The Role of Ndc80 and Ska Complexes in Maturation of Kinetochore-microtubule Attachments

In eukaryotes, segregation of genetic material is a complex and highly regulated process. From the condensation of the chromosomes, through the kinetochore assembly, formation of microtubule attachments, congression and segregation, until the completion of cytokinesis, each step has to be precisely coordinated to allow for faithful distribution of the DNA to daughter cells. Although our understanding of those processes significantly improved over recent years, one important mechanism is yet to be properly described: maturation of kinetochore-microtubule attachments. Several kinetochore components have been identified as essential for formation and maintenance of end-on microtubule attachments. However, it is still unclear how kinetochore-microtubule attachments mature to allow for mitosis completion. The Ndc80 complex was identified as an essential microtubule binder that allows for graded stability of the attachments by a phosphorylation-dependent regulation mechanism. Ten years ago a novel complex, Ska, was discovered as a component of the mitotic spindle and implicated in the attachment maintenance and chromosomal segregation. While some studies indicate that Ska serves as the coupler for depolymerization driven movements, other suggest it is crucial for spindle checkpoint silencing. In this dissertation I investigate the dependencies between Ndc80, Ska and PP1 phosphatase, their roles in the maturation and recognition of the microtubule attachments. I present data indicating that Ska is recruited to the kinetochores through the N-terminal tail of the Ndc80 protein and that the resulting complex forms characteristic structures on the surface of the microtubules. Moreover, the Ska complex binds to and recruits PP1, one of the major mitotic phosphatases required for dephosphorylation of mitotic substrates and, as a result, mitotic exit. Overall, the data presented here strongly suggest that Ndc80-Ska-PP1 network allows kinetochores to mature end-on attachments and couple the binding of microtubules to spindle checkpoint silencing.