The exocytotic fusion pore

T hE fusion pore is the molecular structure that transiently connects the lumens of two membrane compartments during their fusion. The fusion of membrane compartments occurs in all cells because intracellular trafficking of vesicles in both endocytic and exocytic pathways and in consti-tutive exocytosis are ubiquitous cellular processes. In addition, many other cells including endocrine, exocrine and neuronal cells have a specialized case where exocytosis only occurs in response to specific cellular stimuli. Despite its importance, the nature of the fusion pore is unknown. In some ways, this is surprising given how intensively the regulatory mechanisms of membrane fusion have been studied for both intracellular traffic The principal difficulty has been the lack of techniques to directly monitor the activity of single fusion pores in isolation: the reductionist approach that has proven uniquely suited for the study of ion channels. Recently, the application of patch clamp techniques to monitor the activity of individual fusion pores in mast cells has generated a wealth of novel and unexpected observations. Our goal here is to review these findings and speculate about their significance to our understanding of exocytotic fusion. Although the majority of the results come from mast cells, many of the observations are now being reproduced in other secretory cells suggesting that the conclusions may have wider significance in other membrane fusion reactions. <2 ms and are thus ideal for observing the transient intermediate states associated with vesicle fusion (Chandler, 1992). In a remarkable series of micrographs, Chandler and Heuser (1980) captured the formation of exocytotic fusion pores in mast cells. Similar fusion pores have since been seen, thus, providing a common picture of the exo-cytotic event in rapidly frozen cells. The micrographs clearly show membrane lined pores, 20-100 nm in diameter, that provide a water filled pathway between the lumen of the secretory granule and the extracellular environment. As can be seen from Fig. 1 C, these pores appear to be made of a curved bilayer which spans the granule and plasma membranes. quick freeze experiments also give a hint of the events preceding the formation of a fusion pore. In unstimulated mast cells the plasma membrane and secretory granule membranes are kept at least 100 nm apart (Fig. 1 A; Chandler and Heuser, 1980). After stimulation, a new pattern is observed in which the plasma membrane invaginates to form a dimple that approaches the granule membrane (Fig. 1 B; Chandler …