Aspects of signal transduction in stimulus exocytosis‐coupling in Paramecium

This paper deals with the detailed mechanisms of signal transduction that lead to exocytosis during regulative secretion induced by specific secretagogues in a eukaryotic cell, Paramecium tetraurelia. There are at least three cellular compartments involved in the process: (I) the plasma membrane, which contains secretagogue receptors and other transmembrane proteins, (II) the cytoplasm, particularly in the region between the cell and secretory vesicle membranes, where molecules may influence interactions of the membranes, and (III) the secretory vesicle itself. The ciliated protozoan Paramecium tetraurelia is very well suited for the study of signal transduction events associated with exocystosis because this eukaryotic cell contains thousands of docked secretory vesicles (trychocysts) below the cell membrane which can be induced to release synchronously when trioggered with secretagogue. This ensures a high signal‐to‐noise ratio for events associated with this process. Upon release the trichocyst membrane fuses with the cell membrane fuses with the cell membrane and the trichocyst content undergoes a Ca²⁺‐dependent irreversible expansion. Secretory mutants are available which are blocked at different points in the signal transduction pathway. Aspects of the three components mentioned above that will be discussed here include (a) the properties of the vesicle content, its pH, and its membrane; (b) the role of phosphorylation/dephosphorylation of a cytosolic 63‐kilodalton (kDa)M_r protein in membrane fusion; and (c) how influx of extracellular Ca²⁺ required for exocytosis may take place via exocytic Ca²⁺ channels which may be associated with specific membrane microdomains (fusion rosettes).