EXOCYTOTIC FUSION PORES EXHIBIT SEMI-STABLE STATES

Rapid-freezing/freeze-fracture electron microscopy and whole-cell capa citance techniques were used to study degranulation in peritoneal mast cells of the rat and the mutant beige mouse. These studies allowed us to create a time-resolved picture for fusion pore formation. After st imulation, a dimple in the plasma membrane formed a small contact area with the secretory granule membrane. Within this zone of apposition n o ordered proteinaceous specializations were seen. Electrophysiologica l technique measured a small fusion pore which widened rapidly to 1 nS . Thereafter, the fusion pore remained at semi-stable conductances bet ween 1 and 20 nS for a wide range of times, between 10 and 15,000 msec . These conductances correspond to pore diameters 25-36 nm. Ultrastruc tural data confirmed small pores of hourglass morphology, composed of biological membrane co-planar with both the plasma and granular membra nes. Later, the fusion pore rapidly increased in conductance, consiste nt with the observed morphology of omega-figures. The hallmarks of cha nnel-like behavior, instantaneous jumps in pore conductance between de fined levels, and sharp peaks in histograms of conductance dwell-time, were not seen. Since the morphology of small pores shows contiguous f racture planes, the electrical data represent pores that contain lipid . These combined morphological and electrophysiological data are consi stent with a lipid/protein complex mediating both the initial and late r stages of membrane fusion.