Structure of the Sec23p/24p and Sec13p/31p complexes of COPII

COPII-coated vesicles carry proteins from the endoplasmic reticulum to the Golgi complex. This vesicular transport can be reconstituted by using three cytosolic components containing five proteins: the small GTPase Sar1p, the Sec23p/24p complex, and the Sec13p/Sec31p complex. We have used a combinat ion of biochemistry and electron microscopy to investigate the molecular or ganization and structure of Sec23p/24p and Sec13p/31p complexes. The three- dimensional reconstruction of Sec23p/24p reveals that it has a bone-shaped structure, (17 nm in length), composed of two similar globular domains, one corresponding to Sec23p and the other to Sec24p. Sec13p/31p is a heterotet ramer composed of two copies of Sec13p and two copies of Sec31p. It has an elongated shape, is 28-30 nm in length, and contains five consecutive globu lar domains linked by relatively flexible joints. Putting together the arch itecture of these Sec complexes with the interactions between their subunit s and the appearance of the coat in COPII-coated vesicles, we present a mod el for COPII-coat organization.