The Bub1-Bub3 and BubR1-Bub3 checkpoint complexes, or the Bubs, contribute to the accurate segregation of chromosomes during mitosis by promoting chromosome bi-orientation and halting exit from mitosis if this fails. The complexes associate with kinetochores during mitosis, which is required for proper chromosome segregation. The outer kinetochore protein KNL1 (also known as CASC5, Blinkin and AF15Q14) is the receptor for Bub proteins, but the exact nature of the functional binding sites on KNL1 are yet to be determined. Here, we show that KNL1 contains multiple binding sites for the Bub proteins, with the Mps1-phosphorylated MELT repeats constituting individual functional docking sites for direct binding of Bub3. Surprisingly, chromosome congression and the spindle assembly checkpoint (SAC) are still functional when KNL1 is deleted of all but four of its twelve MELT repeats. Systematically reducing the number of MELT repeats to less than four reduced KNL1 functionality. Furthermore, we show that protein phosphatase 1 (PP1) binding to KNL1 during prometaphase reduces the levels of Bub proteins at kinetochores to approximately the level recruited by four active MELT repeats.
Keywords: Bub1; BubR1; KNL1; Kinetochore; SAC; Spindle assembly checkpoint.