Highlights d The spindle checkpoint protein Mad2 has a conserved role in the G2-to-M transition d Mad2's role in G2-to-M requires Mad1 but is independent of kinetochores d Mad2 enables cyclin B accumulation by restraining its degradation by APC/C-Cdc20 d Mad2 and Cdk phosphorylation act in parallel to inhibit APC/ C-Cdc20 in G2

Protein Phosphatase 2A (PP2A) is a heterotrimer composed of scaffolding (A), catalytic (C), and regulatory (B) subunits. PP2A complexes with B56 subunits are targeted by Shugoshin and BUBR1 to protect centromeric cohesion and stabilise kinetochore–microtubule attachments in yeast and mouse meiosis. In Caenorhabditis elegans, the closest BUBR1 orthologue lacks the B56-interaction domain and Shugoshin is not required for meiotic segregation. Therefore, the role of PP2A in C. elegans female meiosis is unknown. We report that PP2A is essential for meiotic spindle assembly and chromosome dynamics during C. elegans female meiosis. BUB-1 is the main chromosome-targeting factor for B56 subunits during prometaphase I. BUB-1 recruits PP2A:B56 to the chromosomes via a newly identified LxxIxE motif in a phosphorylation-dependent manner, and this recruitment is important for proper chromosome congression. Our results highlight a novel mechanism for B56 recruitment, essential for recruiting a pool of PP2A involved in chromosome congression during meiosis I.

During cell division, a balance between protein kinases and phosphatases is required to achieve specific and highly regulated phosphorylation levels. Protein Phosphatase 2A (PP2A) is a heterotrimeric enzyme composed of Scaffold (A), Catalytic (C), and Regulatory (B) subunits. PP2A can be targeted to different locations by specific interactions with one of several possible B subunits. B56-type subunits play important roles during meiosis in yeast and mice, including the protection of centromeric cohesion (targeted by Shugoshin) and kinetochore-microtubule attachments (targeted by BubR1). Protection of sister chromatid cohesion during meiosis I in Caenorhabditis elegans (C. elegans) does not involve the classic Shugoshin-dependent pathway and the C. elegans BubR1 orthologue Mad3SAN-1 lacks the PP2A-recruiting domain. We exploited these features to address the role(s) and regulation of PP2A:B56 during C. elegans oocyte meiosis. We report here that PP2A:B56 is recruited to chromosomes and spindle and is essential for proper chromosome segregation during oocyte meiosis in C. elegans. Recruitment of PP2A:B56 is regulated temporally and spatially by the kinase BUB-1 and is dependent on a previously unrecognised LxxIxE short linear motif (SLiM) in BUB-1. Our results highlight a novel, BUB-1-dependent mechanism for PP2A:B56 recruitment, essential for proper chromosome segregation during meiosis I.

A landmark event in the transition from interphase to mitosis in metazoans is nuclear envelope breakdown (NEBD). Important mitotic events occur prior to NEBD, including condensation of replicated chromosomes and assembly of kinetochores to rapidly engage spindle microtubules. Here, we show that nuclear-enriched protein phosphatase 4 (PP4) ensures robust assembly of the microtubule-coupling outer kinetochore prior to NEBD. In the absence of PP4, chromosomes exhibit extended monopolar orientation after NEBD and subsequently mis-segregate. A secondary consequence of diminished outer kinetochore assembly is defective sister chromatid resolution. After NEBD, a cytoplasmic activity compensates for PP4 loss, leading to outer kinetochore assembly and recovery of chromosomes from monopolar orientation to significant bi-orientation. The Ndc80-Ska microtubule-binding module of the outer kinetochore is required for this recovery. PP4 associates with the inner kinetochore protein CENP-C; however, disrupting the PP4–CENP-C interaction does not perturb chromosome segregation. These results establish that PP4-dependent outer kinetochore assembly prior to NEBD is critical for timely and proper engagement of chromosomes with spindle microtubules.

A landmark event in the transition from interphase to mitosis in metazoans is nuclear envelope breakdown (NEBD). Many events important for mitosis occur prior to NEBD, including condensation of replicated chromosomes and assembly of kinetochores to rapidly engage spindle microtubules. Here we show that nuclear-enriched protein phosphatase 4 (PP4) ensures robust assembly of the microtubule-coupling outer kinetochore prior to NEBD. In the absence of PP4, chromosomes exhibit extended monopolar orientation after NEBD and subsequently mis-segregate. A secondary consequence of diminished outer kinetochore assembly is defective sister chromatid resolution. After NEBD, a cytoplasmic activity compensates for PP4 loss, leading to outer kinetochore assembly and recovery of chromosomes from monopolar orientation to significant biorientation. The Ndc80-Ska microtubule-binding module of the outer kinetochore is required for this recovery. PP4 associates with the inner kinetochore protein CENP-C; however, disrupting the PP4-CENP-C interaction does not perturb chromosome segregation. These results establish that PP4-dependent outer kinetochore assembly prior to NEBD is critical for timely and proper engagement of chromosomes with spindle microtubules.

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