COPIIcoated

COPII-coated vesicles are transport carriers that bud from the endoplasmic reticulum (ER) and deliver their cargo to the Golgi apparatus in most eukaryotic cells. The coat is built from the COPII protein complex and assembles at ER exit sites (ERES) where cargo proteins are selected for export.

Formation begins with the small GTPase Sar1 activated by the ER-bound guanine-nucleotide exchange factor Sec12, which anchors Sar1–GTP to the ER membrane. The Sar1–GTP recruits the inner coat subunits Sec23 and Sec24, which together select cargo directly or via cargo receptors. The Sec23/24 layer then nucleates the outer coat formed by Sec13–Sec31, which polymerizes into a cage that drives membrane curvature and vesicle budding. Typical COPII vesicles are about 60–90 nm in diameter.

Following scission from the ER, Sar1 hydrolyzes GTP, triggering disassembly of the coat and allowing the vesicle to fuse with the Golgi network with the help of SNARE proteins. The coat components are recycled for subsequent rounds of transport.

Cargo selection involves signals on soluble and membrane proteins and, in many cases, cargo receptors such as p24 family members. Multiple isoforms of COPII components in vertebrates (for example Sec23A/B, Sec24A-D, Sec13A/B, Sec31A/B) provide cargo-specificities. Sec16 and other ER exit site proteins help organize COPII assembly at the ER.