Satir B H, Busch G, Vuoso A, Murtaugh T J
Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, New York 10461.
J Cell Biochem. 1988 Apr;36(4):429-43. doi: 10.1002/jcb.240360411.
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 cytoplasms, 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 exocytosis because this eukaryotic cell contains thousands of docked secretory vesicles (trichocysts) below the cell membrane which can be induced to release synchronously when triggered 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 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)Mr 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).
本文探讨了在真核细胞四膜虫中，由特定促分泌素诱导的调节性分泌过程中导致胞吐作用的信号转导详细机制。该过程至少涉及三个细胞区室：I）质膜，其含有促分泌素受体和其他跨膜蛋白；II）细胞质，特别是在细胞与分泌囊泡膜之间的区域，分子可能在此影响膜的相互作用；III）分泌囊泡本身。纤毛原生动物四膜虫非常适合用于研究与胞吐作用相关的信号转导事件，因为这种真核细胞在细胞膜下方含有数千个停靠的分泌囊泡（刺丝泡），当用促分泌素触发时，这些囊泡可被诱导同步释放。这确保了与该过程相关事件的高信噪比。释放后，刺丝泡膜与细胞膜融合，刺丝泡内容物经历钙离子依赖性不可逆膨胀。存在在信号转导途径不同点被阻断的分泌突变体。这里将讨论的上述三个组分的方面包括：a）囊泡内容物的性质、其pH值及其膜；b）细胞质中一种63千道尔顿（kDa）蛋白质的磷酸化/去磷酸化在膜融合中的作用；c）胞吐作用所需的细胞外钙离子内流如何通过可能与特定膜微区（融合玫瑰花结）相关的胞吐钙离子通道发生。
