Munc18-bound syntaxin readily forms SNARE complexes with synaptobrevin in native plasma membranes

Area of Science:

• Cell Biology
• Neuroscience
• Molecular Biology

Background:

• Munc18-1 is a Sec1/Munc18-like (SM) protein vital for regulated exocytosis in neurons and neuroendocrine cells.
• Munc18-1 binds syntaxin 1, stabilizing it in a closed conformation, which seemingly inhibits SNARE complex formation required for membrane fusion.

Purpose of the Study:

• To investigate the role of Munc18-1 in SNARE complex formation and regulated exocytosis in intact cellular membranes.
• To reconcile the in vitro observation of Munc18-1 inhibiting syntaxin 1 function with its essential biological role.

Main Methods:

• Experiments were conducted on intact, exocytosis-competent plasma membrane lawns.
• Recombinant proteins (synaptobrevin, syntaxin 1, SNAP-25) were used to probe Munc18-1 displacement from syntaxin 1.
• Studies utilized membranes from SNAP-25-deficient mice to assess the requirement of endogenous SNAP-25.

Main Results:

• Munc18-1 forms a complex with syntaxin 1 on the plasma membrane that permits SNARE complex assembly.
• Munc18-1 bound to syntaxin 1 can be displaced by synaptobrevin, but not syntaxin 1 or SNAP-25.
• This displacement is dependent on the presence of endogenous SNAP-25.

Conclusions:

• Munc18-1 facilitates the formation of a syntaxin 1-SNAP-25 complex, acting as a crucial intermediate in the exocytosis pathway.
• This Munc18-1-mediated complex serves as an acceptor for vesicle-associated synaptobrevin, promoting membrane fusion.
• The findings resolve the paradox of Munc18-1's function in regulated exocytosis.