COPII-coated vesicles mediate the first leg of the journey
through the biosynthetic pathway—from the ER to the
ERGIC and CGN (Figure 8.25a,b). The COPII coat contains
a number of proteins that were first identified in mutant yeast
cells that were unable to carry out transport from the ER to the
Golgi complex. Homologues of the yeast proteins were subsequently
found in the coats of vesicles budding from the ER in
mammalian cells. Antibodies to COPII-coat proteins block
budding of vesicles from ER membranes but have no effect on
movement of cargo at other stages in the secretory pathway.