COPImediated

COPI-mediated transport refers to the vesicular trafficking processes driven by the coat protein complex I (COPI). This machinery predominantly mediates retrograde transport from the Golgi apparatus back to the endoplasmic reticulum (ER) and contributes to intra-Golgi trafficking, helping maintain compartmental organization and protein composition.

Mechanism and process: COPI coats assemble on Golgi membranes in response to the activation of ARF1, a small GTPase. In its GTP-bound form ARF1 recruits the seven-subunit coatomer complex to the membrane, promoting membrane curvature, vesicle budding, and cargo selection either directly or via cargo receptors. After vesicle scission, the coat is shed (uncoating), allowing vesicles to fuse with target membranes, guided by tethering factors, SNARE proteins, and Rab GTPases.

Composition and cargo: The COPI coat is a seven-subunit complex known as coatomer, including alpha-COP (COPA), beta-COP (COPB1), beta'-COP (COPB2), gamma-COP (COPG1), delta-COP (COPD), epsilon-COP (COPE), and zeta-COP (COPZ1/ COPZ2). The complex associates with ARF and interacts with retrieving signals on cargo proteins, such as the C-terminal KDEL sequence on soluble ER residents and KKXX motifs on membrane proteins, enabling sorting into COPI vesicles.

Functional significance: COPI-mediated transport is essential for maintaining ER and Golgi compartmentalization, recycling resident enzymes, and ensuring proper glycosylation and protein sorting. Disturbances in COPI function can disrupt ER retrieval and Golgi homeostasis, with implications for cellular physiology and disease in various organisms.