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J. Cell Biol.,
Volume 145, Number 5, May 31, 1999 979-991
Dipartimento di Genetica e Biologia dei Microrganismi, 20133 Milano, Italy
The mitotic checkpoint blocks cell cycle progression before anaphase in case of mistakes in the
alignment of chromosomes on the mitotic spindle. In
budding yeast, the Mad1, 2, 3, and Bub1, 2, 3 proteins
mediate this arrest. Vertebrate homologues of Mad1, 2, 3, and Bub1, 3 bind to unattached kinetochores and
prevent progression through mitosis by inhibiting
Cdc20/APC-mediated proteolysis of anaphase inhibitors, like Pds1 and B-type cyclins. We investigated the
role of Bub2 in budding yeast mitotic checkpoint. The
following observations indicate that Bub2 and Mad1, 2 probably activate the checkpoint via different pathways: (a) unlike the other Mad and Bub proteins, Bub2
localizes at the spindle pole body (SPB) throughout the
cell cycle; (b) the effect of concomitant lack of Mad1 or
Mad2 and Bub2 is additive, since nocodazole-treated mad1 bub2 and mad2 bub2 double mutants rereplicate
DNA more rapidly and efficiently than either single
mutant; (c) cell cycle progression of bub2 cells in the
presence of nocodazole requires the Cdc26 APC subunit, which, conversely, is not required for mad2 cells in
the same conditions. Altogether, our data suggest that
activation of the mitotic checkpoint blocks progression
through mitosis by independent and partially redundant mechanisms.
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