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 degrees C, the nonpermissive, nonreleasing temperature at which fusion rosettes do not assemble. Exocytosis could be triggered, nonetheless, by addition of 40 micrometer ionophore A23187 and 15 mM Ca2+ but not Mg+. 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.