Self-assembly of minimal COPII cages.

The small G-protein Sar1 and the cytosolic complexes Sec23/24 and Sec13/31 associate sequentially on endoplasmic reticulum membranes to form a protein coat named COPII, which drives the formation of transport vesicles. Using dynamic light scattering, we show that Sec23/24 and Sec13/31 can self-assemble in a stoichiometric manner in solution to form particles with hydrodynamic radii in the range of 40-60 nm. Self-assembly is favoured by lowering the pH, the ionic strength and/or the temperature. Electron microscopy reveals the formation of spherical particles 60-120 nm in diameter with a tight, rough mesh on their surfaces. We suggest that these structures, which represent a minimal COPII cage, mimic the molecular organization of the membrane-associated COPII coat.