Minimization of cross-talk between Spindle Assembly Checkpoint silencing and error correction

Accurate chromosome segregation during cell division requires that sister kinetochores attach to microtubules originating from opposite spindle poles. This is ensured by the Spindle Assembly Checkpoint (SAC), which detects unattached kinetochores, and an error correction mechanism that detects and destabilizes incorrect attachment of both sister kinetochores to the same spindle pole. Both these processes are regulated by Protein Phosphatase 1 (PP1). PP1 silences the SAC and stabilizes correct, bipolar kinetochore-microtubule attachments. We show that this dual role of PP1 can be problematic. Premature recruitment of PP1 to the kinetochore for SAC silencing interferes with efficient error correction, because PP1 can then inadvertently stabilize incorrect attachments. The yeast kinetochore mitigates this cross-talk by: (a) using independent sources of PP1 for attachment stabilization and SAC silencing, and (b) delaying PP1 recruitment for SAC silencing until correct attachments are established. Thus, error correction precedes SAC silencing, and ensures accurate chromosome segregation.