Genetic control of membrane mosaicism.

The mosaic and dynamic character of cellular membranes is illustrated by the specific intramembrane particle array, the fusion rosette, found to be essential for membrane fusion and secretion in the ciliated protozoa, Tetrahymena and Paramecium. The rosette is not a permanent site within the membrane. When secretion of mucocysts is synchronized by treating cells with the local anesthetic dibucaine, all rosettes disappear, only to reassemble as new mucocysts mature. Assembly of the functional rosette is under genetic control. A series of secretory mutants of Paramecium, blocked in various stages of the secretory cycle, has been studied (11). Mutants that do not secrete lack the fusion rosette, although other intramembrane particle components of the fusion site are present. Certain properties of the rosette, in particular its particle partition coefficient, are temperature-dependent, which may affect the ability of the rosette particles to assemble. A temperature-sensitive mutant, nd9, secretes normally, and has rosettes at 18 degrees C, but fails to secrete attached trichocysts at 27 degrees C.