GAPs in the context of COPI: Enzymes, coat components or both?

TRAFFICKING IN THE EARLY SECRETORY PATHWAY AT FIRST GLANCE IS WELL UNDERSTOOD ACCORDING TO TEXTBOOK KNOWLEDGE: To achieve secretion and to maintain organelle homeostasis, protein and lipid cargo need to be transported constitutively from their origins of biosynthesis to their respective destinations. Thus, secretory cargo exits the ER and is shuttled to the Golgi via vesicular COPII carriers. Lipid and protein cargo is enzymatically modified in the Golgi, transported from cis- to trans- (by mechanisms that are still debated today), and from there travel to their final destinations. The best established roles for COPI vesicles, simply spoken, is to mediate retrograde trafficking of cargo molecules that were transported forward, but need to be transported back.