Ipl1p is the budding yeast member of the Aurora family of protein kinases, critical regulators of genomic stability that are required for chromosome segregation, the spindle checkpoint, and cytokinesis. Using time-lapse microscopy, we found that Ipl1p also has a function in mitotic spindle disassembly that is separable from its previously identified roles. Ipl1–GFP localizes to kinetochores from G1 to metaphase, transfers to the spindle after metaphase, and accumulates at the spindle midzone late in anaphase. Ipl1p kinase activity increases at anaphase, and ipl1 mutants can stabilize fragile spindles. As the spindle disassembles, Ipl1p follows the plus ends of the depolymerizing spindle microtubules. Many Ipl1p substrates colocalize with Ipl1p to the spindle midzone, identifying additional proteins that may regulate spindle disassembly. We propose that Ipl1p regulates both the kinetochore and interpolar microtubule plus ends to regulate its various mitotic functions.

In order to identify regulators of the Schizosaccharomyces pombe septation initiation network (SIN), which signals the onset of cell division, we have isolated extragenic suppressors of mutations in the GTPase spg1p, which is a central element in this pathway. One of these encodes the protein phosphatase 2A (PP2A) BP P-regulatory subunit par1p. Loss of par1p function rescues mutants in cdc11, cdc7, and spg1, but no other SIN mutants. Our data suggest that PP2A-par1p acts as a negative regulator of SIN signalling. ß

Accurate cell division depends on the proper coordination of a number of events. Once all of the chromosomes align on the metaphase plate, anaphase is initiated so that chromosomes are equally segregated to daughter cells. The metaphase to anaphase transition is controlled by the anaphase promoting complex (APC), a ubiquitin ligase that targets the anaphase inhibitor Pds1 for destruction. This process is controlled by the Cdc20 activator protein. We have found that the budding yeast protein kinase Ipl1/Aurora inhibits the metaphase to anaphase transition independently of its role in the spindle checkpoint. Cells arrested in metaphase due to a defect in Cdc20 activity will progress into anaphase due to low Pds1 levels when Ipl1 function is impaired. Consistent with this, the overexpression of the opposing phosphatase drives metaphase‐arrested cells into anaphase. We are currently determining whether Pds1, Cdc20 or the APC are direct targets of Ipl1 and Glc7. This work was supported by grants from the NIH, Leukemia and Lymphoma Society and the Department of Defense.