Sar1p N-terminal helix initiates membrane curvature and completes the fission of a COPII vesicle

Marcus C S Lee; Lelio Orci; Susan Hamamoto; Eugene Futai; Mariella Ravazzola; Randy Schekman

doi:10.1016/j.cell.2005.07.025

Sar1p N-terminal helix initiates membrane curvature and completes the fission of a COPII vesicle

Cell. 2005 Aug 26;122(4):605-17. doi: 10.1016/j.cell.2005.07.025.

Authors

Marcus C S Lee¹, Lelio Orci, Susan Hamamoto, Eugene Futai, Mariella Ravazzola, Randy Schekman

Affiliation

¹ Department of Molecular and Cell Biology and Howard Hughes Medical Institute, University of California, Berkeley, California 94702, USA.

PMID: 16122427
DOI: 10.1016/j.cell.2005.07.025

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.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Binding Sites / physiology
COP-Coated Vesicles / metabolism*
COP-Coated Vesicles / ultrastructure
Endoplasmic Reticulum / metabolism*
Endoplasmic Reticulum / ultrastructure
Golgi Apparatus / metabolism
Guanosine Triphosphate / metabolism
Intracellular Membranes / metabolism*
Intracellular Membranes / ultrastructure
Lipid Bilayers / metabolism
Microscopy, Electron, Transmission
Monomeric GTP-Binding Proteins / chemistry
Monomeric GTP-Binding Proteins / genetics
Monomeric GTP-Binding Proteins / metabolism*
Mutation / physiology
Protein Structure, Secondary / physiology
Protein Transport / physiology
Saccharomyces cerevisiae / genetics
Saccharomyces cerevisiae / metabolism*
Saccharomyces cerevisiae / ultrastructure
Saccharomyces cerevisiae Proteins / chemistry
Saccharomyces cerevisiae Proteins / genetics
Saccharomyces cerevisiae Proteins / metabolism*
Vesicular Transport Proteins

Substances

Lipid Bilayers
Saccharomyces cerevisiae Proteins
Vesicular Transport Proteins
Guanosine Triphosphate
Monomeric GTP-Binding Proteins
SAR1 protein, S cerevisiae