Coalescence of Golgi fragments in microtubule-deprived living cells

The process of stack coalescence, an important mechanism of Golgi recovery from mitosis, was examined using novel experimental paradigms. In living cells with disrupted (by nocodazole) microtubules, galactosyl transferase-GFP-labelled Golgi fragments constantly appeared, grew, sometimes moved with a speed of 1-2 microns/min, coalesced or gradually diminished and disappeared. The rate of Golgi fragment turnover and coalescence was highly balanced to maintain a constant number of Golgi units per cell. Moreover some Golgi islands appear and some received new GalTase-GFP after photobleaching of cell cytoplasm. Short tubules extending from the rims of scattered Golgi fragments frequently formed bridges between ministacks, inducing their coalescence. The frequency of coalescence could also be inhibited by disruption of actin microfilaments. After the Golgi redistribution into endoplasmic reticulum induced by brefeldin A, either the growth of small Golgi fragments or their coalescence leads to compartmentalized stack formation without the participation of microtubules. These results demonstrate that this coalescence between isolated Golgi stacks is microtubule-independent and could thus be mediated by membranous tubules.