E3-ubiquitin ligase, FBXW7 regulates mitotic progression by targeting BubR1 for ubiquitin-mediated degradation.

Faithful chromosome segregation requires correct attachment of kinetochores with the spindle microtubules. Erroneously-attached kinetochores recruit proteins to activate Spindle assembly checkpoint (SAC), which senses the errors and signals cells to delay anaphase progression for error correction. Temporal control of the levels of SAC activating-proteins is critical for checkpoint activation and silencing, but its mechanism is not fully understood. Here, we show that E3 ubiquitin ligase, SCF-FBXW7 targets BubR1 for ubiquitin-mediated degradation and thereby controls SAC in human cells. Depletion of FBXW7 results in prolonged metaphase arrest with increased stabilization of BubR1 at kinetochores. Similar kinetochore stabilization is also observed for BubR1-interacting protein, CENP-E. FBXW7 induced ubiquitination of both BubR1 and the BubR1-interacting kinetochore-targeting domain of CENP-E, but CENP-E domain degradation is dependent on BubR1. Interestingly, Cdk1 inhibition disrupts FBXW7-mediated BubR1 targeting and further, phospho-resistant mutation of Cdk1-targeted phosphorylation site, Thr 620 impairs BubR1-FBXW7 interaction and FBXW7-mediated BubR1 ubiquitination, supporting its role as a phosphodegron for FBXW7. The results demonstrate SCF-FBXW7 as a key regulator of spindle assembly checkpoint that controls stability of BubR1 and its associated CENP-E at kinetochores. They also support that upstream Cdk1 specific BubR1 phosphorylation signals the ligase to activate the process.