thecoatomer

The coatomer is a protein complex involved in vesicular transport within eukaryotic cells. It plays a crucial role in the retrograde transport pathway, moving proteins from the Golgi apparatus back to the endoplasmic reticulum (ER). This process is essential for maintaining the correct localization of ER-resident proteins, which may have inadvertently escaped to the Golgi during their synthesis and initial trafficking.

The coatomer is composed of seven different subunits: $\alpha$, $\beta$, $\beta'$, $\gamma$, $\delta$, $\epsilon$, and $\zeta$. These subunits assemble into a large, dynamic structure that is recruited to specific membrane domains. The assembly and disassembly of the coatomer complex are tightly regulated by small GTPases, particularly members of the ARF (ADP-ribosylation factor) family.

When ARF proteins are in their GTP-bound form, they promote the recruitment of coatomer to the Golgi membrane, initiating the formation of COPI-coated vesicles. These vesicles bud off from the Golgi and carry their cargo, including ER-resident proteins, back to the ER. The binding of coatomer to cargo receptors is mediated by specific interactions, ensuring that only the correct proteins are retrieved.

Once the COPI-coated vesicle reaches the ER, the GTPase-activating protein (GAP) activity of the ER membrane hydrolyzes GTP bound to ARF, leading to ARF inactivation and the subsequent disassembly of the coatomer. This releases the vesicle and its cargo, allowing for their fusion with the ER and the reintegration of ER-resident proteins. The coatomer system is therefore vital for the proper functioning and organization of the secretory pathway.