The function of a specific intramembrane particle array, "the fusion rosette," an essential requirement for exocytosis of trichocysts in Paramecium, was probed with a temperature sensitive secretory mutant (nd9). The cells were grown at 27°C, the nonpermissive, nonreleasing temperature at which fusion rosettes do not assemble. Exocytosis could be triggered, nonetheless, by addition of 40 μM ionophore A23187 and 15 mM Ca2+ but not Mg2+. Rosette function is bypassed by this procedure, suggesting that during normal release, the rosette acts as a Ca2+ channel that allows development of a site-specific increase in Ca2+, which in turn induces fusion and release.
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