Cdc7p-Dbf4p Regulates Mitotic Exit by Inhibiting Polo Kinase

Charles T Miller,Michael Weinreich,Ying-Chou Chen,Carrie Gabrielse,Orna Cohen-Fix

doi:10.1371/journal.pgen.1000498

Charles T Miller, Michael Weinreich + Show 3 more

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https://doi.org/10.1371/journal.pgen.1000498

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Abstract

Cdc7p-Dbf4p is a conserved protein kinase required for the initiation of DNA replication. The Dbf4p regulatory subunit binds Cdc7p and is essential for Cdc7p kinase activation, however, the N-terminal third of Dbf4p is dispensable for its essential replication activities. Here, we define a short N-terminal Dbf4p region that targets Cdc7p-Dbf4p kinase to Cdc5p, the single Polo kinase in budding yeast that regulates mitotic progression and cytokinesis. Dbf4p mediates an interaction with the Polo substrate-binding domain to inhibit its essential role during mitosis. Although Dbf4p does not inhibit Polo kinase activity, it nonetheless inhibits Polo-mediated activation of the mitotic exit network (MEN), presumably by altering Polo substrate targeting. In addition, although dbf4 mutants defective for interaction with Polo transit S-phase normally, they aberrantly segregate chromosomes following nuclear misorientation. Therefore, Cdc7p-Dbf4p prevents inappropriate exit from mitosis by inhibiting Polo kinase and functions in the spindle position checkpoint.

Highlights

Accurate ordering of cell cycle events is an important requirement for the viability of all eukaryotic organisms
We found that Dbf4p physically interacts with another protein called Polo that acts during mitosis, a later step in the cell cycle when the newly copied chromosomes are divided to mother and daughter cells
We conducted a yeast two-hybrid screen to identify proteins that interact with the Dbf4p N-terminus and recovered multiple clones encoding the polo-box domain (PBD) of Cdc5p

Summary

Introduction

Accurate ordering of cell cycle events is an important requirement for the viability of all eukaryotic organisms. Since daughter cell growth is highly polarized in the budding yeast, exit from mitosis is prevented until sister chromatids segregate through the bud neck and into the daughter cell [5,6,7] This ensures that spindle disassembly and mitotic exit are not initiated until accurate chromosome partitioning between mother and daughter cells has occurred. Failure to block mitotic exit when nuclear division takes place within the mother cell results in polyploid and anucleate progeny [8,9] It is not surprising that both entry into and exit from mitosis are delayed by cellular checkpoints that respond to replication stress, chromosome damage, or spindle disruption [1]. Errors in these mitotic checkpoints are catastrophic and result in ploidy defects and genetic alterations, which are frequently observed in human cancers (reviewed by [10])

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