Chapter 12 Exocytosis: The Pulsing Fusion Pore
The elaborate intracellular membrane system of eukaryotic cells participates in vesicle trafficking and represents an important basis exploited in cell-to-cell signaling. Communication between cells involves the release of neurotransmitters, hormones and other chemical messengers that are stored in secretory vesicles and granules. A key event in the release of these primary messengers is exocytosis, consisting of fusion between the vesicle and the plasma membrane. This leads to the formation of a fusion pore through which a diffusional continuum between the vesicle lumen and the extracellular space is established. In the past, in vitro studies of biological membrane fusion considered this an almost impossible process, because large pressures had to be delivered to counteract the electrostatic repulsion owing to negatively charged membrane surfaces. It is only a decade or so that the omnipresent fusion between biological membranes started to be understood in greater detail. Since the SNARE hypothesis was proposed about a decade ago, several proteins have been identified to play a role in exocytosis, and attempts to define minimal molecular machinery for regulated exocytosis have been considered. However, several studies provided evidence for multiple modes of exocytosis, and that exocytosis may not necessarily lead to the release of vesicle cargo. The aim of this chapter is to review the results obtained on pituitary cells, specialized to release a number of important hormones and to highlight that there are multiple mechanisms of exocytosis present in the same cell. Moreover, the goal is to address elementary properties of exocytosis, consisting of the interaction between a single vesicle and the plasma membrane. These studies indicate that the long-thought concept of membrane fusion as an irreversible process will have to be changed. Here we discuss an unusually regular reversible opening of the fusion pore termed “the pulsing pore”.
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