Temperature-sensitive ipl1-2/Aurora B mutation is suppressed by mutations in TOR complex 1 via the Glc7/PP1 phosphatase

Ipl1/Aurora B is the catalytic subunit of a complex that is required for chromosome segregation and nuclear division. Before anaphase, Ipl1 localizes to kinetochores, where it is required to establish proper kinetochore–microtubule associations and regulate the spindle assembly checkpoint. The protein phosphatase Glc7/PP1 opposes Ipl1 for some of these activities. To more thoroughly characterize the Glc7 phosphatase that opposes Ipl1, we have identified mutations that suppress the thermosensitivity of an ipl1-2 mutant. In addition to mutations in genes previously associated with ipl1 suppression, we recovered a null mutant in TCO89, which encodes a subunit of the TOR complex 1 (TORC1), the conserved rapamycin-sensitive kinase activity that regulates cell growth in response to nutritional status. The temperature sensitivity of ipl1-2 can also be suppressed by null mutation of TOR1 or by administration of pharmacological TORC1 inhibitors, indicating that reduced TORC1 activity is responsible for the suppression. Suppression of the ipl1-2 growth defect is accompanied by increased fidelity of chromosome segregation and increased phosphorylation of the Ipl1 substrates histone H3 and Dam1. Nuclear Glc7 levels are reduced in a tco89 mutant, suggesting that TORC1 activity is required for the nuclear accumulation of Glc7. In addition, several mutant GLC7 alleles that suppress the temperature sensitivity of ipl1-2 exhibit negative synthetic genetic interactions with TORC1 mutants. Together, our results suggest that TORC1 positively regulates the Glc7 activity that opposes Ipl1 and provide a mechanism to tie nutritional status with mitotic regulation.