Phosphorylation of Sli15 by Ipl1 is important for proper CPC localization and chromosome stability in Saccharomyces cerevisiae

Chromosome compaction is triggered by an autonomous DNA-binding module within condensin

The compaction of chromatin into mitotic chromosomes is essential for faithful transmission of the genome during cell division. In eukaryotes, chromosome morphogenesis is regulated by the condensin complex, though the exact mechanism used to target condensin to chromatin and initiate condensation is not understood. Here, we reveal that condensin contains an intrinsically disordered region (IDR) that modulates its association with chromatin in early mitosis and exhibits phase separation. We describe DNA-binding motifs within the IDR that, upon deletion, inflict striking defects in chromosome condensation and segregation, ill-timed condensin turnover on chromatin, and cell death. Importantly, we demonstrate that the condensin IDR can impart cell cycle regulatory functions when transferred to other subunits within the complex, indicating its autonomous nature. Collectively, our study unveils the molecular basis for the initiation of chromosome condensation in early mitosis and how this process ultimately promotes genomic stability and faultless cell division.

A new layer of regulation of chromosomal passenger complex (CPC) translocation in budding yeast

The conserved chromosomal passenger complex (CPC) consists of Ipl1Aurora-B, Sli15INCENP, Bir1Survivin, and Nbl1Borealin, and localizes at the kinetochore/centromere to correct kinetochore attachment errors and to prevent checkpoint silencing. After anaphase entry, the CPC moves from the kinetochore/centromere to the spindle. In budding yeast, CPC subunit Sli15 is phosphorylated by both cyclin-dependent kinase (CDK) and Ipl1 kinase. Following anaphase onset, activated Cdc14 phosphatase reverses Sli15 phosphorylation imposed by CDK to promote CPC translocation. Although abolished Sli15 phosphorylation imposed by Ipl1 also causes CPC translocation, the regulation of Ipl1-imposed Sli15 phosphorylation remains unclear. In addition to Sli15, Cdc14 also dephosphorylates Fin1, a regulatory subunit of protein phosphatase 1 (PP1), to enable kinetochore localization of Fin1-PP1. Here, we present evidence supporting the notion that kinetochore-localized Fin1-PP1 likely reverses Ipl1-imposed Sli15 phosphorylation to promote CPC translocation from the kinetochore/centromere to the spindle. Importantly, premature Fin1 kinetochore localization or phospho-deficient sli15 mutation causes checkpoint defects in response to tensionless attachments, resulting in chromosome missegregation. In addition, our data indicate that reversion of CDK- and Ipl1-imposed Sli15 phosphorylation shows an additive effect on CPC translocation. Together, these results reveal a previously unidentified pathway to regulate CPC translocation, which is important for accurate chromosome segregation.

Swe1 and Mih1 regulate mitotic spindle dynamics in budding yeast via Bik1

The mitotic spindle is a very dynamic structure that is built de novo and destroyed at each round of cell division. In order to perform its fundamental function during chromosome segregation, mitotic spindle dynamics must be tightly coordinated with other cell cycle events. These changes are driven by several protein kinases, phosphatases and microtubule-associated proteins. In budding yeast, the kinase Swe1 and the phosphatase Mih1 act in concert in controlling the phosphorylation state of Cdc28, the catalytic subunit of Cdk1, the major regulator of the cell cycle. In this study we show that Swe1 and Mih1 are also involved in the control of mitotic spindle dynamics. Our data indicate that Swe1 and the Polo-like kinase Cdc5 control the balance between phosphorylated and unphosphorylated forms of Mih1, which is, in turn, important for mitotic spindle elongation. Moreover, we show that the microtubule-associated protein Bik1 is a phosphoprotein, and that Swe1 and Mih1 are both involved in controlling phosphorylation of Bik1. These results uncover new players and provide insights into the complex regulation of mitotic spindle dynamics.

