Synchronous exocytosis in Paramecium cells involves the rapid (less than or equal to 1 s) dephosphorylation of a 65-kD phosphoprotein, which, after a lag phase of approximately 5 s, is reversed within approximately 20 s. Exocytosis inhibitors suppress this reaction; stimulatory and inhibitory effects are dose dependent. The dephosphorylation of the 65-kD phosphoprotein occurs only in exocytosis-competent strains, but not in mutant strains that cannot carry out membrane fusion, or that are devoid of secretory organelles or cannot transport them to the cell membrane. Since under all conditions analyzed the transient dephosphorylation of the 65-kD phosphoprotein strictly parallels the actual amount of exocytosed organelles, this process might be involved in exocytosis performance, perhaps in its initiation.

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