Figure 4. Events that precede inappropriate mitotic exit in ts-arp1 bim1D cells. (A) Still images from Video 4. A representative field of ts- arp1 bim1D cells. The cells of interest are outlined in the first frame and again after they exit mitosis or 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. Inappropriate mitotic exit occurs after abortive spindle movements into the neck (*; cell in inset taken from a different field in the same movie) or when cytoplasmic microtubules are lost from the neck (#). (B) Still images from Videos 5 and 6. In some ts-arp1 bim1D cells, spindle movement into the neck is severely delayed and so is exit from mitosis, as indicated by maintenance of Hof1–GFP on the septin ring (arrowhead). Once the spindle moves into the neck, mitotic exit and spindle disassembly proceed (arrow). (C) Still images from Videos 7 and 8. In ts-arp1 bim1D cells, the spindle delays movement into the neck and Hof1– GFP remains at the septin rings (arrowheads). Inappropriate mitotic exit occurs in the absence of SPB–neck interactions. Instead, cy- tokinesis and spindle disassembly (arrow) occur after the loss of cytoplasmic microtubules from the neck (indented arrowhead). In the bottom cell of the lower series, cytoplasmic microtubules fail to penetrate or touch the neck at any time during anaphase. (D) Still im- ages from Videos 9–11. Examples of ts-arp1 bim1D cells, in which inappropriate mitotic exit occurred after the spindle moved toward and touched the neck without completely penetrating the neck. Spindle disassembly and cytokinesis occurred while the SPB was at the neck and often resulted in the shearing of a microtubule within the bud. The Hof1–GFP disappears from the neck after spindle disassembly. (E) Still images from Video 12. In ts-arp1 bub2D cells, inappropriate mitotic exit occurred without SPB–neck interactions or loss of cytoplasmic microtubules from the neck. Times are indicated in the upper right. Supplemental videos are available at http:// www.jcb.org/cgi/content/full/153/1/159/DC1. Bars: (A) 5 mm; (B–E) 2 mm. mutants undergo inappropriate exit from mitosis on occa- sion (albeit at very low frequency), suggesting that the checkpoint has a low but discernible error rate. The frac- tion of cells that show delayed spindle movement into the neck is higher in ts-arp1 bim1D and ts-arp1 stt4-7 cells than in ts-arp1 cells (Fig. 3 B), so that on average there is a longer time between anaphase onset and spindle move- ment into the neck (Fig. 3 A). Thus, it could be that the The Journal of Cell Biology, Volume 153, 2001 164