Evolution and regulation of mitotic checkpoint protein recruitment by the kinetochore scaffold KNL1

Abstract

Faithful chromosome segregation in mitosis depends on a tight coordination between the formation of proper kinetochore-microtubule attachments and mitotic checkpoint (MC) activation and silencing. The outer kinetochore scaffold KNL1 plays an essential integrative role in these processes since it forms an assembly platform for the main microtubule-binding site (the KMN network) and recruits proteins involved in MC signaling. Key to its role in MC activation and the formation of stable kinetochore-microtubule attachments is the recruitment of BUB1, BUBR1 and BUB3 (collectively called the BUB proteins). Our phylogenomics analyses show that KNL1 is a rapidly evolving protein that contains an extensive array of repetitive modules. The 19 repetitive modules in human KNL1 include of TΩ-MELT-SHT motifs and function as independent BUB protein recruitment sites. Whereas only a few active modules are needed for MC activation, increasing the number of functional BUB recruitment sites up to 6 gradually increases chromosome segregation fidelity to wild-type levels. When studying these modules in molecular detail we find that only few BUB recruitment modules are active while the majority have degenerated. We show that the MC kinase MPS1 phosphorylates several residues within MELT- and SHT- motifs, which cooperate in BUB1/BUB3 binding. Phosphorylation sites within SHT motifs are unique for vertebrate KNL1 orthologs and we identify a basic surface on human BUB3 that is responsible for interacting with these phosphor-residues. Finally, we address the role of BUB1-BUB3 in MC signaling. We show that this complex functions as a platform for recruiting the MC proteins BUBR1 and MAD1 and their target CDC20. Whereas BUBR1 kinetochore recruitment is dispensable for the MC, the combined recruitment of MAD1 and CDC20 facilitates the formation of an inhibitory complex (the mitotic checkpoint complex) that prevents cell-cycle progression until all chromosomes have formed stable attachments with microtubules of the mitotic spindle.