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

It is known that the kinase activity of MPS1 is essential for maintaining mitotic arrest even when MAD1 constitutively localizes at kinetochores as a fusion with mCherry-Mis12 (25) (26) (27) (28) . We hypothesize that MPS1 might phosphorylate MAD1 or MAD2 to regulate the efficiency of the MAD1:C-MAD2 catalyst. In vitro kinase assays found that MPS1 indeed phosphorylates MAD1-NTD and CTD (Fig. 4a ). The specificity of the kinase assay was validated by that reversine, a previously characterized MPS1 inhibitor (44) , reduces in vitro phosphorylation of an artificial MPS1 substrate myelin basic protein (Fig. 4a , compare lanes 1 and 2 in the right panel). MAD1-MIM and MAD2 conformers are not good substrates for MPS1 under the experimental condition. Interestingly, recombinant MPS1 kinase binds to MAD1-NTD or CTD but not MIM in the absence of ATP (Fig. 4b) .