Single-Molecule Optical Tweezers Study of Regulated SNARE Assembly

Single-Molecule Optical Tweezers Study of Protein-Membrane Interactions

Numerous proteins directly or indirectly bind membranes to exert their roles in a wide variety of biological processes. Such membrane binding often occurs in the presence of an external mechanical force. It remains challenging to quantify these interactions using traditional experimental approaches based on a large number of molecules, due to ensemble averaging or the lack of mechanical force. Here we described a new single-molecule approach based on high-resolution optical tweezers to characterize protein-membrane interactions. A single membrane binding protein is attached to the lipid bilayer coated on a silica bead via a flexible polypeptide linker, tethered to another bead via a long DNA handle, and pulled away from the bilayer using optical tweezers. Dynamic protein binding and unbinding is detected by the corresponding changes in the extension of the protein-DNA tether with high spatiotemporal resolution, which reveals the membrane binding affinity, kinetics, and intermediates. We demonstrated the method using C2 domains of extended synaptotagmin 2 (E-Syt2) with a detailed protocol. The method can be widely applied to investigate complex protein-membrane interactions under well-controlled experimental conditions.

Single-Molecule Manipulation Study of Chaperoned SNARE Folding and Assembly with Optical Tweezers

Intracellular membrane fusion is primarily driven by coupled folding and assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). SNARE assembly is intrinsically inefficient and must be chaperoned by a family of evolutionarily and structurally conserved Sec1/Munc-18 (SM) proteins. The physiological pathway of the chaperoned SNARE assembly has not been well understood, partly due to the difficulty in dissecting the many intermediates and pathways of SNARE assembly and measure their associated energetics and kinetics. Optical tweezers have proven to be a powerful tool to characterize the intermediates involved in the chaperoned SNARE assembly. Here, we demonstrate the application of optical tweezers combined with a homemade microfluidic system into studies of synaptic SNARE assembly chaperoned by their cognate SM protein Munc18-1. Three synaptic SNAREs and Munc18-1 constitute the core machinery for synaptic vesicle fusion involved in neurotransmitter release. Many other proteins further regulate the core machinery to enable fusion at the right time and location. The methods described here can be applied to other proteins that regulate SNARE assembly to control membrane fusion involved in numerous biological and physiological processes.

Munc18-1 catalyzes neuronal SNARE assembly by templating SNARE association

Sec1/Munc18-family (SM) proteins are required for SNARE-mediated membrane fusion, but their mechanism(s) of action remain controversial. Using single-molecule force spectroscopy, we found that the SM protein Munc18-1 catalyzes step-wise zippering of three synaptic SNAREs (syntaxin, VAMP2, and SNAP-25) into a four-helix bundle. Catalysis requires formation of an intermediate template complex in which Munc18-1 juxtaposes the N-terminal regions of the SNARE motifs of syntaxin and VAMP2, while keeping their C-terminal regions separated. SNAP-25 binds the templated SNAREs to induce full SNARE zippering. Munc18-1 mutations modulate the stability of the template complex in a manner consistent with their effects on membrane fusion, indicating that chaperoned SNARE assembly is essential for exocytosis. Two other SM proteins, Munc18-3 and Vps33, similarly chaperone SNARE assembly via a template complex, suggesting that SM protein mechanism is conserved.