The copy-number and varied strengths of MELT motifs in Spc105 balance the strength and responsiveness the Spindle Assembly Checkpoint

The Spindle Assembly Checkpoint (SAC) maintains genome stability while enabling timely anaphase onset. To maintain genome stability, the SAC must be strong so that it delays cell division even if one chromosome is unattached, but for timely anaphase onset, it must be responsive to silencing mechanisms. How it meets these potentially antagonistic requirements is unclear. Here we show that the balance between SAC strength and responsiveness is determined by the number of ‘MELT’ motifs in the kinetochore protein Spc105/KNL1 and their Bub3-Bub1 binding affinities. Spc105/KNL1 must contain many strong MELT motifs to prevent chromosome missegregation, but not too many, because this delays SAC silencing and anaphase onset. We demonstrate this by constructing a Spc105 variant that trades SAC responsiveness for significantly improved chromosome segregation accuracy. We propose that the necessity of balancing SAC strength with responsiveness drives the evolutionary trend of MELT motif number amplification and degeneration of their functionally optimal amino acid sequence.