title: Sar1p N-terminal helix initiates membrane curvature and completes the fission of a COPII vesicle.
abstract: Secretory proteins traffic from the ER to the Golgi via COPII-coated transport vesicles. The five core COPII proteins (Sar1p, Sec23/24p, and Sec13/31p) act in concert to capture cargo proteins and sculpt the ER membrane into vesicles of defined geometry. The molecular details of how the coat proteins deform the lipid bilayer into vesicles are not known. Here we show that the small GTPase Sar1p directly initiates membrane curvature during vesicle biogenesis. Upon GTP binding by Sar1p, membrane insertion of the N-terminal amphipathic alpha helix deforms synthetic liposomes into narrow tubules. Replacement of bulky hydrophobic residues in the alpha helix with alanine yields Sar1p mutants that are unable to generate highly curved membranes and are defective in vesicle formation from native ER membranes despite normal recruitment of coat and cargo proteins. Thus, the initiation of vesicle budding by Sar1p couples the generation of membrane curvature with coat-protein assembly and cargo capture.
authors.affiliation: Department of Molecular and Cell Biology and Howard Hughes Medical Institute, University of California, Berkeley, California 94702, USA.
authors.forename: Marcus C S
authors.initials: MC
authors.lastname: Lee
authors.forename: Lelio
authors.initials: L
authors.lastname: Orci
authors.forename: Susan
authors.initials: S
authors.lastname: Hamamoto
authors.forename: Eugene
authors.initials: E
authors.lastname: Futai
authors.forename: Mariella
authors.initials: M
authors.lastname: Ravazzola
authors.forename: Randy
authors.initials: R
authors.lastname: Schekman
mesh_terms: D001665:Binding Sites / Q000502:physiology; D022181:COP-Coated Vesicles / Q000378:metabolism* / Q000648:ultrastructure; D004721:Endoplasmic Reticulum / Q000378:metabolism* / Q000648:ultrastructure; D006056:Golgi Apparatus / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism* / Q000648:ultrastructure; D008051:Lipid Bilayers / Q000378:metabolism; D046529:Microscopy, Electron, Transmission; D020559:Monomeric GTP-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D009154:Mutation / Q000502:physiology; D017433:Protein Structure, Secondary / Q000502:physiology; D021381:Protein Transport / Q000502:physiology; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D033921:Vesicular Transport Proteins
chemical_list: D008051:Lipid Bilayers; D029701:Saccharomyces cerevisiae Proteins; D033921:Vesicular Transport Proteins; D006160:Guanosine Triphosphate; D020559:Monomeric GTP-Binding Proteins; C077395:SAR1 protein, S cerevisiae