The spindle checkpoint of Saccharomyces cerevisiae responds to separable microtubule-dependent events

The spindle checkpoint regulates microtubule-based chromosome segregation a nd helps to maintain genomic stability [1,2]. Mutational inactivation of sp indle checkpoint genes has been implicated in the progression of several ty pes of human cancer. Recent evidence from budding yeast suggests that the s pindle checkpoint is complex, Order-of-function experiments have defined tw o separable pathways within the checkpoint, One pathway, defined by MAD2, c ontrols the metaphase-to-anaphase transition and the other, defined by BUB2 controls the exit from mitosis [3-6], The relationships between the separa te branches of the checkpoint, and especially the events that trigger the p athways, have not been defined, We localized a Bub2p-GFP fusion protein to the cytoplasmic side of the spindle pole body and used a kar9 mutant to sho w that cells with misoriented spindles are arrested in anaphase of mitosis, We used a kar9 bub2 double mutant to show that the arrest is BUB2 dependen t. We conclude that the separate pathways of the spindle checkpoint respond to different classes of microtubules. The MAD2 branch of the pathway respo nds to kinetochore microtubule interactions and the BUB2 branch of the path way operates within the cytoplasm, responding to spindle misorientation.