Transcriptional intermediary factor 1γ binds to the anaphase-promoting complex/cyclosome and promotes mitosis

The anaphase-promoting complex/cyclosome (APC/C) is an ubiquitin ligase that functions during mitosis. Here we identify the transcriptional regulator, transcriptional intermediary factor 1 gamma, TIF1 gamma, as an APC/C-interacting protein that regulates APC/C function. TIF1 gamma is not a substrate for APC/C-dependent ubiquitylation but instead, associates specifically with the APC/C holoenzyme and Cdc20 to affect APC/C activity and progression through mitosis. RNA interference studies indicate that TIF1 gamma knockdown results in a specific reduction in APC/C ubiquitin ligase activity, the stabilization of APC/C substrates, and an increase in the time taken for cells to progress through mitosis from nuclear envelope breakdown to anaphase. TIF1 gamma knockdown cells are also characterized by the inappropriate presence of cyclin A at metaphase, and an increase in the number of cells that fail to undergo metaphase-to-anaphase transition. Expression of a small interfering RNA-resistant TIF1 gamma species relieves the mitotic phenotype imposed by TIF1 gamma knockdown and allows for mitotic progression. Binding studies indicate that TIF1 gamma is also a component of the APC/C-mitotic checkpoint complex (MCC), but is not required for MCC dissociation from the APC/C once the spindle assembly checkpoint (SAC) is satisfied. TIF1 gamma inactivation also results in chromosome misalignment at metaphase and SAC activation; inactivation of the SAC relieves the mitotic block imposed by TIF1 gamma knockdown. Together these data define novel functions for TIF1 gamma during mitosis and suggest that a reduction in APC/C ubiquitin ligase activity promotes SAC activation.