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Structural details of GEF‐mediated Sar1 activation
Author(s) -
Joiner Aaron,
Fromme Chris
Publication year - 2020
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.2020.34.s1.03474
Subject(s) - copii , gtpase , endoplasmic reticulum , guanine nucleotide exchange factor , microbiology and biotechnology , transport protein , vesicle , copi , chemistry , biology , secretory pathway , golgi apparatus , biochemistry , membrane
The first trafficking step of the eukaryotic secretory pathway is the packaging of cargo proteins into COPII‐coated vesicles that exit the endoplasmic reticulum (ER). COPII vesicle coat proteins are recruited to the ER membrane via direct interaction with the activated (GTP‐bound) form of the Sar1 GTPase. Sar1 is activated on the surface of the ER by Sec12, a membrane‐anchored GEF (guanine nucleotide exchange factor). Here we report the crystal structure of the complex between Sar1 and the cytoplasmic domain of Sec12, representing a snapshot of the activation reaction. This structure provides detailed insight into the mechanism of Sar1 activation. The structure reveals that despite structural homology to the RCC1 GEF, a distinct surface of the beta‐propeller fold of Sec12 interacts with Sar1 to displace nucleotide. The structural results suggest that the cytoplasmic domain of Sec12 may adopt an unexpected orientation relative to the membrane surface. This orientation would position Sar1 on the ER membrane so that binding to GTP and engagement of its amphipathic N‐terminal helix with the lipid leaflet can occur simultaneously. Together with accompanying functional data, our results provide a structural explanation for the critical regulatory step of COPII vesicle biogenesis. Support or Funding Information This work was funded by the National Institutes of Health grant R01GM116942 to J.C.F. Alfred P. Sloan Foundation Fellowship and NSF GRFP DGE‐1650441 to A.J.