Insights into COPII coat nucleation from the structure of Sec23.Sar1 complexed with the active fragment of Sec31.
The COPII vesicular coat forms on the endoplasmic reticulum from Sar1-GTP, Sec23/24 and Sec13/31 protein subunits. Here, we define the interaction between Sec23/24.Sar1 and Sec13/31, involving a 40 residue Sec31 fragment. In the crystal structure of the ternary complex, Sec31 binds as an extended polypeptide across a composite surface of the Sec23 and Sar1-GTP molecules, explaining the stepwise character of Sec23/24.Sar1 and Sec13/31 recruitment to the membrane. The Sec31 fragment stimulates GAP activity of Sec23/24, and a convergence of Sec31 and Sec23 residues at the Sar1 GTPase active site explains how GTP hydrolysis is triggered leading to COPII coat disassembly. The Sec31 active fragment is accommodated in a binding groove supported in part by Sec23 residue Phe380. Substitution of the corresponding residue F382L in human Sec23A causes cranio-lenticulo-sutural dysplasia, and we suggest that this mutation disrupts the nucleation of COPII coat proteins at endoplasmic reticulum exit sites.
Bi
Xiping
X
Howard Hughes Medical Institute and the Structural Biology Program, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021, USA.
Mancias
Joseph D
JD
Howard Hughes Medical Institute and the Structural Biology Program, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021, USA.
Goldberg
Jonathan
J
Howard Hughes Medical Institute and the Structural Biology Program, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021, USA. Electronic address: jonathan@ximpact4.ski.mskcc.org.
D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D001665:Binding Sites; D022181:COP-Coated Vesicles / Q000378:metabolism*; D018360:Crystallography, X-Ray; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D020559:Monomeric GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D033921:Vesicular Transport Proteins / Q000235:genetics / Q000378:metabolism*
C512739:SEC23A protein, human; C507455:SEC31A protein, human; D033921:Vesicular Transport Proteins; C493083:SAR1A protein, human; D020559:Monomeric GTP-Binding Proteins