Docking of secretory granules containing insulin occurs by interaction between EXOC3/Sec6 located on the membrane of the secretory granule and EXOC4/Sec8 located at the plasma membrane. Additional components (EXOC1, EXOC2, EXOC5, EXOC6, EXOC7, EXOC8) form the Exocyst Complex. A beta cell contains about 10 000 secretory granules. Of these, about 1000 are docked at the inner surface of the plasma membrane and a subset of about 100 docked granules form the "readily releasable" pool (granules which are released within about 5 minutes of glucose stimulation).
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