Biophysical Society Thematic Meeting| Padova 2019

Quantitative Aspects of Membrane Fusion and Fission

Thursday Speaker Abstracts

VISUALIZING MEMBRANE STRUCTURAL REMODELING DURING FUSION AND FISSION IN LIVE CELLS Ling-Gang Wu 1 ; 1 NIH, NINDS, Bethesda, Maryland, USA Fusion and fission mediate many biological processes, such as exocytosis, endocytosis, intracellular trafficking, and viral entry. Despite intense studies, membrane structural remodeling mediating fusion and fission had not been real-time observed, and is thus poorly understood. With super-resolution STED microscopy, We observed the hemi-fused ?-shaped structure for the first time in live cells, the neuroendocrine chromaffin cells and pancreatic b-cells. This structure was generated from fusion pore opening or closure (fission) at the plasma membrane. Unexpectedly, its transition to full fusion or fission was determined by competition between fusion and calcium/dynamin-dependent fission mechanisms, and was surprisingly slow (seconds to tens of seconds) in a significant fraction of the events. These results provide key missing evidence over the past three decades proving the hemi-fusion and hemi-fission hypothesis in live cells, and reveal the hemi-fused intermediate as a key structure controlling fusion/fission, as fusion and fission mechanisms compete to determine its transition to fusion or fission. Passing through hemi-fusion, vesicle fusion at the plasma membrane generates an W-shape membrane profile, which may enlarge or shrink while maintaining vesicular membrane proteins. Closure of fusion-generated ?-profiles, which produces various sizes of vesicles, is a major mechanism mediating rapid and slow endocytosis. Strong calcium influx triggers dynamin-mediated closure, whereas weak calcium influx facilitates the merging of ?-profiles with the plasma membrane via shrinking rather than full-collapse as generally believed. W-profile shrinking and merging are mediated by strong membrane tension provided by dynamic assembly of filamentous actin, involving ATP hydrolysis, N-WASP and formin. In summary, we have visualized a series of membrane structural changes that mediate vesicle fusion, merging with the plasma have membrane, fission, their crucial function in exo- and endocytosis, and a part of their underlying molecular mechanisms.

30