Insights into COPII coat nucleation from the structure of Sec23.Sar1 complexed with the active fragment of Sec31

Dev Cell. 2007 Nov;13(5):635-45. doi: 10.1016/j.devcel.2007.10.006.


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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Amino Acid Substitution
  • Binding Sites
  • COP-Coated Vesicles / metabolism*
  • Crystallography, X-Ray
  • Humans
  • Models, Molecular*
  • Molecular Sequence Data
  • Monomeric GTP-Binding Proteins / genetics
  • Monomeric GTP-Binding Proteins / metabolism*
  • Protein Binding
  • Protein Conformation
  • Vesicular Transport Proteins / genetics
  • Vesicular Transport Proteins / metabolism*


  • SEC23A protein, human
  • SEC31A protein, human
  • Vesicular Transport Proteins
  • SAR1A protein, human
  • Monomeric GTP-Binding Proteins

Associated data

  • PDB/2QTV