COPIIvesicles

COPIIvesicles are coated transport vesicles that mediate the movement of proteins from the endoplasmic reticulum (ER) to the Golgi apparatus in the early secretory pathway. They are assembled at ER exit sites by the COPII coat protein complex, which consists of the small GTPase Sar1 and two sequential coat layers: Sec23/Sec24 as the inner coat and Sec13/Sec31 as the outer coat.

Formation begins when Sec12, a guanine nucleotide exchange factor on the ER, activates Sar1 by promoting GDP-to-GTP exchange. GTP-bound Sar1 inserts its amphipathic helix into the ER membrane and recruits the Sec23/Sec24 inner coat, which selects cargo through Sec24’s cargo-binding sites and may involve cargo receptors. The outer coat Sec13/Sec31 then assembles to drive membrane curvature and vesicle budding. GTP hydrolysis by Sar1 promotes coat disassembly after the vesicle is released from the ER.

COPIIvesicles are typically small, about 60 to 90 nanometers in diameter, and bud from ER exit sites—specialized domains organized by scaffolding proteins such as Sec16. After budding, the coat is shed, and the vesicles are targeted to the Golgi by tethering factors and Rab1, followed by SNARE-mediated fusion.

Cargo selection is a key feature of COPII trafficking; Sec24 isoforms provide cargo specificity, enabling recognition of soluble proteins and membrane proteins, often via cargo receptors. Regulation of COPII traffic integrates quality control within the ER and coordinates with other trafficking pathways. Mutations in COPII components can cause human disease by disrupting protein secretion and organelle biogenesis.