In mammalian cells, the Golgi apparatus disassembles into vesicles and short tubules before mitosis. Golgi breakdown requires that two membrane fusion pathways necessary for Golgi reassembly after mitosis be temporarily inhibited. For one, the NSF pathway, this inhibition is achieved by Cdc2-mediated phosphorylation of an NSF-associated factor at early mitosis. But how the cell cycle regulates the second, the p47/p97 pathway, was not known.
Uchiyama et al. have determined that Cdc2-mediated phosphorylation also inhibits the p47/p97 pathway. p97 is an ATPase that binds to the Golgi through its associated factor, p47. Uchiyama et al. find that p47 is phosphorylated by Cdc2 upon entry into mitosis. Phosphorylated p47 bound to p97 in mitotic cells and had a decreased binding affinity for Golgi membranes, thus releasing the complex from the Golgi.
Addition of a phosphorylation-insensitive version of p47 to mitotic cells blocked Golgi breakdown. However, the mutant did not impair proper separation of the Golgi into the daughter cells. How the Golgi can partition without first disassembling in mammalian cells is unclear. Perhaps they are able to undergo fission, as in Toxoplasma. Additionally, we are left to ponder why Golgi disassembly is essential for entry into mitosis, and indeed why Golgi complexes disassemble at all. ▪