Recent Progress on the Localization of the Spindle Assembly Checkpoint Machinery to Kinetochores

Abstract

Faithful chromosome segregation during mitosis is crucial for maintaining genome stability. The spindle assembly checkpoint (SAC) is a surveillance mechanism that ensures accurate mitotic progression. Defective SAC signaling leads to premature sister chromatid separation and aneuploid daughter cells. Mechanistically, the SAC couples the kinetochore microtubule attachment status to the cell cycle progression machinery. In the presence of abnormal kinetochore microtubule attachments, the SAC prevents the metaphase-to-anaphase transition through a complex kinase-phosphatase signaling cascade which results in the correct balance of SAC components recruited to the kinetochore. The correct kinetochore localization of SAC proteins is a prerequisite for robust SAC signaling and, hence, accurate chromosome segregation. Here, we review recent progresses on the kinetochore recruitment of core SAC factors.

Keywords: kinase; kinetochore; localization; microtubule; spindle assembly checkpoint (SAC).

Figure 1

The Knl1-Mis12-Ndc80 (KMN) network recruits spindle assembly checkpoint (SAC) proteins to the kinetochores without (correct) microtubule attachment. The diagram shows the kinetochore key components assembly in early mitosis. Upon mitotic entry, the Cdk1-Cyclin B phosphorylation of multiple substrates promotes outer kinetochore assembly. Specifically, the Cdk1 phosphorylation towards CENP-T significantly enhances the binding of Ndc80C to CENP-T (A). Aurora B phosphorylation towards multiple substrates also plays a crucial role in kinetochore assembly and SAC factors recruitment. It phosphorylates Mis12C and enhances the loading of Mis12C to CENP-T (B). Most importantly, the Aurora B phosphorylation of Ndc80/Hec1 N-tail plays dual roles: destabilizing microtubule attachment and enhancing Mps1 recruitment (C). Once activated, Mps1 can phosphorylate Knl1. Then, phosphorylated Knl1 targets the Bub1-Bub3 complex, and then, Bub1 acts as a scaffold to recruit BubR1-Bub3 and Mad1-Mad2. Mps1 phosphorylates Bub1 and Mad1 subsequently to boost Mad1-Mad2 kinetochore recruitment (D). We note that there are multiple phosphorylation-mediated protein docking/assembly events on kinetochores, and we only labeled the key events highly related with this review.

Figure 2

The molecular models to couple microtubule attachment with SAC signaling: (A) A model of the competitive binding to Ndc80C between microtubule and Mps1 kinase. Due to the competitive binding with Ndc80C, the microtubule attachment leads to the dissociation of Mps1 from the kinetochore. As a consequence, PP1 and PP2A dephosphorylate Mps1 substrates Knl1, Bub1 and Mad1. SAC signaling is quenched quickly. (B) A model of the spatial separation of Aurora B with the outer kinetochore substrates upon tension establishment. Upon the establishment of bipolar spindle attachment, Aurora B is unable to phosphorylate its outer kinetochore substrates (most importantly, Hec1) due to tension-caused spatial separation. Without the persistent phosphorylation by Aurora B, dephosphorylated Hec1 results in the dissociation of Mps1 and, finally, the silence of SAC signaling.