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Vol. 18, Issue 2, 594-604, February 2007

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Mitogen-activated Protein Kinase Kinase 1-dependent Golgi Unlinking Occurs in G₂ Phase and Promotes the G₂/M Cell Cycle Transition^,

Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213

Submitted June 19, 2006; Revised November 22, 2006; Accepted November 30, 2006
Monitoring Editor: Benjamin Glick

Two controversies have emerged regarding the signaling pathways that regulate Golgi disassembly at the G₂/M cell cycle transition. The first controversy concerns the role of mitogen-activated protein kinase activator mitogen-activated protein kinase kinase (MEK)1, and the second controversy concerns the participation of Golgi structure in a novel cell cycle "checkpoint." A potential simultaneous resolution is suggested by the hypothesis that MEK1 triggers Golgi unlinking in late G₂ to control G₂/M kinetics. Here, we show that inhibition of MEK1 by RNA interference or by using the MEK1/2-specific inhibitor U0126 delayed the passage of synchronized HeLa cells into M phase. The MEK1 requirement for normal mitotic entry was abrogated if Golgi proteins were dispersed before M phase by treatment of cells with brefeldin A or if GRASP65, which links Golgi stacks into a ribbon network, was depleted. Imaging revealed that unlinking of the Golgi apparatus begins before M phase, is independent of cyclin-dependent kinase 1 activation, and requires MEK signaling. Furthermore, expression of the GRASP family member GRASP55 after alanine substitution of its MEK1-dependent mitotic phosphorylation sites inhibited both late G₂ Golgi unlinking and the G₂/M transition. Thus, MEK1 plays an in vivo role in Golgi reorganization, which regulates cell cycle progression.

The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

Address correspondence to: Adam D. Linstedt (linstedt{at}andrew.cmu.edu)

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