(A) In the crystal structure, β- and δ-COP share an extensive interface with a size of 1737 Å2. The longin domain, β–turn, and helix a of δ-COP are part of the interface with the inside of the β-COP α-solenoid, whereas helix b traverses the α-solenoid along the connection of helices α7 and α6 of β-COP and protrudes away from the solenoid (see also Figure 1). F133 in helix a is in close contact with α-solenoid residues Y114 and Y149. The side chain of δ-M136 anchors the short region between helix a and b onto β. Additionally, the side-chain of β-N112 forms a hydrogen bond to the main-chain oxygen of δ-M136. Other interactions are a salt bridge between δ-E141, which is located at the beginning of helix b, and side chain of β-K71, and a hydrogen bond between side chains of δ-S138 and β-E113. The side chains of I143, I146, and I147 in helix b are shown. These residues are essential for correct trafficking of HDEL motif containing cargo by COPI (Arakel et al., 2016). (B) In the crystal, δ-COP helix b (red) is involved in a contact with helix b of a symmetry-related molecule (grey). The crystal contact comprises an interface area of 371 Å2. We cannot assess whether the crystal contact influences the orientation of helix b, but superposition of corresponding parts of β- and δ-COP from the X-ray and cryo-ET structures (not shown) suggests that there is some flexibility in this region. (C) In the X-ray structure of γζ-COP with bound Arf1 (Yu et al., 2012), helices α2, α4, and α6 of γ-COP interact with Arf1. γζ-COP helix α4 shows standard α-helical geometry from residues T63 to T74 and ends in a 310 helical conformation (K75–Q78). In contrast, in our structure of βδ-COP, downstream of residue I61, β-COP helix α4 is shorter, and between residues I61 and F64 it forms a widened helix with a rare π-helix-like geometry. To highlight the quality of the structure in this region, electron density at contour level 4σ from the anomalous diffraction experiment with SeMet-labelled protein is shown for the two methionine residues M59 and M66. (D) The main chain oxygen of β-COP F64 in helix α4 forms a hydrogen bond with the amino group of the side chain of K37, which is located in the preceding helix α3. 2mFo-DFc electron density is shown at contour level 1σ. (E) The x-ray structure of Chaetomium thermophilum βδ-COP can be fit as a rigid body into the cryo-ET map of the Mus musculus COPI leaf at 9.2 Å resolution, indicating a high degree of structural conservation. β-COP is shown in dark green, δ-COP in orange.