thumb|dark green: Protein transport protein Sec23A, Homo sapiens, Gene names: [[SEC23A<br>brown: Protein transport protein Sec24A, Homo sapiens, Gene names: SEC24A<br>blue: Vesicle-trafficking protein SEC22b, Mus musculus, Gene names: SEC22B, Sec22l1<br>pink: C-terminal sorting motif ILE-ILE, Homo sapiens]]
The coat protein complex II, or COPII, is a group of proteins that facilitate the formation of vesicles to transport proteins from the endoplasmic reticulum to the Golgi apparatus or endoplasmic-reticulum–Golgi intermediate compartment. This process is termed anterograde transport, in contrast to the retrograde transport associated with the COPI complex. COPII is assembled in two parts: first an inner layer of Sar1, Sec23, and Sec24 forms; then the inner coat is surrounded by an outer lattice of Sec13 and Sec31.
Function
The COPII coat is responsible for the formation of vesicles from the endoplasmic reticulum (ER). These vesicles transport cargo proteins to the Golgi apparatus (in yeast) or the endoplasmic-reticulum-Golgi intermediate compartment (ERGIC, in mammals).
Sec24 proteins recognize various cargo proteins, packaging them into the budding vesicles.
Structure
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. Sar1 resembles other Ras-family GTPases, with a core of six beta strands flanked by three 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).
Regulation
The signal(s) that triggers Sec12 to initiate COPII assembly remains unclear, though some regulators of coat formation are now known. The frequency of COPII formation is regulated in part by Sec16A and Tango1 proteins, likely by concentrating Sec12 in a given location, so it can more efficiently activate Sar1.). In cultured mammalian cells the two Sar1 genes appear redundant; however, in animals SAR1B is uniquely required for the formation of large (over 1 micrometer across) COPII-coated vesicles. In humans, inheriting two copies of certain SAR1B variants results in Chylomicron retention disease,
Conformational changes
CopII has three specific binding sites that can each be complexed. The adjacent picture (Sed5) uses the Sec22 t-SNARE complex to bind. This site is more strongly bound, and therefore is more favored. (Embo)