Structural basis for the initiation of COPII vesicle biogenesis

The first stage of the eukaryotic secretory pathway is the packaging of cargo proteins into COPII vesicles exiting the endoplasmic reticulum (ER). The cytoplasmic COPII vesicle coat machinery is recruited to the ER membrane by the activated, GTP-bound, form of the conserved Sar1 GTPase. Activation of Sar1 on the surface of the ER by Sec12, a membrane-anchored GEF (guanine nucleotide exchange factor), is therefore the initiating step of the secretory pathway. Here we report the structure of the complex between Sar1 and the cytoplasmic GEF domain of Sec12, both from Saccharomyces cerevisiae . This structure, representing the key nucleotide-free activation intermediate, reveals how the potassium ion-binding K-loop disrupts the nucleotide binding site of Sar1. We describe an unexpected orientation of the GEF domain relative to the membrane surface and propose a mechanism for how Sec12 facilitates membrane insertion of the amphipathic helix exposed by Sar1 upon GTP-binding.