The kinetochore, a large protein complex that connects chromosomes to the mitotic spindle, is divided into the inner kinetochore, which persists at centromeres throughout the cell cycle, and the outer kinetochore, which establishes attachments to spindle microtubules. The Polo-like kinase 1 (PLK-1) regulates chromosome segregation and is targeted to the inner kinetochore by the constitutive centromere associated network (CCAN) and to the outer kinetochore by the kinase BUB1 and the mitotic regulator CDC20. Here (Bel Borja et al., 2024), Federico Pelisch and colleagues dissect the mechanisms and function of PLK-1 recruitment to the inner kinetochore in C. elegans embryos. CENP-C is the only CCAN component in C. elegans, and the authors show that mutations in its Polo docking (PD) domain (CENP-C PDmut) reduce PLK-1 levels at kinetochores. Interestingly, CENP-C PDmut causes defective kinetochore–microtubule attachment and chromosome congression without significantly affecting CDC20 levels, which depend on outer kinetochore-bound PLK-1. In addition, outer kinetochore protein levels are increased in CENP-C PDmut embryos, suggesting that PLK-1 at the inner kinetochore plays a role in regulating outer kinetochore protein assembly. Finally, disruption of PLK-1 recruitment to both the inner and the outer kinetochore causes severe mitotic defects. Overall, this study uncovers two distinct PLK-1 populations at the kinetochore, each of which plays separate roles during mitosis, providing new insights into cell division mechanisms.
