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 and also cell migration and growth.

Subunits
The exocyst is composed of eight subunits that aggregate in a polar fashion. In mammalian cells and in Saccharomyces cerevisiae, there are eight subunits: EXOC1, EXOC2, EXOC3, EXOC4, EXOC5, EXOC6, EXOC7, and EXOC8. These subunits were formerly known as Sec3, Sec5, Sec6, Sec8, Sec10, Sec15, Exo70, and Exo84, although they have been recently renamed.

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 (EXOC1) and Exo70 (EXOC7) 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 (EXOC6) 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.