Mph1 kinetochore localization is crucial and upstream in the hierarchy of spindle assembly checkpoint protein recruitment to kinetochores.

The spindle assembly checkpoint (SAC) blocks entry into anaphase until all chromosomes have stably attached to the mitotic spindle through their kinetochores. The checkpoint signal originates from unattached kinetochores, where there is an enrichment of SAC proteins. Whether the enrichment of all SAC proteins is crucial for SAC signaling is unclear. Here, we provide evidence that, in fission yeast, recruitment of the kinase Mph1 is of vital importance for a stable SAC arrest. An Mph1 mutant that eliminates kinetochore enrichment abolishes SAC signaling, whereas forced recruitment of this mutant to kinetochores restores SAC signaling. In bub3Δ cells, the SAC is functional when only Mph1 and the Aurora kinase Ark1, but no other SAC proteins, are enriched at kinetochores. We analyzed the network of dependencies for SAC protein localization to kinetochores and identify a three-layered hierarchy with Ark1 and Mph1 on top, Bub1 and Bub3 in the middle, and Mad3 as well as the Mad1-Mad2 complex at the lower end of the hierarchy. If Mph1 is artificially recruited to kinetochores, Ark1 becomes dispensable for SAC activity. Our results highlight the crucial role of Mph1 at kinetochores and suggest that the Mad1-Mad2 complex does not necessarily need to be enriched at kinetochores for functional SAC signaling.