Practice question

Spindle assembly checkpoint activation hinges on kinase Mps1, also known as TTK, a dual specificity protein kinase recruited to unattached kinetochores. In prometaphase, Mps1 binds Ndc80 complex through competition with microtubules, positioning it near scaffold KNL1. Mps1 phosphorylates multiple MELT repeats on KNL1 at conserved methionine-glutamate-leucine-threonine motifs, converting them into phospho-docking sites for Bub3-Bub1 and Bub3-BubR1 complexes. Additionally, Mps1 phosphorylates Mad1 at C-terminus facilitating catalytic conversion of open O-Mad2 to closed C-Mad2, the conformer capable of entrapping Cdc20. Phosphorylation of Cdc20 itself and other checkpoint components augments MCC assembly, effectively inhibiting APC/C. As microtubules attach, they displace Mps1 through steric exclusion and activate PP1 phosphatase via KNL1 RVSF motifs, terminating MELT phosphorylation and checkpoint signaling. Experimental Mps1 inhibition by reversine or NMS-P715 overrides checkpoint, causes massive chromosome missegregation and cell death, highlighting its upstream regulatory role and therapeutic relevance in targeting chromosomally unstable tumors and chemosensitization strategies. This regulatory circuit illustrates integration of checkpoint kinases, ubiquitin ligases, phosphatases and structural proteins coordinating accurate cell division and preventing aneuploidy associated with tumorigenesis.