Coiled-coil interactions drive ectopic condensation of overexpressed MAD1 to promote mitotic slippage in cancer cells.

The mitotic checkpoint protein MAD1 is significantly overexpressed in several cancers, weakening the checkpoint and promoting mitotic slippage. Overexpressed MAD1 forms ectopic foci in mitotic cells, yet the biophysical nature of these foci and their contributions to mitotic slippage remain unclear. Here, we report that MAD1 foci are phase-separated condensates that shorten the mitotic duration by sequestering checkpoint proteins. Our biophysical quantifications reveal that MAD1 ectopic foci in mitotic cells exhibit dynamic condensate properties rather than those of a solid aggregate. Using an inducible phase separation assay in live cells, we show that MAD1 phase separation is driven by interactions between its coiled-coil and disordered domains at the N-terminus. We decouple the contributions of condensation from concentration by inducing the formation of MAD1 ectopic condensates in mitotic cells with low levels of MAD1, demonstrating that the condensation process directly drives mitotic slippage. Mechanistically, the MAD1 ectopic condensate traps the diffusive pool of MAD2, an interaction partner of MAD1, thereby weakening the MAD2 conversion cycle necessary for a robust mitotic checkpoint. Our work illustrates a loss of function caused by ectopic condensates in MAD1-overexpressed cancer cells.