Genetic analysis of regulated exocytosis can be accomplished in ciliates, since mutants defective in stimulus-dependent secretion of dense-core vesicles can be identified. In Tetrahymena thermophila, secretion in wild-type cells can result in their encapsulation by the proteins released from vesicle cores. Cells with defects in secretion were isolated from mutagenized homozygous cells that were generated using a highly efficient method. Screening was based both on a visual assay for encapsulation, and on a novel panning step using differential centrifugation to take advantage of the selective mobility of mutants that fail to encapsulate upon stimulation. 18 mutants with defects in several ordered steps have been identified. Defects in a set of these could be localized to three stages: granule formation, transport to cell surface docking sites, and exocytosis itself. Mutants with defects in this last stage can be ordered into successive steps based on several criteria, including their responsiveness to multiple secretagogues and Ca2+ ionophores. The results of both somatic and genetic complementation on selected pairs also help to characterize the defective factors.

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