The coat protein complex I (COPI) allows the precise sorting of lipids and proteins between Golgi cisternae and retrieval from the Golgi to the ER. This essential role maintains the identity of the early secretory pathway and impinges on key cellular processes, such as protein quality control. In this Cell Science at a Glance and accompanying poster, we illustrate the different stages of COPI-coated vesicle formation and revisit decades of research in the context of recent advances in the elucidation of COPI coat structure. By calling attention to an array of questions that have remained unresolved, this review attempts to refocus the perspectives of the field.

The copy number of membrane proteins at the cell surface is tightly regulated. Many ion channels and receptors present retrieval motifs to COPI vesicle coats and are retained in the early secretory pathway. In some cases, the interaction with COPI is prevented by binding to 14-3-3 proteins. However, the functional significance of this antagonism between COPI and 14-3-3 in terminally differentiated cells is unknown. Here, we show that ATP-sensitive K⁺ (K_ATP) channels, which are composed of Kir6.2 and SUR1 subunits, are stalled in the Golgi complex of ventricular, but not atrial, cardiomyocytes. Upon sustained β-adrenergic stimulation, which leads to activation of protein kinase A (PKA), SUR1-containing channels reach the plasma membrane of ventricular cells. We show that PKA-dependent phosphorylation of the C-terminus of Kir6.2 decreases binding to COPI and, thereby, silences the arginine-based retrieval signal. Thus, activation of the sympathetic nervous system releases this population of K_ATP channels from storage in the Golgi and, hence, might facilitate the adaptive response to metabolic challenges.

The transport of the potassium channels TASK-1 and TASK-3 to the cell surface is controlled by the binding of 14-3-3 proteins to a trafficking control region at the extreme C-terminus of the channels. The current model proposes that phosphorylation-dependent binding of 14-3-3 sterically masks a COPI binding motif. However, the direct effects of phosphorylation on COPI binding and on the binding parameters of 14-3-3 are still unknown. We found that phosphorylation of the trafficking control region prevents COPI binding even in the absence of 14-3-3, and we present a quantitative analysis of the binding of all human 14-3-3 isoforms to the trafficking control regions of TASK-1 and TASK-3. Surprisingly, the affinities of 14‑3‑3 proteins for TASK-1 were two orders of magnitude lower than for TASK-3. Furthermore, we found that phosphorylation of a second serine in the C-terminus of TASK-1 inhibited 14-3-3 binding. Thus, phosphorylation of the trafficking control region can stimulate or inhibit transport of TASK-1 to the cell surface depending on the target serine. Our findings suggest highly dynamic control of TASK-1 trafficking by COPI, kinases, phosphatases, and 14-3-3.

Significance: We systematically dissect the structural elements of the δ subunit of the coat protein I (COPI) vesicle coat. We demonstrate that the µ-homology domain is dispensable for essential COPI functions. We map a key helix, positionally analogous to a helix observed in clathrin adaptor (AP) structures but without assigned function in AP, let alone COPI. This helix shows all features of an amphipathic helix that can enable interactions of a protein with the membrane. Indeed, the residues on its hydrophobic surface were specifically required for its function. Both findings substantially refine current models of coat formation and restrict the mechanisms of how the coat polymer is constructed or how HDEL cargo is recognized by COPI.