The effect of phosphorylation on the salt-tolerance-related functions of the soybean protein PM18, a member of the group-3 LEA protein family

Enzymatically driven post-translated modifications (PTMs) usually happen within the intrinsically disordered regions of a target protein and can modulate variety of protein functions. Late embryogenesis abundant (LEA) proteins are a family of the plant intrinsically disordered proteins (IDPs). Despite their important roles in plant stress response, there is currently limited knowledge on the presence and functional and structural effects of phosphorylation on LEA proteins. In this study, we identified three phosphorylation sites (Ser90, Tyr136, and Thr266) in the soybean PM18 protein that belongs to the group-3 LEA proteins. In yeast expression system, PM18 protein increased the salt tolerance of yeast, and the phosphorylation of this protein further enhanced its protective function. Further analysis revealed that Ser90 and Tyr136 are more important than Thr266, and these two sites might work cooperatively in regulating the salt resistance function of PM18. The circular dichroism analysis showed that PM18 protein was disordered in aqueous media, and phosphorylation did not affect the disordered status of this protein. However, phosphorylation promoted formation of more helical structure in the presence of sodium dodecyl sulfate (SDS) or trifluoroethanol (TFE). Furthermore, in dedicated in vitro experiments, phosphorylated PM18 protein was able to better protect lactate dehydrogenase (LDH) from the inactivation induced by the freeze-thaw cycles than its un- or dephosphorylated forms. All these data indicate that phosphorylation may have regulatory effects on the stress-tolerance-related function of LEA proteins. Therefore, further studies are needed to shed more light on functional and structural roles of phosphorylation in LEA proteins.

Microtubule cross-linking activity of She1 ensures spindle stability for spindle positioning

Dynein orients the spindle by pulling on astral microtubules from the cortex. In Saccharomyces cerevisiae, the microtubule-associated protein She1 specifically inhibits dynein in the mother compartment to promote spindle movements toward the bud. Zhu et al. demonstrate that She1 also stabilizes interpolar microtubules, ensuring spindle integrity during dynein-mediated spindle positioning.

Dynein mediates spindle positioning in budding yeast by pulling on astral microtubules (MTs) from the cell cortex. The MT-associated protein She1 regulates dynein activity along astral MTs and directs spindle movements toward the bud cell. In addition to localizing to astral MTs, She1 also targets to the spindle, but its role on the spindle remains unknown. Using function-separating alleles, live-cell spindle assays, and in vitro biochemical analyses, we show that She1 is required for the maintenance of metaphase spindle stability. She1 binds and cross-links MTs via a C-terminal MT-binding site. She1 can also self-assemble into ring-shaped oligomers. In cells, She1 stabilizes interpolar MTs, preventing spindle deformations during movement, and we show that this activity is regulated by Ipl1/Aurora B phosphorylation during cell cycle progression. Our data reveal how She1 ensures spindle integrity during spindle movement across the bud neck and suggest a potential link between regulation of spindle integrity and dynein pathway activity.

An engineered minimal chromosomal passenger complex reveals a role for INCENP/Sli15 spindle association in chromosome biorientation

The four-subunit chromosomal passenger complex (CPC), whose enzymatic subunit is Aurora B kinase, promotes chromosome biorientation by detaching incorrect kinetochore-microtubule attachments. In this study, we use a combination of truncations and artificial dimerization in budding yeast to define the minimal CPC elements essential for its biorientation function. We engineered a minimal CPC comprised of the dimerized last third of the kinase-activating Sli15/INCENP scaffold and the catalytic subunit Ipl1/Aurora B. Although native Sli15 is not oligomeric, artificial dimerization suppressed the biorientation defect and lethality associated with deletion of a majority of its microtubule-binding domain. Dimerization did not act through a physical clustering-based kinase activation mechanism but instead promoted spindle association, likely via a putative helical domain in Sli15 that is essential even when dimerized and is required to target kinetochore substrates. Based on the engineering and characterization of a minimal CPC, we suggest that spindle association is important for active Ipl1/Aurora B complexes to preferentially destabilize misattached kinetochores.

