The experiments described in this thesis have investigated the mitotic function of a protein complex called the Chromosomal Passenger Complex (CPC). The CPC is required for the detection and correction of defective (non bi-oriented) microtubule-kinetochore attachments, spindle checkpoint function and cytokinesis. First, a specific role for the CPC-subunit Survivin within the complex was found. Survivin, through binding to the NH2-terminus of INCENP, directs the CPC to the centromeres and central spindle during mitosis. As such, Survivin plays an important role in localising the enzymatic core of the CPC, the Aurora-B kinase, at the right place at the right time during mitosis to allow it to function properly. Additionally, it was found that the CPC plays two separatable roles in response to defective attachments. First, it destabilises such attachments and creates unattachments and second, it is required to strengthen the checkpoint signal to sustain an efficient arrest. Importantly, a specific domain in INCENP is required for the second function of the CPC. Finally, experimental evidence is provided that restriction of the CPC from the kinetochore upon bi-orientation of chromosomes is required for checkpoint silencing and mitotic progression. By fusing INCENP to the centromeric protein CENP-B, the CPC could not be separated from the kinetechore upon bi-orientation. Additionally, the CPC is normally partially displaced from bi-oriented chromosomes during metaphase. This displacement is impaired upon expression of the fusion. Thus, partial displacement and spatial restriction cooperatively act to restrict CPC activity upon bi-orientation to allow spindle checkpoint silencing during metaphase and mitotic progression
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