Mechanisms of Golgi Vesiculation during Mitosis

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DESCRIPTION (provided by applicant): The pericentriolar inter connected stacks of Golgi cisternae are fragmented in mitotic mammalian cells. Why do these membranes undergo such a drastic change in their organization and by what mechanism? Do Golgi membranes fuse with the endoplasmic reticulum (ER) during mitosis and therefore use ER for their transmission into daughter cells? Our recent findings reveal that Golgi membranes do not fuse with the ER during mitosis. These findings resolve a long-standing controversy. We have found that inhibiting Golgi fragmentation prevents entry into mitosis. We have proposed the existence of a Golgi membrane specific cell-cycle checkpoint. We have found that kinases MEK1 and Polo like kinase (Plk); and Golgi associated proteins GRASP55 and GRASP65 are required for Golgi fragmentation. Our hypothesis is that MEK1 activates a novel spliced variant of ERK called ERK1c, which phosphorylates GRASP55 to disconnect Golgi stacks. This event occurs in G2 and it is necessary for entry into mitosis. Plk on the other hand, we suggest phosphorylates GRASP65. The phosphorylated GRASP65 recruits a novel protein called Gip (GRASP interacting protein) and this event is necessary for GRASP65 dependent regulation of spindle dynamics in mitosis. A new screen has revealed a number of proteins that regulate the dimensions of Golgi stacks and our hypothesis is that these proteins are modified (by phosphorylation/dephosphorylation) to reorganize Golgi membranes during mitosis. Finally, we are screening 5000 human enzymes (kinase, phosphatase, lipases, GTPases etc) to identify components that control, through the organization of Golgi apparatus, entry of cells into mitosis. Our findings will reveal the mechanism by which, cells duplicate their essential organelles during the cell cycle, and how Golgi membranes in turn through their organization, control mitotic entry