Figure 2. The spindle posi- tion checkpoint is completely abolished in bub2D and bfa1D mutants, but not in mad2D mutants. (A) Cytokinesis was delayed in ts-arp1 and ts-arp1 mad2D cells, but not in the ts- arp1 bfa1D and ts-arp1 bub2D mutants. Shown are the mean time intervals from the start of anaphase to spindle move- ment into the neck (white col- umns), from spindle move- ment into the neck to mitotic exit (gray columns), and the total time for anaphase (black columns). In the bfa1D and bub2D mutants, mitotic exit occurred in about half the time (P , 0.001), after spin- dle movement into the neck, compared with the wild-type strain. Number of events for each column from left to right on the chart: wild type, 108, 83, 83; ts-arp1, 120, 116, 114; ts- arp1 mad2D, 65, 71, 64; ts-arp1 bfa1D, 94, 91, 114; ts-arp1 bub2D, 68, 66, 92. Error bars represent SEM. (B) In the ts-arp1 and ts-arp1 mad2D strains, only a small fraction of cells that delayed spindle movement into the neck ex- ited mitosis inappropriately. In the ts-arp1 bfa1D and ts- arp1 bub2D strains, all cells that delayed spindle move- ment into the neck underwent inappropriate mitotic exit. Comparison of the percentage of cells that underwent inap- propriate mitotic exit (white columns) to the percentage of cells that delayed spindle movement into the neck by .20 min (gray columns) is shown. The frequency of in- appropriate mitotic exit in cells that delayed spindle movement is also shown (black columns). Cell numbers: wild type, 217; ts-arp1, 287; ts-arp1 mad2D, 95; ts-arp1 bfa1D, 117; ts-arp1 bub2D, 93. Error bars represent standard error of the proportion (SEP). (C) Still images from Video 2. Mitosis in rep- resentative cells of a ts-arp1 mutant. The cells of interest are outlined in the first frame and again after they divide, or they are outlined in the last frame. Some cells (x) move the spindle into the neck within the normal time for mitotic exit. Other cells (y) delay spindle movement into the neck beyond the normal time for mitotic exit, but then correct the spindle position and divide. A third type of cell (z) does not move the spindle into the neck or exit mitosis during the course of the movie. Time (in min) is indicated. (D) Still images from Video 3. Mitosis in representative cells of a ts-arp1 bub2D mutant. Most cells (x) move the spindle into the neck within the normal time for mitotic exit. Those cells that do not move the spindle into the neck in time (*), exit mitosis at the normal time producing a binucleate mother and an anucleate daughter. Time (in min) is indicated. Supplemental videos are available at http://www.jcb.org/cgi/content/full/ 153/1/159/DC1. Bars, 5 mm. move into the neck within 25 min of the initial interaction of the SPB with the neck, inappropriate exit from mitosis oc- curred (57/57 incidents in all aforementioned strains). Therefore, mitotic exit appears to be activated within z25 min of SPB interaction with the neck, regardless of whether the spindle subsequently elongates successfully into the bud. Inappropriate Mitotic Exit in bim1D Mutants To test any model for how the spindle position checkpoint monitors the position of the spindle, we needed to identify mutants that would affect the surveillance mechanism of the checkpoint while leaving intact the signaling pathway leading to Bub2p–Bfa1p down regulation of Tem1p. Such mutants The Journal of Cell Biology, Volume 153, 2001 162