FIGURE 14-12 Three-dimensional structure of the ternary complex comprising the COPII coat proteins Sec23 and Sec24 and Sar1·GTP. Early in the formation of the COPII coat, Sec23 (orange)/Sec24 (green) complexes are recruited to the ER membrane by Sar1 (red) in its GTP-bound state. In order to form a stable ternary complex in solution for structural studies, the nonhydrolyzable GTP analog GppNHp is used. A cargo protein in the ER membrane can be recruited to COPII vesicles by the interaction of a tripeptide di-acidic sorting signal (purple) in the cargo protein’s cytosolic domain with Sec24. The likely positions of the COPII vesicle membrane and the transmembrane segment of the cargo protein are indicated. The N-terminal segment of Sar1 that tethers it to the membrane is not shown.

[Republished with permission of Nature, from Bi, X. et al., “Structure of the Sec23/24-Sar1 pre-budding complex of the COPII vesicle coat,” 2002, Nature 419(6904):271–7; permission conveyed through the Copyright Clearance Center, Inc.]