Exocyst


The exocyst is an octameric protein complex involved in vesicle trafficking, specifically the tethering and spatial targeting of post-Golgi vesicles to the plasma membrane prior to vesicle fusion. It is implicated in a number of cell processes, including exocytosis, cell migration, and growth.

Subunits

The exocyst is composed of eight subunits, whose nomenclature differs between mammalian cells and Saccharomyces cerevisiae.
SubunitMammalian cellsSaccharomyces cerevisiae
1EXOC1Sec3
2EXOC2Sec5
3EXOC3Sec6
4EXOC4Sec8
5EXOC5Sec10
6EXOC6Sec15
7EXOC7Exo70
8EXOC8Exo84

Function

The exocyst complex serves to direct vesicles after the Golgi complex to specific locations on the plasma membrane and to mediate their tethering and localization to the membrane immediately before fusion. Because of this function, the exocyst complex is heavily involved in exocytosis. Sec3 and Exo70 are localized to the plasma membrane, and are physically attached to the membrane by Rho GTPases such as CDC42. Other complementary exocyst components such as Sec15 and Sec4 are localized to the vesicle membrane. Exocyst proteins on the plasma membrane bind vesicular exocyst proteins, bringing the vesicle very close to the plasma membrane in a fashion similar to the SNARE interactions to facilitate fusion.
The exocyst also interacts with Rho GTPases responsible for controlling cell polarity and the activity of the cytoskeleton.