Surface structure of the COPII-coated vesicle.

The spatial arrangement of COPII coat protein subunits was analyzed by crosslinking to an artificial membrane surface and by electron microscopy of coat proteins and coated vesicle surfaces. The efficiency of COPII subunit crosslinking to phospholipids declined in order of protein recruitment to the coat: Sar1p > Sec23/24p >> Sec13/31p. Deep-etch rotary shadowing and electron microscopy were used to explore the COPII subunit structure with isolated proteins and coated vesicles. Sec23/24 resembles a bow tie, and Sec13/31p contains terminal bilobed globular structures bordering a central rod. The surface structure of COPII vesicles revealed a coat built with polygonal units. The length of the side of the hexagonal/pentagonal units is close to the dimension of the central rod-like segment of Sec13/31. Partially uncoated profiles revealed strands of Sec13/31p stripped from the vesicle surface. We conclude that the coat subunits form layers displaced from the membrane surface in reverse order of addition to the coat.