Figure 2.

Cisternal progression/maturation. Secretory cargoes exit the ER in dissociative carriers, which coalesce with one another and with COPI vesicles derived from the cis-Golgi to form the intermediate compartment, which coalesces in turn to form a new cis-cisterna. In subsequent rounds of COPI-mediated recycling, the new cisterna matures by receiving medial and then trans-Golgi proteins from older cisternae while exporting cis and then medial-Golgi proteins to younger cisternae. Meanwhile, the cisterna progresses through the stack, carrying forward both small and large secretory cargoes. In the final stage of maturation, the cisterna is a TGN element that breaks down into anterograde and retrograde transport carriers.