Phosphorylation Events in Regulation of Exocytosis
The ciliated protozoan Paramecium tetraurelia is an especially well suited organism with which to study the process of signal transduction associated with stimulus exocytosis coupling. Wild type (wt) Paramecium contains thousands of membrane-bounded secretory organelles, called trichocysts. These organelles are docked at specific sites below the plasma membrane. The site of a docked trichocyst is marked in freeze-fracture electron microscopy by a specific membrane microdomain consisting of an outer ring of intramembrane particles (IMPs) with a central rosette composed of 9–11 IMPs (Janish, 1972, Satir 1974). Upon stimulation with secretagogue, such as trinitrophenol (TNP), in the presence of Ca2+, the trichocyst membrane and the plasma membrane fuse and the trichocyst content, called trichocyst matrix (tmx), expands lengthwise, leaving the cell as a paracrystalline needle-like structure 40μm long. Both membrane fusion and matrix expansion are Ca2+-dependent irreversible events. The released paracrystalline tmx is easily observed in the light microscope. This has been used to assay the isolation of several secretory mutants of this cell (Beisson et al., 1976; Pollack, S., 1974; Beisson and Rossignol, 1975). In addition to the two Ca2+-dependent events mentioned above, a third event has been described which involves a covalent modification of a protein.
KeywordsMembrane Fusion Intramembrane Particle Dephosphorylated Form 63kDa Phosphoprotein Plasma Membrane Fuse
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