A SUMOylation wave to anchor the genome

Chromatin tethers to the nuclear envelope are lost during mitosis to facilitate chromosome segregation. How these connections are reestablished to ensure functional genome organization in interphase is unclear. Ptak et al. (2021. J. Cell Biol.https://doi.org/10.1083/jcb.202103036) identify a phosphorylation and SUMOylation-dependent cascade that links chromatin to the nuclear membrane during late mitosis.

Gradual compaction of the central spindle decreases its dynamicity in PRC1 and EB1 gene-edited cells

During mitosis, the spindle undergoes morphological and dynamic changes. It reorganizes at the onset of the anaphase when the antiparallel bundler PRC1 accumulates and recruits central spindle proteins to the midzone. Little is known about how the dynamic properties of the central spindle change during its morphological changes in human cells. Using gene editing, we generated human cells that express from their endogenous locus fluorescent PRC1 and EB1 to quantify their native spindle distribution and binding/unbinding turnover. EB1 plus end tracking revealed a general slowdown of microtubule growth, whereas PRC1, similar to its yeast orthologue Ase1, binds increasingly strongly to compacting antiparallel microtubule overlaps. KIF4A and CLASP1 bind more dynamically to the central spindle, but also show slowing down turnover. These results show that the central spindle gradually becomes more stable during mitosis, in agreement with a recent “bundling, sliding, and compaction” model of antiparallel midzone bundle formation in the central spindle during late mitosis.

Simultaneous regulation of cytokinetic furrow and nucleus positions by cortical tension contributes to proper DNA segregation during late mitosis

Coordinating mitotic spindle and cytokinetic furrow positioning is essential to ensure proper DNA segregation. Here we present a novel mechanism, which corrects DNA segregation defects due to cytokinetic furrow mispositioning. We show that DNA segregation defects following the abnormal displacement of the cytokinetic furrow towards the anterior side of C. elegans one-cell embryos are unexpectedly corrected at the end of cytokinesis. This correction relies on the concomitant displacement of the furrow and of the anterior nucleus towards the posterior and anterior poles, respectively. It also coincides with cortical blebbing and an anteriorly directed flow of cytoplasmic particles. While microtubules contribute to nuclear displacement, relaxation of an excessive tension at the anterior cortex plays a central role in the correction process and simultaneously regulates cytoplasmic flow as well as nuclear and furrow displacements. This work thus reveals the existence of a so far uncharacterized correction mechanism, which is critical to correct DNA segregation defects due to cytokinetic furrow mispositioning.

KNL1 facilitates phosphorylation of outer kinetochore proteins by promoting Aurora B kinase activity

Aurora B kinase phosphorylates kinetochore proteins during early mitosis, increasing kinetochore–microtubule (MT) turnover and preventing premature stabilization of kinetochore–MT attachments. Phosphorylation of kinetochore proteins during late mitosis is low, promoting attachment stabilization, which is required for anaphase onset. The kinetochore protein KNL1 recruits Aurora B–counteracting phosphatases and the Aurora B–targeting factor Bub1, yet the consequences of KNL1 depletion on Aurora B phospho-regulation remain unknown. Here, we demonstrate that the KNL1 N terminus is essential for Aurora B activity at kinetochores. This region of KNL1 is also required for Bub1 kinase activity at kinetochores, suggesting that KNL1 promotes Aurora B activity through Bub1-mediated Aurora B targeting. However, ectopic targeting of Aurora B to kinetochores does not fully rescue Aurora B activity in KNL1-depleted cells, suggesting KNL1 influences Aurora B activity through an additional pathway. Our findings establish KNL1 as a requirement for Aurora B activity at kinetochores and for wild-type kinetochore–MT attachment dynamics.

Orthologues of the Anaphase-Promoting Complex/Cyclosome Coactivators Cdc20p and Cdh1p Are Important for Mitotic Progression and Morphogenesis in Candida albicans

ABSTRACTThe conserved anaphase-promoting complex/cyclosome (APC/C) system mediates protein degradation during mitotic progression. Conserved coactivators Cdc20p and Cdh1p regulate the APC/C during early to late mitosis and G1phase.Candida albicansis an important fungal pathogen of humans, and it forms highly polarized cells when mitosis is blocked through depletion of the polo-like kinase Cdc5p or other treatments. However, the mechanisms governing mitotic progression and associated polarized growth in the pathogen are poorly understood. In order to gain insights into these processes, we characterizedC. albicansorthologues of Cdc20p and Cdh1p. Cdc20p-depleted cells were blocked in early or late mitosis with elevated levels of Cdc5p and the mitotic cyclin Clb2p, suggesting that Cdc20p is essential and has some conserved functions during mitosis. However, the yeast cells formed highly polarized buds in contrast to the large doublets ofS. cerevisiaecdc20mutants, implying a distinct role in morphogenesis. In comparison,cdh1Δ/cdh1Δcells were viable but showed enrichment of Clb2p and Cdc5p, suggesting that Cdh1p may influence mitotic exit. Thecdh1Δ/cdh1Δphenotype was pleiotropic, consisting of normal or enlarged yeast, pseudohyphae, and some elongated buds, whereasS. cerevisiaecdh1Δyeast cells were reduced in size. Thus,C. albicansCdh1p may have some distinct functions. Finally, absence of Cdh1p or Cdc20p had a minor or no effect on hyphal development, respectively. Overall, the results suggest that Cdc20p and Cdh1p may be APC/C activators that are important for mitosis but also morphogenesis inC. albicans. Their novel features imply additional variations in function and underscore rewiring in the emerging mitotic regulatory networks of the pathogen.

Megakaryocytes survive without survivin

Survivin has been described as an important regulator of late mitosis and as antiapoptotic in various cells. In this issue of Blood, Wen and colleagues describe a megakaryocyte-specific knockout of survivin, with no influence on megakaryocyte survival, and a stimulating effect on polyploidy.1

Anaphase-Promoting Complex/Cyclosome-Cdh1-Mediated Proteolysis of the Forkhead Box M1 Transcription Factor Is Critical for Regulated Entry into S Phase

ABSTRACT The forkhead box M1 (FoxM1) transcription factor is overexpressed in many cancers, and in mouse models it is required for tumor progression. FoxM1 activates expression of the cell cycle genes required for both S and M phase progression. Here we demonstrate that FoxM1 is degraded in late mitosis and early G1 phase by the anaphase-promoting complex/cyclosome (APC/C) E3 ubiquitin ligase. FoxM1 interacts with the APC/C complex and its adaptor, Cdh1. Expression of Cdh1 stimulated degradation of the FoxM1 protein, and depletion of Cdh1 resulted in stabilization of the FoxM1 protein in late mitosis and in early G1 phase of the cell cycle. Cdh1 has been implicated in regulating S phase entry. We show that codepletion of FoxM1 inhibits early S phase entry observed in Cdh1-depleted cells. The N-terminal region of FoxM1 contains both destruction box (D box) and KEN box sequences that are required for targeting by Cdh1. Mutation of either the D box sequence or the KEN box sequence stabilized FoxM1 and blocked Cdh1-induced proteolysis. Cells expressing a nondegradable form of FoxM1 entered S phase rapidly following release from M phase arrest. Together, our observations show that FoxM1 is one of the targets of Cdh1 in late M or early G1 phase and that its proteolysis is important for regulated entry into S phase.