transport in vesicles. Certain integral membrane proteins of
the ER are selectively captured because they contain “ER
export” signals as part of their cytosolic tail. These signals
interact specifically with COPII proteins of the vesicle coat
(Figure 8.25b). Proteins selected by COPII-coated vesicles include
(1) enzymes that act at later stages in the biosynthetic
pathway, such as the glycosyltransferases of the Golgi complex
(indicated as orange membrane proteins in Figure 8.25b),
(2) membrane proteins involved in the docking and fusion of
the vesicle with the target compartment, and (3) membrane
proteins that are able to bind soluble cargo (such as the secretory
proteins, indicated by the red spheres in Figure 8.25b).
Mutations in one of these cargo receptors has been linked to an
inherited bleeding disorder. Persons with this disorder fail to
secrete certain coagulation factors that promote blood