A ROLE FAR TUBULAR NETWORKS AND A COP I-INDEPENDENT PATHWAY IN THE MITOTIC FRAGMENTATION OF GOLGI STACKS IN A CELL-FREE SYSTEM

Golgi stacks were previously shown to be converted into tubular networ ks when incubated in mitotic cytosol depleted of the coatomer subunit of COP I coats (Misteli and Warren, 1994), Similar, though smaller, ne tworks are now shown to be an early intermediate on the Golgi fragment ation 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 mec hanism that leads eventually to a vesicle fraction that is larger in s ize and more heterogeneous than that produced by the COP I-mechanism. We suggest that both pathways operate concurrently at the onset of mit otic fragmentation. The COP I-independent pathway converts cisternae i nto tubular networks that then fragment. The COP I-dependent pathway p artially consumes first the cisternae at the beginning of the incubati on and then the tubular networks that form from them.