COPIImedierad

COPII-mediated transport refers to the process by which proteins and certain lipids are packaged into COPII-coated vesicles that bud from the endoplasmic reticulum (ER) and travel to the Golgi apparatus in most eukaryotic cells. The COPII coat is formed from a defined set of cytosolic proteins that assemble at ER exit sites to create a transport carrier capable of carrying cargo to the early secretory pathway.

The process begins with Sec12, an ER-embedded guanine-nucleotide exchange factor (GEF) that activates Sar1 by promoting GTP binding. Membrane-associated Sar1-GTP recruits the inner coat proteins Sec23 and Sec24, where Sec24 provides cargo selection by recognizing export signals on cargo proteins or cargo receptors. The outer coat composed of Sec13 and Sec31 then assembles to form a cage-like lattice that drives membrane curvature and vesicle budding. Cargo selection is coordinated, with Sec24 contributing to specific cargo recognition, including soluble proteins and membrane proteins that need to reach the Golgi.

After vesicle formation, the coat is disassembled as Sar1 hydrolyzes GTP, allowing the vesicle to uncoat and become fusion-competent. Vesicles are subsequently tethered and fused with the Golgi apparatus through a series of Rab, tethering factors and SNARE proteins, enabling delivery of their cargo.

Regulation of COPII transport involves additional factors that define ER exit sites, such as Sec16, and adaptors like TANGO1 for bulky cargo. COPII-mediated transport is the major anterograde route from the ER to the Golgi, distinct from COPI-mediated retrograde trafficking and clathrin-mediated pathways. Disruptions in COPII components can affect protein secretion and are linked to various cellular and developmental disorders.