Editing COPII (section)

==Structure==
[[File:PDB 2gao EBI.jpg|thumb|Human Sar1A bound to GDP]]
The COPII coat consists of an inner layer – a flexible meshwork of Sar1, Sec23, and Sec24 – and an outer layer made of Sec13 and Sec31.<ref name=Peotter2019>{{cite journal |vauthors=Peotter J, Kasberg W, Pustova I, Audhya A |title=COPII-mediated trafficking at the ER/ERGIC interface |journal=Traffic |volume=20 |issue=7 |pages=491–503 |date=July 2019 |pmid=31059169 |pmc=6640837 |doi=10.1111/tra.12654}}</ref> Sar1 resembles other Ras-family GTPases, with a core of six [[beta strand]]s flanked by three [[alpha helix|alpha helices]], and two flexible "switch domains". Unlike other Ras GTPases, Sar1 inserts into membranes via an N-terminal helix (rather than [[myristoylation]] or [[prenylation]]).<ref name=Peotter2019/>

These coat proteins are necessary but insufficient to direct or dock the vesicle to the correct target membrane.  [[SNARE (protein)|SNARE]], cargo, and other proteins are also needed for these processes to occur.

Pre-budding complex (composed of Sar1-GTP and Sec23/24) recruits the flexible Sec13p/31p complex, characterized by polymerization of the Sec13/31 complex with other Sec13/31 complexes to form a [[cuboctahedron]] with a broader lattice than its [[Clathrin]] vesicle analog. The formation of the cuboctahedron deforms the ER membrane and detaches the COPII vesicle (alongside cargo proteins and v-SNAREs), completing the COPII vesicle budding process.<ref name="Fath 2007">{{cite journal | vauthors = Fath S, Mancias JD, Bi X, Goldberg J | title = Structure and organization of coat proteins in the COPII cage | journal = Cell | volume = 129 | issue = 7 | pages = 1325–36 | date = June 2007 | pmid = 17604721 | doi = 10.1016/j.cell.2007.05.036 | s2cid = 10692166 | doi-access = free }}</ref>