminals / Q000378:metabolism; D021381:Protein Transport / Q000502:physiology; D050600:SNARE Proteins; D013570:Synaptic Membranes / Q000378:metabolism*; D009435:Synaptic Transmission / Q000502:physiology; D050827:Syntaxin 1; D033921:Vesicular Transport Proteins / Q000235:genetics / Q000378:metabolism*
D000954:Antigens, Surface; D046911:Macromolecular Substances; D018832:Molecular Chaperones; D051938:Munc18 Proteins; D009419:Nerve Tissue Proteins; D050600:SNARE Proteins; C494212:STX1A protein, human; C497937:STXBP1 protein, human; C494214:Stx1a protein, mouse; C497938:Stxbp1 protein, mouse; D050827:Syntaxin 1; D033921:Vesicular Transport Proteins