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 500-900A vesicles 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 ∼3000A long molecules. We now present a cryogenic electron tomographic reconstruction of a Sec13/31 tubule that is approximately 3300A long containing a hollow cylindrical interior that is 300A in 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.
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