Stability, folding dynamics, and long-range conformational transition of the synaptic t-SNARE complex

Significance Intracellular membrane fusion is mediated by coupled folding and assembly of three or four soluble N -ethylmaleimide–sensitive factor attachment protein receptor (SNARE) proteins into a four-helix bundle. A rate-limiting step is the formation of a partial complex containing three helixes called the target (t)-SNARE complex on the target plasma membrane. The t-SNARE complex then serves as a template to guide stepwise zippering of the fourth helix, a process that is further regulated by other proteins. The synaptic t-SNARE complex readily misfolds. Consequently, its conformation, stability, and dynamics have not been well understood. Using optical tweezers and theoretical modeling, we elucidated the folding intermediates and kinetics of the t-SNARE complex and discovered a long-range conformational switch of t-SNAREs during SNARE zippering, which is essential for regulated SNARE assembly during synaptic vesicle fusion.