Coordination between discrete Mitotic Arrest Deficient 1 (MAD1) domains is required for efficient mitotic checkpoint signaling

Wenbin Ji; Yibo Luo; Ejaz Ahmad; Song-Tao Liu

Current model suggests that the conversion is catalyzed by a unusual catalyst: the MAD1:C-MAD2 complex localized at unattached kinetochores. How the catalysis is achieved is still unclear. Together with two very recent publications (29, 30) , our results support the roles of MAD1-CTD and MPS1 kinase in promoting the MAD2 O-C conversion. Our work agreed with the critical role of MAD1 T716 which is phosphorylated by MPS1, but also suggested that S610 and Y634 are potential key residues for regulating MAD2 O-C conversion (Fig 1, Fig 5) . S610 is phosphorylated by MPS1 in vitro (Fig 4) , and Y634 was reported to be phosphorylated in vivo although we have not confirmed this yet in mitotic cells (37) . We have also uncovered additional protein-protein interactions between MPS1, MAD1 and MAD2 (Figs. 2, 3, 4, 6) . Of particular interest, we found that MAD1-NTD and CTD interact with each other and both bind to O-MAD2 and C-MAD2. These results have been integrated into an updated model for the MAD1:C-MAD2 catalyst that promotes MAD2 O-C conversion (Fig. 7) . Many mechanistic details remain to be filled, so we hereby discuss the implications of our results.