Polyploid yeast are dependent on elevated levels of Mps1 for successful chromosome segregation.

Tumor cell lines with elevated chromosome numbers frequently have correlated elevations of Mps1 expression and these tumors are more dependent on Mps1 activity for their survival than control cell lines. Mps1 is a conserved kinase involved in controlling aspects of chromosome segregation in mitosis and meiosis. The mechanistic explanation for the Mps1-addiction of aneuploid cells is unknown. To address this question, we explored Mps1-dependence in yeast cells with increased sets of chromosomes. These experiments revealed that in yeast, increasing ploidy leads to delays and failures in orienting chromosomes on the mitotic spindle. Yeast cells with elevated numbers of chromosomes proved vulnerable to reductions of Mps1 activity. Cells with reduced Mps1 activity exhibit an extended prometaphase with longer spindles and delays in orienting the chromosomes. One known role of Mps1 is in recruiting Bub1 to the kinetochore in meiosis. We found that the Mps1-addiction of polyploid yeast cells is due in part to its role in Bub1 recruitment. Together, the experiments presented here demonstrate that increased ploidy renders cells more dependent on Mps1 for orienting chromosomes on the spindle. The phenomenon described here may be relevant in understanding why hyper-diploid cancer cells exhibit elevated reliance on Mps1 expression for successful chromosome segregation.

Ycs4 Subunit of Saccharomyces cerevisiae Condensin Binds DNA and Modulates the Enzyme Turnover.

Condensins play a key role in higher order chromosome organization. In budding yeast Saccharomyces cerevisiae, a condensin complex consists of five subunits: two conserved structural maintenance of chromosome subunits, Smc2 and Smc4, a kleisin Brn1, and two HEAT repeat subunits, Ycg1, which possesses a DNA binding activity, and Ycs4, which can transiently associate with Smc4 and thereby disrupt its association with the Smc2 head. We characterized here DNA binding activity of the non-SMC subunits using an agnostic, model-independent approach. To this end, we mapped the DNA interface of the complex using sulfo-NHS biotin labeling. Besides the known site on Ycg1, we found a patch of lysines at the C-terminal domain of Ycs4 that were protected from biotinylation in the presence of DNA. Point mutations at the predicted protein-DNA interface reduced both Ycs4 binding to DNA and the DNA stimulated ATPase activity of the reconstituted condensin, whereas overproduction of the mutant Ycs4 was detrimental for yeast viability. Notably, the DNA binding site on Ycs4 partially overlapped with its interface with SMC4, revealing an intricate interplay between DNA binding, engagement of the Smc2-Smc4 heads, and ATP hydrolysis and suggesting a mechanism for ATP-modulated loading and translocation of condensins on DNA.

FEAR but not MEN genes are required for exit from meiosis I.

Exit from mitosis is regulated by Cdc14, which plays an essential role in triggering cyclin-dependent kinase inactivation. Throughout most of the cell cycle, Cdc14 is sequestered in the nucleolus where it remains inactive. After the completion of anaphase, an essential signaling cascade, named the Mitotic Exit Network, or MEN, promotes Cdc14 release. Cdc14 is also released from the nucleolus in early anaphase by another, nonessential, pathway called FEAR (CdcFourteen Early Anaphase Release). Separase (Esp1), polo kinase (Cdc5), the kinetochore protein Slk19, and Spo12, whose molecular function remains unknown, have been identified as members of the FEAR pathway. In meiosis, mutations in CDC14 and its FEAR pathway regulators, CDC5, SLK19, and SPO12, all result it asci that contain only two diploid spores because of a defect in the ability to exit meiosis I. Thus although the FEAR pathway is dispensible for mitotic exit, it is essential for meiosis I exit. The way that the genes of the Mitotic Exit Network contribute to coordinating meiotic progression is less clear. Here, we explore this issue. Our results demonstrate that the orderly transition from meiosis I to meiosis II is accomplished by eliminating MEN function and using the FEAR pathway to modulate cyclin dependent kinase activity, in part through the actions of SIC1.