Continuum architecture dynamics of vesicle tethering in exocytosis

Essential for eukaryotes, multiple copies of the exocyst complex tether each secretory vesicle to the plasma membrane (PM) in constitutive exocytosis. The exocyst higher-order structure (ExHOS) that coordinates the action of these multiple exocysts remains unexplored. We integrated particle tracking, super-resolution microscopy, and cryo-electron tomography to time-resolve the continuum conformational landscape of the ExHOS and to functionally annotate its different conformations. We found that 7 exocysts form a flexible ring-shaped ExHOS that tethers vesicles at <45 nm from the PM. The ExHOS rapidly expands while pulling the vesicle toward the PM in a stepwise mechanism comprising three metastable states at 27, 18, and 5 nm from the PM. After fusion, Sec18 mediates the disassembly of the stationary ExHOS, a function that controls the rate of exocytosis. By resolving biophysical principles in situ, we reconstructed the spatiotemporal dynamics of the multimeric architecture controlling vesicle tethering in exocytosis.