Spindle checkpoint: trapped by the corona, cyclin B1 goes MAD

Abstract

The spindle checkpoint protects against aneuploidy by ensuring that dividing cells only proceed with chromosome segregation once all kinetochores are stably attached to spindle microtubules. The checkpoint protein MAD1 localizes to the corona, a structural expansion of the kinetochore forming in the absence of microtubule attachment, but molecular mechanism or functional significance of this localization remains unknown. Recent results now show that cyclin B1 recruits MAD1 to the corona and that this MAD1 pool is required for robust checkpoint signaling.

Figure 1. The cyclin B1‐MAD1 interaction promotes checkpoint signaling at kinetochores

Cyclin B1‐CDK1 is imported into the prophase nucleus, where cyclin B1 binds to the MAD1 N‐terminus (i) and CDK1 phosphorylates the APC/C, which results in APC/C activation (ii). Cyclin B1 binding to MAD1 releases MAD1 along with MAD2 from the inner nuclear pore component TPR, perhaps by locally potentiating MPS1 activity toward TPR (i, iii). Free MAD1‐MAD2 is then recruited to the outer kinetochore by BUB1 to catalyze assembly of the MCC (iv), which is needed to antagonize rising APC/C activity (v). An MPS1 phosphorylation cascade plays a key role in SAC signaling: Phospho‐KNL1 recruits BUB1, phospho‐BUB1 recruits MAD1, and phosphorylation of the MAD1 C‐terminus is needed for MCC catalysis (iv). Cyclin B1‐CDK1 in turn phosphorylates MPS1 to promote its kinetochore localization and BUB1 to prime it for MPS1 phosphorylation. Thus, feedback between MPS1 and cyclin B1‐CDK1 activities ensures efficient SAC activation. In early prometaphase, kinetochores assemble an outermost domain, the corona, which facilitates microtubule capture (vi). Cyclin B1 anchors the MAD1 N‐terminus in the corona, and the highly elongated shape of the MAD1 coiled‐coil may allow its C‐terminus (and the associated MAD2) to reach the outer kinetochore to participate in MCC assembly. Since kinetochore recruitment of corona‐anchored MAD1‐MAD2 no longer depends on the KNL1‐BUB1 pathway, MAD1‐MAD2 can persist at kinetochores to catalyze MCC assembly even when MPS1 activity is decreased with small molecule inhibitors (“low MPS1 activity”). Thus, corona‐localized MAD1‐MAD2 makes SAC signaling more robust. This may become physiologically relevant when a single unattached kinetochore in late prometaphase needs to generate a strong enough checkpoint signal to inhibit the APC/C until that last chromosome is incorporated into the spindle.