Dexamethasone Spindle positioning is governed by the

Spindle positioning is governed by the actin cytoskeleton. After a transient metaphase arrest, cells lacking haspin exhibit a misdistribution of actin, which accumulates within the bud. Actin dynamics is modulated by an intricate network including septins, the polarisome complex, and formins. We show that, similar to what happens with actin, loss of haspin causes the accumulation of formins and polarisome components in the bud. The mechanism controlling the recruitment of these proteins to their proper location is still largely unknown, making it difficult to determine the direct target of haspin kinase. Preliminary results suggest that the phosphorylation status of Bud6 and Bnr1 is not influenced by loss of haspin (not shown). High-throughput screenings have suggested physical and genetic interactions between polarization proteins and yeast haspin (Bodenmiller et al., 2010, Breitkreutz et al., 2010, Fiedler et al., 2009, Sharifpoor et al., 2012); further studies will be required to identify other players involved in the establishment of cell polarity that could be targeted by haspin. Polarity factors have to redistribute during mitosis (Geymonat et al., 2009), and haspin-defective cells show a clear defect in such redistribution, suggesting that haspin plays an important function in maintaining the coupling between cell-cycle progression and redistribution of cell polarity factors that are crucial for correct cell division after cell-cycle restart. Failure to properly localize polarity cues causes a prolonged hyperpolarization during mitotic arrest, which sustain the forces pulling the spindle toward the daughter cell. The absence of Bud6 from the bud neck and the restriction of Dexamethasone and polarisome within the daughter affect the establishment of a balancing force pulling toward the mother, resulting in mispositioning of the mitotic spindle. A similar imbalance of these forces has been suggested to explain the “daughterly” nuclear positioning observed in the absence of sister chromatid separation in esp1-1 mutants during an unperturbed cell cycle (Ross and Cohen-Fix, 2004). Intriguingly, we found that expression of A. thaliana haspin in alk1Δalk2Δ cells suppresses the benomyl-sensitive phenotype and partially rescues the nuclear segregation defect (Figure S5), strongly suggesting that haspin function in cell polarity is conserved throughout evolution.
Experimental Procedures
Acknowledgments
Introduction Epigenetic changes brought about by post-translational modifications (PTM) are widely used to generate on/off switches for protein–protein interactions, which serve as important gene controls in cells. Phosphorylation, methylation, and acetylation are the three most frequent chemical processes involved in PTMs. In addition, two or more PTM events can be employed simultaneously in order to modulate signals. N-Terminal histone proteins have extensive epigenetically varied sequences, especially in H3, that are decorated by different types of PTMs (Fig. 1). In order to accomplish multiple modifications, one change should be carried out in the presence of other PTMs and, as a result, the possibility exists that one PTM could influence the nature and efficiency of a subsequent PTM. Since they are accompanied by relatively large structural and environmental changes and they are easily recognized, phosphorylation reactions are often used as initial signals to promote other PTMs. Indeed, phosphorylation-mediated signaling is known to affect methylation or acetylation based processes in H3. For example, phosphorylation at S10 results in suppression of mono- and dimethylation at K9 and an enhancement of acetylation at K14. The results of a recent study also showed that a variety of methylations, taking place in the presence of phosphorylation mimicry of S10, are retarded significantly. However, few examples exist where phosphorylation processes are affected by other PTMs. In terms of molecular recognition, methylation reactions do not bring about as dramatic changes in structure or environment as do phosphorylation processes. Furthermore, the different sites as well as multiple patterns of methylation that occur in Lys (mono-, di-, and tri-) and Arg (symmetric and asymmetric di-), which are components of ‘histone codes,’ complicate investigation of the roles of methylation as signal events. Single methylation/demethylation processes are not easily detected since generation of antibodies against individual, complex methylation patterns are difficult if not impossible. Also, the old belief that methylations are static changes has hampered investigations of methylation-mediated signalling.