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Nature. 2002 Sep 19;419(6904):271-7.

Structure of the Sec23/24-Sar1 pre-budding complex of the COPII vesicle coat.

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  • 1Howard Hughes Medical Institute and the Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, New York 10021, USA.

Abstract

COPII-coated vesicles form on the endoplasmic reticulum by the stepwise recruitment of three cytosolic components: Sar1-GTP to initiate coat formation, Sec23/24 heterodimer to select SNARE and cargo molecules, and Sec13/31 to induce coat polymerization and membrane deformation. Crystallographic analysis of the Saccharomyces cerevisiae Sec23/24-Sar1 complex reveals a bow-tie-shaped structure, 15 nm long, with a membrane-proximal surface that is concave and positively charged to conform to the size and acidic-phospholipid composition of the COPII vesicle. Sec23 and Sar1 form a continuous surface stabilized by a non-hydrolysable GTP analogue, and Sar1 has rearranged from the GDP conformation to expose amino-terminal residues that will probably embed in the bilayer. The GTPase-activating protein (GAP) activity of Sec23 involves an arginine side chain inserted into the Sar1 active site. These observations establish the structural basis for GTP-dependent recruitment of a vesicular coat complex, and for uncoating through coat-controlled GTP hydrolysis.

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