Protein Export Form The ER and The COP II Coat
Once proteins have folded and are ready to be transported out of the ER it is thought that they assemble at specific site within the ER called "ER exit sites". These sites can be transient but are most likely situated in the ER where the ER is close to the next transport compartment, the vesicular-tubular cluster (VTC) (also referred to as the ER-Golgi Intermediate Compartment (ERGIC)). The details of how proteins are concentrated or localized to the exit site is unclear (some aspects are emerging!) but the actual process of budding a vesicle containing these proteins start with a protein called Sec12. This protein recruits a small GTPase called Sar1 (think of Sar1 as a switch, it is active when bound to GTP and inactive when it hydrolyses the GTP to GDP). This in turn leads to the recruitment of a protein complex, the Sec23/Sec24 and the Sec13/Sec31 complex (also known as the COPII coat). In a nutshell what these proteins do is form a mesh at the ER exit site and the mesh through mechanical curvature forms a little "blob" that pinches off from the ER with proteins inside (think, playdough as the ER and your hand as the Sec complex pinching off little bits). The mesh disassembles off the budded vesicle when Sar1 hydrolyses the GTP to GDP. Interestingly, this activity of Sar1 is enhanced by Sec23/24.
Read more about this topic: TRAPP Complex
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