Protein kinase required for the spindle assembly checkpoint; prevents anaphase onset in the presence of spindle damage; Cdc28p-mediated phosphorylation (T566) is essential for anaphase degradation and checkpoint adaptation; associates with CEN DNA via Skp1p; relocalizes Sgo1p in response to sister kinetochore tension; BUB1 has a highly diverged paralog, MAD3, that arose from the whole genome duplication; N-terminal (Nt) TPR domain-containing region is homologous with the Nt region of Mad3p