title: Structure and organization of coat proteins in the COPII cage.
abstract: 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.
authors.affiliation: Howard Hughes Medical Institute and the Structural Biology Program, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA.
authors.forename: Stephan
authors.initials: S
authors.lastname: Fath
authors.forename: Joseph D
authors.initials: JD
authors.lastname: Mancias
authors.forename: Xiping
authors.initials: X
authors.lastname: Bi
authors.forename: Jonathan
authors.initials: J
authors.lastname: Goldberg
mesh_terms: D000818:Animals; D022181:COP-Coated Vesicles / Q000737:chemistry* / Q000648:ultrastructure*; D002478:Cells, Cultured; D002966:Clathrin / Q000378:metabolism; D018360:Crystallography, X-Ray; D004721:Endoplasmic Reticulum / Q000378:metabolism* / Q000648:ultrastructure; D006056:Golgi Apparatus / Q000378:metabolism / Q000648:ultrastructure; D046911:Macromolecular Substances; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D028861:Nuclear Pore Complex Proteins / Q000737:chemistry* / Q000378:metabolism; D017433:Protein Structure, Secondary / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000502:physiology; D021381:Protein Transport / Q000502:physiology; D012441:Saccharomyces cerevisiae; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry* / Q000378:metabolism; D033921:Vesicular Transport Proteins / Q000737:chemistry* / Q000378:metabolism
chemical_list: D002966:Clathrin; D046911:Macromolecular Substances; D008565:Membrane Proteins; D028861:Nuclear Pore Complex Proteins; C079620:SEC13 protein, S cerevisiae; C106113:SEC31 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D033921:Vesicular Transport Proteins