Fig. 3.

Sequential assembly of the COPII coat structure. Following activation of Sar1-GDP to Sar1-GTP by the membrane-bound GEF Sec12, Sec23/24 is recruited to the ER membrane. Sec23/24 provides the major cargo binding function of the coat. Sec12 could be geometrically excluded from ERES because of membrane curvature. The final outer layer coat components Sec13/13 are recruited just prior to scission events, forming a vesicle. The minimal cage structure is shown; vesicle size would be dictated by cargo incorporation and potentially by other factors. These are schematic illustrations generated using Autodesk Maya 8.0 and are not intended as accurate rendering of the precise atomic coordinates from crystallographic data