Rat peritoneal mast cells: a model system for studying membrane fusion.

We used the mast cell as a model system for studying some of the membrane events which occur during exocytosis. Our observations indicate that the maximum cluster size of IgE molecules necessary for the "on" signal to activate a mast cell is 10 or less and that the "off" signal is not associated with the gross patching or pinocytosis of IgE and its Fc receptors. Furthermore, the use of Con A-Sepharose beads to stimulate mast cells has shown that such signaling is localized to the areas of stimulus, but this localization is not a function of desensitization over the rest of the cell since the subsequent addition of soluble Con A to locally released cells induced generalized degranulation. Ca2+ influx therefore acts in a localized manner to initiate degranulation. Following receptor cross-linking, most of the membrane proteins and the layer of intervening cytoplasm are laterally displaced away from the areas of membrane interaction. This displacement may act as the signal for fusion to occur. The resulting fused bilayers are predominantly lipid, a situation which may be common in all transient membrane fusion. The mechanism of exposing histamine-containing granules to the extracellular space by blebbing is discussed.