| Summary: | <p>The spindle assembly checkpoint (SAC), a conserved surveillance system, ensures genomic stability by delaying anaphase entry until all sister chromatids are attached to the mitotic spindle. The SAC network is active at unattached kinetochores and a complex signalling cascade culminates in the production of a diffusible ‘wait anaphase’ signal. Regulation of protein:protein interactions by reversible phosphorylation is critical in this signalling pathway. The checkpoint kinase Mps1, a central regulator of the SAC, plays a positive role in enabling protein interactions. Mps1 builds the SAC network by phosphorylating multiple kinetochore targets and in doing so it coordinates mitotic exit with microtubule binding. Given the critical role Mps1 plays in SAC activation, precise regulation of the activity and localisation of Mps1 must be key in controlling the activation status of the SAC. </p> <p>Mps1 binds to unattached kinetochores and is activated by autophosphorylation of its T-loop. However, once chromosomes are attached to the spindle and the SAC is satisfied Mps1 is switched off. The mechanisms controlling the inactivation of Mps1 were unknown. Here, it is shown that phosphorylation of T676, located on the activation loop of Mps1, is important in controlling Mps1 activity and SAC signalling. A PP2A-B56 phosphatase complex opposes Mps1 activation by directly dephosphorylating its activation loop both in vitro and in vivo. The interaction of PP2A-B56 with BubR1 is essential for this dephosphorylation event. Moreover, preliminary results from experiments combining Mps1 immunoprecipitations with mass spectrometry indicate that phosphate groups are globally lost from Mps1 when the SAC is inactive. The binding of interaction partners is a common mechanism used by kinases to regulate their function. Knl1 is present in Mps1 complexes by immunoprecipitation and mass spectrometry. While these results are still preliminary it is clear from siRNA depletion that Knl1 plays a previously unidentified role in regulating the localisation of Mps1. Together, these results signify an important advance in our knowledge of how the function of Mps1 is controlled.</p>
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