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A more recent version of this article appeared on January 1, 2005
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Submitted on July 6, 2004
Revised on October 19, 2004
Accepted on October 21, 2004
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Verna and Marrs McClean Department of Biochemistry and Molecular Biology and *Department of Molecular and Cellular Biology, Baylor College of Medicine, Houston, TX 77030; Wellcome Trust Centre for Cell Biology, University of Edinburgh, Edinburgh, EH9 3JR United Kingdom
Monitoring Editor: Mark Solomon
During mitosis, the spindle assembly checkpoint (SAC) responds to faulty attachments between kinetochores and the mitotic spindle by imposing a metaphase arrest until the defect is corrected, thereby preventing chromosome mis-segregation. A genetic screen to isolate SAC mutants in fission yeast yielded point mutations in three fission yeast SAC genes: mad1, bub3 and bub1. The bub1-A78V mutant is of particular interest because it produces a wild-type amount of protein that is mutated in the conserved but uncharacterized Mad3-like region of Bub1p. Characterization of mutant cells demonstrates that the alanine at position 78 in the Mad3-like domain of Bub1p is required for: 1) cell cycle arrest induced by SAC activation; 2) kinetochore accumulation of Bub1p in checkpoint-activated cells; 3) recruitment of Bub3p and Mad3p, but not Mad1p, to kinetochores in checkpoint-activated cells; and 4) nuclear accumulation of Bub1p, Bub3p, and Mad3p, but not Mad1p, in cycling cells. Increased targeting of Bub1p-A78V to the nucleus by an exogenous NLS does not significantly increase kinetochore localization or SAC function. These data indicate that Bub1p-A78V is defective in both nuclear accumulation and kinetochore targeting and that a threshold level of nuclear Bub1p is necessary for the nuclear accumulation of Bub3p and Mad3p.
Present address: Department of Human and Molecular Genetics, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030.
||Corresponding author.
E-mail: ssazer{at}bcm.tmc.edu
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