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Concept
Vesicle fusion
Summary
Vesicle fusion is the merging of a vesicle with other vesicles or a part of a cell membrane. In the latter case, it is the end stage of secretion from secretory vesicles, where their contents are expelled from the cell through exocytosis. Vesicles can also fuse with other target cell compartments, such as a lysosome. Exocytosis occurs when secretory vesicles transiently dock and fuse at the base of cup-shaped structures at the cell plasma membrane called porosome, the universal secretory machinery in cells. Vesicle fusion may depend on SNARE proteins in the presence of increased intracellular calcium (Ca2+) concentration.
Stimuli that trigger vesicle fusion act by increasing intracellular Ca2+.
Synaptic vesicles commit vesicle fusion by a nerve impulse reaching the synapse, activating voltage-dependent calcium channels that cause influx of Ca2+ into the cell.
In the endocrine system, many hormones are released by their releasing hormones binding to G protein coupled receptors coupled to the Gq alpha subunit, activating the IP3/DAG pathway to increase Ca2+. Examples of this mechanism include:
Gonadotropin releasing hormone
Thyrotropin releasing hormone
Growth hormone releasing hormone (minor pathway - main one is cAMP dependent pathway)
Model systems consisting of a single phospholipid or a mixture have been studied by physical chemists. Cardiolipin is found mainly in mitochondrial membranes, and calcium ions play an important role in the respiratory processes mediated by the mitochondrion. The forces involved have been postulated to explain this process in terms of nucleation for agglomeration of smaller supramolecular entities or phase changes in the structure of the biomembranes.
In synaptic vesicle fusion, the vesicle must be within a few nanometers of the target membrane for the fusion process to begin. This closeness allows the cell membrane and the vesicle to exchange lipids which is mediated by certain proteins which remove water that comes between the forming junction.
Official source
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