Plasma membrane resident 'fusion complexes' mediate reconstituted exocytosis

Calcium-triggered exocytosis is thought to be mediated by membrane-associat ed protein complexes. In sea urchin eggs, high concentrations of calcium ac tivate multiple 'fusion complexes' per cortical vesicle-plasma membrane doc king site. Some of these fusion complexes are known to reside in the vesicl e membrane. It is not known if fusion complexes also reside in the plasma m embrane, or if plasma membrane-resident fusion complexes require cognate pa rtners in the vesicle membrane. Using reconstitution, we show that N-ethylm aleimide treatment of either vesicles or plasma membrane fragments prior to reconstitution does not completely inhibit exocytosis. Treatment of both c omponents did result in complete inhibition. Upon reconstitution, cortical vesicles and the early endosomes formed by compensatory endocytosis both co ntributed, on average, two fusion complexes per reconstituted docking site. The plasma membrane contributed, on average, two fusion complexes per dock ing site when assembled with cortical vesicles, but only one complex when r econstituted with endosomes. We conclude that there are at least two types of plasma membrane-resident fusion complexes that participate in reconstitu ted cortical vesicle-plasma membrane fusion. The activity of one of these f usion complexes is target-specific for cortical vesicles, while the second type also supports fusion with endosomes.