B-Cyclin/CDKs regulate mitotic spindle assembly by phosphorylating kinesins-5 in budding yeast

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Although it has been known for many years that B-cyclin/CDK complexes regulate the assembly of the mitotic spindle and entry into mitosis, the full complement of relevant CDK targets has not been identified. It has previously been shown in a variety of model systems that B-type cyclin/CDK complexes, kinesin-5 motors, and the SCFCdc4 ubiquitin ligase are required for the separation of spindle poles and assembly of a bipolar spindle. It has been suggested that, in budding yeast, B-type cyclin/CDK (Clb/Cdc28) complexes promote spindle pole separation by inhibiting the degradation of the kinesins-5 Kip1 and Cin8 by the anaphase-promoting complex (APCCdh1). We have determined, however, that the Kip1 and Cin8 proteins are present at wild-type levels in the absence of Clb/Cdc28 kinase activity. Here, we show that Kip1 and Cin8 are in vitro targets of Clb2/Cdc28 and that the mutation of conserved CDK phosphorylation sites on Kip1 inhibits spindle pole separation without affecting the protein's in vivo localization or abundance. Mass spectrometry analysis confirms that two CDK sites in the tail domain of Kip1 are phosphorylated in vivo. In addition, we have determined that Sic1, a Clb/Cdc28-specific inhibitor, is the SCFCdc4 target that inhibits spindle pole separation in cells lacking functional Cdc4. Based on these findings, we propose that Clb/Cdc28 drives spindle pole separation by direct phosphorylation of kinesin-5 motors. © 2010 Chee, Haase.






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Chee, Mark K, and Steven B Haase (2010). B-Cyclin/CDKs regulate mitotic spindle assembly by phosphorylating kinesins-5 in budding yeast. PLoS Genetics, 6(5). p. 35. 10.1371/journal.pgen.1000935 Retrieved from https://hdl.handle.net/10161/4466.

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Steven B. Haase

Professor of Biology

Our group is broadly interested in understanding the biological clock mechanisms that control the timing of events during the cell division cycle. In 2008, the Haase group proposed a new model in which a complex network of sequentially activated transcription factors regulates the precise timing of gene expression during the cell-cycle, and functions as a robust time-keeping oscillator. Greater than a thousand genes are expressed at distinct phases of the cycle, and the control network itself consists of ~20 components, so this dynamical system is far too complex to understand simply by biological intuition. We rely heavily on the expertise of the Harer group (Dept. of Mathematics, Duke University) for the analysis of complex data, and their understanding of dynamical systems.  Using a collection of tools, including molecular genetics, genomics, mathematical models, and statistical inference, our groups aim to understand how the cell division clock works, how it might be perturbed in proliferative diseases such as cancer, and how the clock components might be targeted for new anti-tumor therapies.  Qualitatively, the clock networks that control the yeast cell cycle look much like the networks controlling circadian rhythms in a variety of organisms. More recently, we have been using our experimental and quantitative approaches to investigate the function of circadian clocks, as well as clocks that control the division and development of pathogenic organisms such as P. falciparum and P. vivax, the causative agents of malaria.

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