Abstract
Mitotic arrest deficient 1 (Mad1) plays a well-characterized role in the major cell-cycle checkpoint that regulates chromosome segregation during mitosis, the mitotic checkpoint (also known as the spindle assembly checkpoint). During mitosis, Mad1 recruits Mad2 to unattached kinetochores, where Mad2 is converted into an inhibitor of the anaphase-promoting complex/cyclosome bound to its specificity factor, Cdc20. During interphase, Mad1 remains tightly bound to Mad2, and both proteins localize to the nucleus and nuclear pores, where they interact with Tpr (translocated promoter region). Recently, it has been shown that interaction with Tpr stabilizes both proteins and that Mad1 binding to Tpr permits Mad2 to associate with Cdc20. However, interphase functions of Mad1 that do not directly affect the mitotic checkpoint have remained largely undefined. Here we identify a previously unrecognized interphase distribution of Mad1 at the Golgi apparatus. Mad1 colocalizes with multiple Golgi markers and cosediments with Golgi membranes. Although Mad1 has previously been thought to constitutively bind Mad2, Golgi-associated Mad1 is Mad2 independent. Depletion of Mad1 impairs secretion of α5 integrin and results in defects in cellular attachment, adhesion, and FAK activation. Additionally, reduction of Mad1 impedes cell motility, while its overexpression accelerates directed cell migration. These results reveal an unexpected role for a mitotic checkpoint protein in secretion, adhesion, and motility. More generally, they demonstrate that, in addition to generating aneuploidy, manipulation of mitotic checkpoint genes can have unexpected interphase effects that influence tumor phenotypes.