Protein Phosphatase 1 inactivates Mps1 to ensure efficient Spindle Assembly Checkpoint silencing

Faithfull genome partitioning during cell division relies on the Spindle Assembly Checkpoint (SAC), a conserved signaling pathway that delays anaphase onset until all chromosomes are attached to spindle microtubules. Mps1 kinase is an upstream SAC regulator that promotes the assembly of an anaphase inhibitor through a sequential multi-target phosphorylation cascade. Thus, the SAC is highly responsive to Mps1, whose activity peaks in early mitosis as a result of its T-loop autophosphorylation. However, the mechanism controlling Mps1 inactivation once kinetochores attach to microtubules and the SAC is satisfied remains unknown. Here we show in vitro and in Drosophila that Protein Phosphatase 1 (PP1) inactivates Mps1 by dephosphorylating its T-loop. PP1-mediated dephosphorylation of Mps1 occurs at kinetochores and in the cytosol, and inactivation of both pools of Mps1 during metaphase is essential to ensure prompt and efficient SAC silencing. Overall, our findings uncover a mechanism of SAC inactivation required for timely mitotic exit.We thank Geert Kops (UMC, Utrecht, The Netherlands), Thomas Maresca (University of Massachu-setts Amherst, USA) and Christian Lehner (University of Zurich, Switzerland) for antibodies, con-structs and fly stocks. We thank Carla Sofia Lopes (i3S, IBMC, University of Porto) and all the members of the Sunkel laboratory for critical and helpful discussions. This article is a result of the project Norte-01-0145-FEDER-000029 - Advancing Cancer Research: From basic knowledge to application, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (FEDER). MO is supported by a fellowship from the GABBA PhD program from the University of Porto, PD/BD/105746/2014. JB is supported by an FCT PhD grant SFRH/BD/87871/2012. CC is sup-ported by an FCT investigator position and funding (IF/01755/2014). HM is funded by PRECISE and CODECHECK grants from the European Research Council, FLAD Life Science 2020, and the Louis-Jeantet Young Investigator Career Award