Exclusion of Golgi residents from transport vesicles budding from Golgi cisternae in intact cells

A central feature of cisternal progression/maturation models for anterograd e transport across the Golgi stack is the requirement that the entire popul ation of steady-state residents of this organelle be continuously transport ed backward to earlier cisternae to avoid loss of these residents as the me mbrane of the oldest (trans-most) cisterna departs the stack. For this to o ccur, resident proteins must be packaged into retrograde-directed transport vesicles, and to occur at the rate of anterograde transport? resident prot eins must be present in vesicles at a higher concentration than in cisterna l membranes. We have tested this prediction by localizing two steady-state residents of medial Golgi cisternae (mannosidase II and N-acetylglucosaminy l transferase I) at the electron microscopic level in intact cells. In both cases, these abundant cisternal constituents were strongly excluded from b uds and vesicles. This result suggests that cisternal progression takes pla ce substantially more slowly than most protein transport and therefore is u nlikely to be the predominant mechanism of anterograde movement.