COPII

These vesicles transport cargo proteins to the Golgi apparatus (in yeast) or the endoplasmic-reticulum-Golgi intermediate compartment (ERGIC, in mammals).

[1] Coat assembly is initiated when the cytosolic Ras GTPase Sar1 is activated by its guanine nucleotide exchange factor Sec12.

[1] Sar1 resembles other Ras-family GTPases, with a core of six beta strands flanked by three alpha helices, and two flexible "switch domains".

[1] These coat proteins are necessary but insufficient to direct or dock the vesicle to the correct target membrane.

[2] The signal(s) that triggers Sec12 to initiate COPII assembly remains unclear, though some regulators of coat formation are now known.

[4] 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.

[6] Halperin-Birk syndrome (HLBKS), a rare autosomal recessive neurodevelopmental disorder, is caused by a null mutation in the SEC31A.