COPII

COPII is a coat protein complex that mediates vesicular transport from the endoplasmic reticulum (ER) to the Golgi apparatus in eukaryotic cells. It assembles at ER exit sites and forms vesicles that carry secretory and membrane proteins to the Golgi. The process is initiated by the small GTPase Sar1, which binds to the ER membrane when GTP-bound and recruits the inner coat Sec23-Sec24. Sec24 provides cargo selection, recognizing export signals on cargo proteins and on cargo receptors; Sec23 acts as a GTPase-activating protein for Sar1, promoting coat maturation. The outer coat, Sec13-Sec31, forms a cage that shapes the vesicle. Sec16 functions as a scaffolding protein at ER exit sites to organize coat assembly, and Sec12 serves as the guanine nucleotide exchange factor that activates Sar1. After budding, hydrolysis of GTP on Sar1 triggers coat disassembly, allowing the vesicle to fuse with Golgi membranes via the usual tethering and SNARE machinery.

COPII vesicles are relatively small, typically about 60 to 80 nanometers in diameter. The complex is conserved from yeast to humans, reflecting a fundamental pathway for early secretory trafficking. COPII not only exports soluble secretory proteins but also selects certain membrane proteins for forward movement. Regulation of COPII function integrates with cellular stress responses and ER quality control to maintain protein folding homeostasis.

Mutations in COPII components have been linked to human disease, including disorders affecting lipid metabolism, skeletal and craniofacial development, and congenital glycosylation pathways, illustrating the essential role of COPII in cellular and organismal health.