Figure 6.
The COPII coat is dynamic. (A) The Sec13•Sec31 hetero-octamer (red) is the repeating unit of the outer shell of the COPII complex. A cuboctahedral arrangement made of 12 modules is depicted, but other larger geometries of COPII cages were also observed by EM.48 Sec13 and Sec31 are shown in orange and grey, respectively. The inner shell of the COPII complex has been omitted for clarity. (B) With increasing cage size, the decreasing curvature of the vesicle is accounted for by a hinge motion that leads to more extended conformations of the Sec13•Sec31 hetero-octamer.49 Spheres represent β-propeller domains, while cylinders represent α-helical solenoid domains. Although Sec13 interacts with Sec31 and Nup145C in an analogous fashion, the higher-order organization differs and is facilitated by an additional β-propeller domain in Sec31 that is absent in Nup145C. The Sec13 binding site in Sec31 is located between the N-terminal β-propeller domain and the C-terminal α-helical solenoid domain.