Paramecium fusion rosettes: Possible function as Ca²⁺ gates

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.