Analysis of Mph1 kinase and its substrates in spindle checkpoint signalling
Accurate chromosome segregation is crucial as mis-segregation results in aneuploidy, which can lead to severe diseases such as cancer. The spindle checkpoint monitors sister-chromatid attachment and inhibits the onset of anaphase until all chromosomes are correctly bi-oriented on the mitotic spindle. The spindle checkpoint machinery of S.pombe is composed of many proteins, one of which is the kinase Mph1 (Mps1p-like pombe homolog). It previously has been shown that Mph1 is essential for the spindle checkpoint but not whether this is due to its kinase activity. In this study we determined the role of Mph1 kinase activity in the spindle checkpoint. To do so a kinase-dead version of Mph1, which had no detectable kinase activity, was analysed. Using this kinase-dead allele we showed that lack of Mph1 kinase activity abolished the spindle checkpoint and led to chromosome missegregation. As a result of these two defects cell viability of cells lacking Mph1 kinase activity was severely impaired. These results led to the question of how Mph1 kinase activity regulates the spindle checkpoint. Spindle checkpoint signalling is thought to mainly take place at two sites, at the kinetochore and at the anaphase promoting complex (APC). The APC is an E3 ubiquitin ligase that drives cells into anaphase by targeting the separase inhibitor securin and cyclin B for degradation by the 26 S proteasome. Upon activation of the spindle checkpoint the APC is inhibited by the mitotic checkpoint complex (MCC) composed of Slp1, Mad2 and Mad3. In this study we wanted to test whether the regulatory role of Mph1 kinase in the spindle checkpoint is via MCC binding to the APC. Using the kinase-dead version of Mph1 we showed that Mad2 and Mad3 binding to the APC is severely impaired in the absence of Mph1 kinase activity. This result led to the hypothesis that Mph1 might regulate Mad2 and Mad3 binding Using kinase assays Mad2 and Mad3 were identified as in vitro substrates of Mph1 and phosphorylation sites in Mad2 and Mad3 were determined by mass spectrometry. Phosphorylation mutants of Mad2 and Mad3 showed spindle checkpoint defects, indicating that they are important Mph1 substrates.