Budding yeast Bub2 is localized at spindle pole bodies and activates the mitotic checkpoint via a different pathway from Mad2

The mitotic checkpoint blocks cell cycle progression before anaphase in cas e of mistakes in the alignment of chromosomes on the mitotic spindle. In bu dding yeast, the Mad1, 2, 3, and Bub1, 2, 3 proteins mediate this arrest. V ertebrate homologues of Mad1, 2, 3, and Bub1, 3 bind to unattached kinetoch ores and prevent progression through mitosis by inhibiting Cdc20/APC-mediat ed proteolysis of anaphase inhibitors, like Pds1 and B-type cyclins. We inv estigated the role of Bub2 in budding yeast mitotic checkpoint. The followi ng observations indicate that Bub2 and Mad1, 2 probably activate the checkp oint via different pathways: (a) unlike the other Mad and Bub proteins, Bub 2 localizes at the spindle pole body (SPB) throughout the cell cycle; (b) t he effect of concomitant lack of Mad1 or Mad2 and Bub2 is additive, since n ocodazole-treated mad1 bub2 and mad2 bub2 double mutants rereplicate DNA mo re rapidly and efficiently than either single mutant; (c) cell cycle progre ssion of bub2 cells in the presence of nocodazole requires the Cdc26 APC su bunit, which, conversely, is not required for mad2 cells in the same condit ions. Altogether, our data suggest that activation of the mitotic checkpoin t blocks progression through mitosis by independent and partially redundant mechanisms.