Employing the one-cell C. elegans embryo to study cell division processes

Cyclin B3 is a dominant fast-acting cyclin that drives rapid early embryonic mitoses

In many species, early embryonic mitoses proceed at a very rapid pace, but how this pace is achieved is not understood. Here we show that in the early C. elegans embryo, cyclin B3 is the dominant driver of rapid embryonic mitoses. Metazoans typically have three cyclin B isoforms that associate with and activate Cdk1 kinase to orchestrate mitotic events: the related cyclins B1 and B2 and the more divergent cyclin B3. We show that whereas embryos expressing cyclins B1 and B2 support slow mitosis (NEBD to Anaphase ~ 600s), the presence of cyclin B3 dominantly drives the ~3-fold faster mitosis observed in wildtype embryos. CYB-1/2-driven mitosis is longer than CYB-3-driven mitosis primarily because the progression of mitotic events itself is slower, rather than delayed anaphase onset due to activation of the spindle checkpoint or inhibitory phosphorylation of the anaphase activator CDC-20. Addition of cyclin B1 to cyclin B3-only mitosis introduces an ~60s delay between the completion of chromosome alignment and anaphase onset, which likely ensures segregation fidelity; this delay is mediated by inhibitory phosphorylation on CDC-20. Thus, the dominance of cyclin B3 in driving mitotic events, coupled to introduction of a short cyclin B1-dependent delay in anaphase onset, sets the rapid pace and ensures fidelity of mitoses in the early C. elegans embryo.

A tripartite mechanism catalyzes Mad2-Cdc20 assembly at unattached kinetochores

During cell division, kinetochores couple chromosomes to spindle microtubules. To protect against chromosome gain or loss, kinetochores lacking microtubule attachment locally catalyze association of the checkpoint proteins Cdc20 and Mad2, which is the key event in the formation of a diffusible checkpoint complex that prevents mitotic exit. We elucidated the mechanism of kinetochore-catalyzed Mad2-Cdc20 assembly with a probe that specifically monitors this assembly reaction at kinetochores in living cells. We found that catalysis occurs through a tripartite mechanism that includes localized delivery of Mad2 and Cdc20 substrates and two phosphorylation-dependent interactions that geometrically constrain their positions and prime Cdc20 for interaction with Mad2. These results reveal how unattached kinetochores create a signal that ensures genome integrity during cell division.

Polo-like kinase 1 independently controls microtubule-nucleating capacity and size of the centrosome

Centrosomes are composed of a centriolar core surrounded by a pericentriolar material (PCM) matrix that docks microtubule-nucleating γ-tubulin complexes. During mitotic entry, the PCM matrix increases in size and nucleating capacity in a process called centrosome maturation. Polo-like kinase 1 (PLK1) is recruited to centrosomes and phosphorylates PCM matrix proteins to drive their self-assembly, which leads to PCM expansion. Here, we show that in addition to controlling PCM expansion, PLK1 independently controls the generation of binding sites for γ-tubulin complexes on the PCM matrix. Selectively preventing the generation of PLK1-dependent γ-tubulin docking sites led to spindle defects and impaired chromosome segregation without affecting PCM expansion, highlighting the importance of phospho-regulated centrosomal γ-tubulin docking sites in spindle assembly. Inhibiting both γ-tubulin docking and PCM expansion by mutating substrate target sites recapitulated the effects of loss of centrosomal PLK1 on the ability of centrosomes to catalyze spindle assembly.

A Non-canonical BRCT-Phosphopeptide Recognition Mechanism Underlies RhoA Activation in Cytokinesis

Cytokinesis partitions the cell contents to complete mitosis. During cytokinesis, polo-like kinase 1 (PLK1) activates the small GTPase RhoA to assemble a contractile actomyosin ring. PLK1 is proposed to pattern RhoA activation by creating a docking site on the central spindle that concentrates the RhoA guanine nucleotide exchange factor ECT2. However, ECT2 targeting to the central spindle is dispensable for cytokinesis, indicating that how PLK1 controls RhoA activation remains unresolved. To address this question, we employed an unbiased approach targeting ∼100 predicted PLK1 sites in two RhoA regulators: ECT2 and the centralspindlin complex, composed of CYK4 and kinesin-6. This comprehensive approach suggested that the only functionally critical PLK1 target sites are in a single cluster in the CYK4 N terminus. Phosphorylation of this cluster promoted direct interaction of CYK4 with the BRCT repeat module of ECT2. However, mutational analysis in vitro and in vivo led to the surprising finding that the interaction was independent of the conserved "canonical" residues in ECT2's BRCT repeat module that, based on structurally characterized BRCT-phosphopeptide interactions, were presumed critical for binding. Instead, we show that the ECT2 BRCT module binds phosphorylated CYK4 via a distinct conserved basic surface. Basic surface mutations mimic the effects on cytokinesis of loss of CYK4 cluster phosphorylation or inhibition of PLK1 activity. Together with evidence for ECT2 autoinhibition limiting interaction with CYK4 in the cytoplasm, these results suggest that a spatial gradient of phosphorylated CYK4 around the central spindle patterns RhoA activation by interacting with ECT2 on the adjacent plasma membrane.

The G2-to-M Transition Is Ensured by a Dual Mechanism that Protects Cyclin B from Degradation by Cdc20-Activated APC/C

In the eukaryotic cell cycle, a threshold level of cyclin B accumulation triggers the G2-to-M transition, and subsequent cyclin B destruction triggers mitotic exit. The anaphase-promoting complex/cyclosome (APC/C) is the E3 ubiquitin ligase that, together with its co-activator Cdc20, targets cyclin B for destruction during mitotic exit. Here, we show that two pathways act in concert to protect cyclin B from Cdc20-activated APC/C in G2, in order to enable cyclin B accumulation and the G2-to-M transition. The first pathway involves the Mad1-Mad2 spindle checkpoint complex, acting in a distinct manner from checkpoint signaling after mitotic entry but employing a common molecular mechanism-the promotion of Mad2-Cdc20 complex formation. The second pathway involves cyclin-dependent kinase phosphorylation of Cdc20, which is known to reduce Cdc20's affinity for the APC/C. Cooperation of these two mechanisms, which target distinct APC/C binding interfaces of Cdc20, enables cyclin B accumulation and the G2-to-M transition.