Studies of haspin-depleted cells reveal that spindle-pole integrity in mitosis requires chromosome cohesion.

Cohesins and their regulators are vital for normal chromosome cohesion and segregation. A number of cohesion proteins have also been localized to centrosomes and proposed to function there. We show that RNAi-mediated depletion of factors required for cohesion, including haspin, Sgo1 and Scc1, leads to the generation of multiple acentriolar centrosome-like foci and disruption of spindle structure in mitosis. Live-cell imaging reveals that, in haspin-depleted cells, these effects occur only as defects in chromosome cohesion become manifest, and they require ongoing microtubule dynamics and kinesin-5 (also known as Eg5) activity. Inhibition of topoisomerase II in mitosis, which prevents decatenation and separation of chromatids, circumvents the loss of cohesion and restores integrity of the spindle poles. Although these results do not rule out roles for cohesin proteins at centrosomes, they suggest that when cohesion is compromised, spindle-pole integrity can be disrupted as an indirect consequence of the failure to properly integrate chromosome- and centrosome-initiated pathways for spindle formation.

Recombination protein Tid1p controls resolution of cohesin-dependent linkages in meiosis in Saccharomyces cerevisiae.

Sister chromatid cohesion and interhomologue recombination are coordinated to promote the segregation of homologous chromosomes instead of sister chromatids at the first meiotic division. During meiotic prophase in Saccharomyces cerevisiae, the meiosis-specific cohesin Rec8p localizes along chromosome axes and mediates most of the cohesion. The mitotic cohesin Mcd1p/Scc1p localizes to discrete spots along chromosome arms, and its function is not clear. In cells lacking Tid1p, which is a member of the SWI2/SNF2 family of helicase-like proteins that are involved in chromatin remodeling, Mcd1p and Rec8p persist abnormally through both meiotic divisions, and chromosome segregation fails in the majority of cells. Genetic results indicate that the primary defect in these cells is a failure to resolve Mcd1p-mediated connections. Tid1p interacts with recombination enzymes Dmc1p and Rad51p and has an established role in recombination repair. We propose that Tid1p remodels Mcd1p-mediated cohesion early in meiotic prophase to facilitate interhomologue recombination and the subsequent segregation of homologous chromosomes.

Haspin inhibitors reveal centromeric functions of Aurora B in chromosome segregation.

Haspin phosphorylates histone H3 at threonine-3 (H3T3ph), providing a docking site for the Aurora B complex at centromeres. Aurora B functions to correct improper kinetochore-microtubule attachments and alert the spindle checkpoint to the presence of misaligned chromosomes. We show that Haspin inhibitors decreased H3T3ph, resulting in loss of centromeric Aurora B and reduced phosphorylation of centromere and kinetochore Aurora B substrates. Consequently, metaphase chromosome alignment and spindle checkpoint signaling were compromised. These effects were phenocopied by microinjection of anti-H3T3ph antibodies. Retargeting Aurora B to centromeres partially restored checkpoint signaling and Aurora B-dependent phosphorylation at centromeres and kinetochores, bypassing the need for Haspin activity. Haspin inhibitors did not obviously affect phosphorylation of histone H3 at serine-10 (H3S10ph) by Aurora B on chromosome arms but, in Aurora B reactivation assays, recovery of H3S10ph was delayed. Haspin inhibitors did not block Aurora B localization to the spindle midzone in anaphase or Aurora B function in cytokinesis. Thus, Haspin inhibitors reveal centromeric roles of Aurora B in chromosome movement and spindle checkpoint signaling.

Shugoshin and PP2A: collaborating to keep chromosomes connected.

Timely release of sister chromatid cohesion is essential for accurate chromosome segregation during cell division. Shugoshin forms a complex with the phosphatase PP2A that has been proposed to dephosphorylate cohesin proteins to prevent premature loss of centromeric cohesion. A recent study in Molecular Cell by Xu et al. presents the structure of Shugoshin bound to PP2A and provides evidence that this interaction is required for cohesion protection.

Sister chromatid cohesion remodeling and meiotic recombination.

Proper control of cohesion along the chromosome arms is essential for segregation of homologous chromosomes in meiosis. In a recent study we reported that Tid1p, a protein previously implicated in recombination, is required for resolution of Mcd1p-dependent cohesion in meiosis. Here we demonstrate that Pds5p and Dmc1p promote this cohesion. Pds5p is known to be required for maintenance of cohesion while Dmc1p is recognized as essential for meiotic recombination. Finding that the same defect in separation of sister chromatids could be suppressed by disrupting the functions of these proteins supports the emerging recognition that cohesion is remodeled during recombination and further indicates that cohesion is modified specifically to regulate meiotic recombination. We also find that overexpression of the regulatory subunit of Cdc7p kinase, Dbf4p, suppresses the tid1delta sporulation defect, suggesting a role for Cdc7p/Dbf4p in regulating cohesion.