Probing MPS1 function in mitosis

The importance of MPS1 in mitosis has been established for several years. Even though recent years have seen quite some development in the elucidation of the molecular pathways concerning MPS1 signaling, many aspects remained to be uncovered at the onset of the research presented in this thesis. In Chapter 2 we report the development of mutant cell lines that allow specific, highly penetrant and reversible inhibition of MPS1, providing us with the opportunity to study the role of MPS1 in mitosis. One of these functions - the recruitment of MAD1 to unattached kinetochores - is studied in more detail in Chapter 3. We provide evidence that feedback control between MPS1, BUB1 and ZW10 regulates MAD1 kinetochore recruitment and subsequent SAC activity. In this chapter, we furthermore show that the predominant function for RZZ and BUB1 in SAC signaling is ensuring MAD1 localization. Chapter 4 describes the identification of PLK1 as an auxiliary factor in the establishment and maintenance of the SAC. Under conditions of maximal SAC activity, PLK1 is dispensable, but when SAC signaling is suboptimal, PLK1 becomes essential to maintain SAC signaling. Our data suggest that under these conditions PLK1 activity promotes MCC stability. The work presented in Chapter 5 describes the efforts put into identification of direct MPS1 targets and provides useful lessons for future investigation in the search for novel MPS1 targets.