CENP-I and Aurora B act as a molecular switch that ties RZZ/Mad1 recruitment to kinetochore attachment status

The RZZ (Rod, ZW10, and Zwilch) complex and Mad1 proteins tightly associate with kinetochores to generate the spindle checkpoint signal, but they are released when a kinetochore forms mature microtubule attachments. Here we demonstrate that the centromere protein CENP-I is required to generate a stable association of RZZ and Mad1 with kinetochores. CENP-I also inhibits their removal by dynein stripping. This regulation of Mad1 and RZZ dissociation functions independently of Aurora B, which regulates their association. We show that the microtubule status of each kinetochore independently dictates the recruitment of Aurora B kinase, kinase activity on a kinetochore substrate, and loading of spindle checkpoint proteins. This dynamic regulation of Mad1 association by Aurora B is only uncovered when CENP-I is depleted, consistent with our finding that CENP-I inhibits the dissociation of Mad1. We conclude that the dual activities of Aurora B and CENP-I generate a molecular switch that maintains a robust spindle checkpoint signal at prometaphase kinetochores until they attain mature attachments to microtubules.