The structure of a COPII tubule.

ABSTRACT

Nearly a third of all eukaryotic proteins are transported from the ER to the Golgi apparatus through the secretory pathway using COPII coated vesicles. Evidence suggests that this transport occurs via 500-900 Švesicles that bud from the ER membrane. It has been shown that procollagen molecules utilize the COPII proteins for transport, but it is unclear how the COPII coat can accommodate these ∼3000 Šlong molecules. We now present a cryogenic electron tomographic reconstruction of a Sec13/31 tubule that is approximately 3300 Šlong containing a hollow cylindrical interior that is 300 Šin diameter, dimensions that are consistent with those that are required to encapsulate a procollagen molecule wrapped in a membrane and accessory COPII components. This structure suggests a novel mechanism that the COPII coat may employ to transport elongated cargo.