Structure and organization of coat proteins in the COPII cage.
Publication Type:Journal Article
Source:Cell, Volume 129, Issue 7, p.1325-36 (2007)
Keywords:Animals, Cells, Cultured, Clathrin, COP-Coated Vesicles, Crystallography, X-Ray, Endoplasmic Reticulum, Golgi Apparatus, Macromolecular Substances, Membrane Proteins, Models, Molecular, Nuclear Pore Complex Proteins, Protein Structure, Secondary, Protein Structure, Tertiary, Protein Transport, Saccharomyces cerevisiae, Saccharomyces cerevisiae Proteins, Vesicular Transport Proteins
<p>COPII-coated vesicles export newly synthesized proteins from the endoplasmic reticulum. The COPII coat consists of the Sec23/24-Sar1 complex that selects cargo and the Sec13/31 assembly unit that can polymerize into an octahedral cage and deform the membrane into a bud. Crystallographic analysis of the assembly unit reveals a 28 nm long rod comprising a central alpha-solenoid dimer capped by two beta-propeller domains at each end. We construct a molecular model of the COPII cage by fitting Sec13/31 crystal structures into a recently determined electron microscopy density map. The vertex geometry involves four copies of the Sec31 beta-propeller that converge through their axial ends; there is no interdigitation of assembly units of the kind seen in clathrin cages. We also propose that the assembly unit has a central hinge-an arrangement of interlocked alpha-solenoids-about which it can bend to adapt to cages of variable curvature.</p>