Involved in cell cycle checkpoint enforcement. The formation of a MAD1-BUB1-BUB3 complex seems to be required for the spindle checkpoint mechanism. Catalyzes the phosphorylation of BUB3 and its autophosphorylation. Associates with centromere (CEN) DNA via interaction with SKP1. The association with SKP1 is required for the mitotic delay induced by kinetochore tension defects; but not for the arrest induced by spindle depolymerization or kinetochore assembly defects. {ECO:0000269|PubMed:12769845}.