Mad1 (green) localizes to the kinetochores of control cells treated with nocodazole (left) but is absent from the kinetochores of cells that lack both CENP-I and Aurora B activity (right).
Mad1 (green) localizes to the kinetochores of control cells treated with nocodazole (left) but is absent from the kinetochores of cells that lack both CENP-I and Aurora B activity (right).
Matson and Stukenberg describe how the centromeric protein CENP-I cooperates with the Aurora B kinase to control the kinetochore localization of spindle checkpoint proteins.
Mad1 and the RZZ complex are critical components of the spindle assembly checkpoint that prevent anaphase onset by binding to kinetochores that aren’t attached to the mitotic spindle correctly. Once spindle microtubules are properly attached, the motor protein dynein strips Mad1 and the RZZ complex away from kinetochores and allows mitosis to proceed. Aurora B helps recruit RZZ and Mad1 to kinetochores in early mitosis, but cells treated with Aurora B inhibitors and the microtubule-depolymerizing drug nocodazole can still activate the spindle checkpoint as long as they express a group of centromeric proteins that includes CENP-I. How CENP-I supports checkpoint activation is unknown, however.
found that CENP-I stabilized RZZ and Mad1’s interaction with kinetochores, limiting their dissociation and preventing dynein from stripping them away prematurely. Thus, when Aurora B activity is lowered by inhibitors, CENP-I helps retain enough RZZ and Mad1 at unattached kinetochores to activate the spindle checkpoint.
On the other hand, Aurora B promoted RZZ and Mad1’s association with kinetochores. The kinase’s activity was enhanced by so-called PreK-fibers, microtubule bundles nucleated by the kinetochores themselves. In prometaphase cells lacking the stabilizing influence of CENP-I, checkpoint proteins only accumulated at kinetochores with PreK-fibers and high levels of Aurora B activity. Under normal circumstances, however, Aurora B and CENP-I combine to regulate checkpoint signaling at individual kinetochores according to their microtubule attachment status.
