Coatomer

Coatomer, or the COPI coatomer, is a cytosolic multiprotein complex that assembles on membranes to form COPI-coated vesicles. These vesicles mediate retrograde transport within the Golgi apparatus and from the Golgi back to the endoplasmic reticulum (ER). The coat is recruited to membranes by the small GTPase ARF1 in its GTP-bound form; subsequent GTP hydrolysis triggers coat disassembly, allowing vesicle fusion with target membranes.

The coatomer is composed of seven subunits, traditionally named alpha-, beta-, beta'-, gamma-, delta-, epsilon-, and zeta-COP. In mammals these subunits are encoded by genes such as COPA (alpha-COP), COPB1 and COPB2 (beta-COP), COPG1 (gamma-COP), COPD (delta-COP), COPE (epsilon-COP), and COPZ1/COPZ2 (zeta-COP). In budding yeast, the counterparts are referred to as Ret1, Ret2, Ret3, Ret4, Ret5, Ret6, and Ret7. The complex can be organized into distinct functional subcomplexes that cooperatively assemble into a cage-like coat on late Golgi or ER-to-Golgi membranes, facilitating cargo selection and vesicle budding.

Function and interactions: Coatomer interacts with cargo receptors, including members of the p24 family, and with SNAREs and other trafficking factors to select cargo and promote vesicle formation. It supports retrieval of ER-resident proteins bearing signals such as KDEL and contributes to maintaining Golgi structure and protein distribution.

Evolution and significance: The COPI coatomer is conserved across eukaryotes and is distinct from other coat systems such as COPII and clathrin. It is essential for proper intracellular trafficking and Golgi-ER compartmentalization, and defects in COPI components can disrupt protein sorting and organelle organization.