Cryo-EM structure of the complete inner kinetochore of the budding yeast point centromere

SummaryThe point centromere of budding yeast specifies assembly of the large multi-subunit kinetochore complex. By direct attachment to the mitotic spindle, kinetochores couple the forces of microtubule dynamics to power chromatid segregation at mitosis. Kinetochores share a conserved architecture comprising the centromere-associated inner kinetochore CCAN (constitutive centromere-associated network) complex and the microtubule-binding outer kinetochore KMN network. The budding yeast inner kinetochore additionally includes the centromere-binding CBF1 and CBF3 complexes. Here, we reconstituted the complete yeast inner kinetochore complex assembled onto the centromere-specific CENP-A nucleosome (CENP-ANuc) and determined its structure using cryo-EM. This revealed a central CENP-ANuc, wrapped by only one turn of DNA, and harboring extensively unwrapped DNA ends. These free DNA duplexes function as binding sites for two CCAN protomers, one of which entraps DNA topologically and is positioned precisely on the centromere by the sequence-specific DNA-binding complex CBF1. The CCAN protomers are connected through CBF3 to form an arch-like configuration, binding 150 bp of DNA. We also define a structural model for a CENP-ANuc-pathway to the outer kinetochore involving only CENP-QU. This study presents a framework for understanding the basis of complete inner kinetochore assembly onto a point centromere, and how it organizes the outer kinetochore for robust chromosome attachment to the mitotic spindle.