Real-Time Visualization of Complexin During Single Exocytic Events

Understanding the fundamental role of SNARE complexes in membrane fusion requires knowledge of the spatiotemporal dynamics of their assembly. To this end, we visualized complexin (cplx), a cytosolic protein that binds assembled SNARE complexes, during single exocytic events in live cells. We show that cplx appears briefly during full fusion. However, a truncated version of cplx containing only the SNARE-complex binding region persists at fusion sites for seconds and causes fusion to be transient. Resealing pores with the mutant cplx only partially release transmitter and lipid probes, indicating they are narrow and not purely lipidic in structure. Depletion of cplx similarly causes secretory cargo to be retained. Thus, complexin is recruited at a late step in exocytosis and modulates fusion pores composed of SNARE complexes.