Golgi stacks were previously shown to be converted into tubular networks when incubated in mitotic cytosol depleted of the coatomer subunit of COP I coats (Misteli and Warren, 1994). Similar, though smaller, networks are now shown to be an early intermediate on the Golgi fragmentation pathway both in vitro and in vivo. Their appearance mirrors the disappearance of Golgi cisternae and at their peak they constitute 35% of total Golgi membrane. They are consumed by two pathways, the first involving the budding of COP I-coated vesicles described previously (Misteli and Warren, 1994). The second involves a COP I-independent mechanism that leads eventually to a vesicle fraction that is larger in size and more heterogeneous than that produced by the COP I-mechanism. We suggest that both pathways operate concurrently at the onset of mitotic fragmentation. The COP I-independent pathway converts cisternae into tubular networks that then fragment. The COP I-dependent pathway partially consumes first the cisternae at the beginning of the incubation and then the tubular networks that form from them.

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