Munc18-lipid interplay and spatial arrangements of Syntaxin-1a

Release of neurotransmitters in synapses is achieved by fusion of synaptic vesicles with the neuronal plasma membrane. Neurotransmitter-bearing synaptic vesicles are docked and primed for fusion at the active zone of the presynaptic plasma membrane, where their fusion is triggered by calcium influx into the cell. Efficient docking of the synaptic vesicles at the neuronal plasma membrane requires Syntaxin-Munc18 interaction. Syntaxin has often been observed to cluster in cell and model membranes and the lipid environment is an important factor regulating Syntaxin's lateral distribution. To determine how Munc18 aids the docking of synaptic vesicles, we investigate Munc18's effect on Syntaxin's lateral distribution, mobility, and orientation in model membranes. Here we show that Munc18 disrupts Syntaxin oligomers, in a manner that depends on the lipid environment, with a strong dependency on the concentration of phosphatidylinositol-4,5-bisphosphate (PIP2). Moreover, previously established interactions between Syntaxin and PIP2 are enhanced by Munc18. Finally, measurements of Syntaxin diffusion in planar supported membranes by single molecule tracking indicate that Munc18 alters Syntaxin's lateral diffusion in a lipid-dependent manner. In conclusion, lipids and Munc18 cooperate to modulate the distribution and mobility of Syntaxin in model membranes of defined composition.