Dual recognition of chromatin and microtubules by INCENP is important for mitotic progression

The chromosomal passenger complex (CPC), composed of inner centromere protein (INCENP), Survivin, Borealin, and the kinase Aurora B, contributes to the activation of the mitotic checkpoint. The regulation of CPC function remains unclear. Here, we reveal that in addition to Survivin and Borealin, the single α-helix (SAH) domain of INCENP supports CPC localization to chromatin and the mitotic checkpoint. The INCENP SAH domain also mediates INCENP's microtubule binding, which is negatively regulated by Cyclin-dependent kinase-mediated phosphorylation of segments flanking the SAH domain. The microtubule-binding capacity of the SAH domain is important for mitotic arrest in conditions of suppressed microtubule dynamics, and the duration of mitotic arrest dictates the probability, but not the timing, of cell death. Although independent targeting of INCENP to microtubules or the kinetochore/centromere promotes the mitotic checkpoint, it is insufficient for a robust mitotic arrest. Altogether, our results demonstrate that dual recognition of chromatin and microtubules by CPC is important for checkpoint maintenance and determination of cell fate in mitosis.

A direct interaction between fascin and microtubules contributes to adhesion dynamics and cell migration

Fascin is an actin-binding and bundling protein that is highly upregulated in most epithelial cancers. Fascin promotes cell migration and adhesion dynamics in vitro and tumour cell metastasis in vivo. However, potential non-actin bundling roles for fascin remain unknown. Here, we show for the first time that fascin can directly interact with the microtubule cytoskeleton and that this does not depend upon fascin-actin bundling. Microtubule binding contributes to fascin-dependent control of focal adhesion dynamics and cell migration speed. We also show that fascin forms a complex with focal adhesion kinase (FAK, also known as PTK2) and Src, and that this signalling pathway lies downstream of fascin-microtubule association in the control of adhesion stability. These findings shed light on new non actin-dependent roles for fascin and might have implications for the design of therapies to target fascin in metastatic disease.

Bub1 autophosphorylation feeds back to regulate kinetochore docking and promote localized substrate phosphorylation

During mitosis, Bub1 kinase phosphorylates histone H2A-T120 to promote centromere sister chromatid cohesion through recruitment of shugoshin (Sgo) proteins. The regulation and dynamics of H2A-T120 phosphorylation are poorly understood. Using quantitative phosphoproteomics we show that Bub1 is autophosphorylated at numerous sites. We confirm mitosis-specific autophosphorylation of a several residues and show that Bub1 activation is primed in interphase but fully achieved only in mitosis. Mutation of a single autophosphorylation site T589 alters kinetochore turnover of Bub1 and results in uniform H2A-T120 phosphorylation and Sgo recruitment along chromosome arms. Consequently, improper sister chromatid resolution and chromosome segregation errors are observed. Kinetochore tethering of Bub1-T589A refocuses H2A-T120 phosphorylation and Sgo1 to centromeres. Recruitment of the Bub1-Bub3-BubR1 axis to kinetochores has recently been extensively studied. Our data provide novel insight into the regulation and kinetochore residency of Bub1 and indicate that its localization is dynamic and tightly controlled through feedback autophosphorylation.

Inter-domain Cooperation in INCENP Promotes Aurora B Relocation from Centromeres to Microtubules

The chromosomal passenger complex is essential for error-free chromosome segregation and proper execution of cytokinesis. To coordinate nuclear division with cytoplasmic division, its enzymatic subunit, Aurora B, relocalizes from centromeres in metaphase to the spindle midzone in anaphase. In budding yeast, this requires dephosphorylation of the microtubule-binding (MTB) domain of the INCENP analog Sli15. The mechanistic basis for this relocalization in metazoans is incompletely understood. We demonstrate that the putative coiled-coil domain within INCENP drives midzone localization of Aurora B via a direct, electrostatic interaction with microtubules. Furthermore, we provide evidence that the CPC multimerizes via INCENP's centromere-targeting domain (CEN box), which increases the MTB affinity of INCENP. In (pro)metaphase, the MTB affinity of INCENP is outcompeted by the affinity of its CEN box for centromeres, while at anaphase onset—when the histone mark H2AT120 is dephosphorylated—INCENP and Aurora B switch from centromere to microtubule localization.