Practice question

Golgi organization is explained by cisternal maturation model where new cis cisternae arise from fusion of ER-derived COPII vesicles at ERGIC and progressively mature into trans cisternae carrying secretory cargo forward toward trans-Golgi network. Resident processing enzymes, including cis mannosidases, medial GlcNAc transferases and trans sialyltransferases, cannot move forward with cargo otherwise compartmental polarity would be lost and glycosylation random. To preserve sequential enzyme gradients, transmembrane residents are selectively extracted into COPI-coated vesicles that transport retrogradely to younger cisternae. Cytosolic tails expose signals recognized by adaptors such as Vps74, GOLPH3 and COG complex concentrating enzymes into COPI buds. Live imaging in yeast shows COPI vesicles carrying medial mannosidase II backward while cargo remains lumenal and anterograde carriers move forward. Anterograde vesicle shuttle models propose stable cisternae, but COPI mutants and Rab effectors and tether assays demonstrate retrograde return essential for maturation, not degradation, lateral diffusion, or default secretion pathway maintaining fidelity and polarity.