Single kinetochores execute an ordered series of molecular events as the Spindle Assembly Checkpoint is silenced

Abstract

The Spindle Assembly Checkpoint (SAC) delays anaphase onset until all kinetochores are stably attached to microtubules, thus promoting error-free chromosome segregation. Multiple molecular events are implicated in SAC silencing, including removal of phospho-marks, protein (un)binding, and structural reorganisation of the kinetochore - but we currently lack a quantitative map of how these events unfold through time. Here, we use the levels of the checkpoint protein MAD2 to create a pseudo-timeline of SAC silencing at single kinetochores. We demonstrate how silencing proceeds through an ordered series of molecular events where MAD2-Spindly unbinds first and then the KNL1 catalytic platform disassembles, with a pool of active MPS1 retained. Coincidently, the NDC80 ensemble reconfigures in response to high microtubule occupancy. Kinetochores next switch into a mature attachment state that then undergoes gradual further stabilisation through NDC80 tail dephosphorylation. By preventing biorientation, we also define otherwise hidden kinetochore states involved in error correction cycles. This includes a "poised" state which we propose allows for error correction and rapid reactivation of the SAC. These results provide a critical temporal framework for understanding the mechanisms of SAC silencing and error correction at single human kinetochores.