TRIP13 Regulates Both the Activation and Inactivation of the Spindle-Assembly Checkpoint

Summary: Biochemical studies have indicated that p31comet and TRIP13 are critical for inactivating MAD2. To address unequivocally whether p31comet and TRIP13 are required for mitotic exit at the cellular level, their genes were ablated either individually or together in human cells. Neither p31comet nor TRIP13 were absolutely required for unperturbed mitosis. MAD2 inactivation was only partially impaired in p31comet-deficient cells. In contrast, TRIP13-deficient cells contained MAD2 exclusively in the C-MAD2 conformation. Our results indicate that although p31comet enhanced TRIP13-mediated MAD2 conversion, it was not absolutely necessary for the process. Paradoxically, TRIP13-deficient cells were unable to activate the spindle-assembly checkpoint, revealing that cells lacking the ability to inactivate MAD2 were incapable in mounting a checkpoint response. These results establish a paradigm of the roles of p31comet and TRIP13 in both checkpoint activation and inactivation. : Mitotic exit requires the inactivation of MAD2. Ma and Poon show that MAD2 inactivation in mammalian cells requires TRIP13 and is stimulated by p31comet. Moreover, TRIP13-deficient cells containing only C-MAD2 are unable to activate the spindle-assembly checkpoint. Keywords: MAD2, mitosis, mitotic exit, p31comet, spindle-assembly checkpoin