Mad1 contribution to spindle assembly checkpoint signalling goes beyond presenting Mad2 at kinetochores

Synopsis image This study, published alongside one from the Nilsson laboratory, shows that Mad1 mutants that still recruit Mad2 to kinetochores cannot activate the spindle assembly checkpoint in yeast. Thus, Mad1 has an additional, hitherto unidentified role in this process. The Mad1 C-terminus and Bub1 conserved motif 1 are required for kinetochore localization of the Schizosaccharomyces pombe Mad1:Mad2 complex. The Mad1 C-terminal "head" is required for checkpoint activity despite being dispensable for Mad1 and Mad2 kinetochore recruitment. Mad1 is not only the scaffold for presenting Mad2 at kinetochores, but its C-terminus additionally promotes checkpoint signalling. Abstract The spindle assembly checkpoint inhibits anaphase until all chromosomes have become attached to the mitotic spindle. A complex between the checkpoint proteins Mad1 and Mad2 provides a platform for Mad2:Mad2 dimerization at unattached kinetochores, which enables Mad2 to delay anaphase. Here, we show that mutations in Bub1 and within the Mad1 C-terminal domain impair the kinetochore localization of Mad1:Mad2 and abrogate checkpoint activity. Artificial kinetochore recruitment of Mad1 in these mutants co-recruits Mad2; however, the checkpoint remains non-functional. We identify specific mutations within the C-terminal head of Mad1 that impair checkpoint activity without affecting the kinetochore localization of Bub1, Mad1 or Mad2. Hence, Mad1 potentially in conjunction with Bub1 has a crucial role in checkpoint signalling in addition to presenting Mad2.