The Spindle Position Checkpoint Requires Positional Feedback from Cytoplasmic Microtubules

The objective of mitosis is to provide a copy of the genome to each progeny of a cell division. This requires the separation of duplicate chromatids by the spindle apparatus and the delivery of one set of chromosomes to each of the daughter cells. In budding yeast, the fidelity of chromosome delivery depends on the spindle position checkpoint, which prolongs mitosis until one end of the anaphase spindle arrives in the bud 1, 2, 3. Here we tested the hypothesis that the activity of the spindle position checkpoint depends on persistent interactions between cytoplasmic microtubules and the mother-bud neck, the future site of cytokinesis. We used laser ablation to disrupt microtubule interactions with the bud neck, and we found that loss of microtubules from the neck leads to mitotic exit in a majority of checkpoint-activated cells. Our findings suggest that cytoplasmic microtubules are used to monitor the location of the spindle in the dividing cell and, in the event of positioning errors, relay a signal to inhibit mitotic exit until the spindle is appropriately positioned.