Nuclear-enriched protein phosphatase 4 ensures outer kinetochore assembly prior to nuclear dissolution

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. Rocha et al. show that C. elegans protein phosphatase 4 ensures robust outer kinetochore assembly prior to nuclear envelope breakdown, which is critical for timely and proper engagement of mitotic chromosomes with spindle microtubules.

Automated summary

This study demonstrates that PP4 protein phosphatase is a crucial factor for robust assembly of the microtubule-coupling outer kinetochore prior to nuclear envelope breakdown in mitotic cells. Absence of PP4 leads to extended monopolar orientation of chromosomes after nuclear envelope breakdown and subsequent mis-segregation. Defective sister chromatid resolution is also observed due to diminished outer kinetochore assembly.