Cell Division: The Unattached Kinetochore Wears an Expansive RZZ Coat

Kinetochore‐catalyzed MCC formation: A structural perspective

The spindle assembly checkpoint (SAC) is a cellular surveillance mechanism that functions to ensure accurate chromosome segregation during mitosis. Macromolecular complexes known as kinetochores, act as the interface of sister chromatid attachment to spindle microtubules. In response to unattached kinetochores, the SAC activates its effector, the mitotic checkpoint complex (MCC), which delays mitotic exit until all sister chromatid pairs have achieved successful attachment to the bipolar mitotic spindle. Formation of the MCC (composed of Mad2, BubR1, Bub3 and Cdc20) is regulated by an Mps1 kinase-dependent phosphorylation signaling cascade which assembles and repositions components of the MCC onto a catalytic scaffold. This scaffold functions to catalyze the conversion of the HORMA-domain protein Mad2 from an "inactive" open-state (O-Mad2) into an "active" closed-Mad2 (C-Mad2), and simultaneous Cdc20 binding. Here, our current understanding of the molecular mechanisms underlying the kinetic barrier to C-Mad2:Cdc20 formation will be reviewed. Recent progress in elucidating the precise molecular choreography orchestrated by the catalytic scaffold to rapidly assemble the MCC will be examined, and unresolved questions will be highlighted. Ultimately, understanding how the SAC rapidly activates the checkpoint not only provides insights into how cells maintain genomic integrity during mitosis, but also provides a paradigm for how cells can utilize molecular switches, including other HORMA domain-containing proteins, to make rapid changes to a cell's physiological state.

Mps1 dimerization and multisite interactions with Ndc80 complex enable responsive spindle assembly checkpoint signaling

Error-free mitosis depends on accurate chromosome attachment to spindle microtubules, which is monitored by the spindle assembly checkpoint (SAC) signaling. As an upstream factor of SAC, the precise and dynamic kinetochore localization of Mps1 kinase is critical for initiating and silencing SAC signaling. However, the underlying molecular mechanism remains elusive. Here, we demonstrated that the multisite interactions between Mps1 and Ndc80 complex (Ndc80C) govern Mps1 kinetochore targeting. Importantly, we identified direct interaction between Mps1 tetratricopeptide repeat domain and Ndc80C. We further identified that Mps1 C-terminal fragment, which contains the protein kinase domain and C-tail, enhances Mps1 kinetochore localization. Mechanistically, Mps1 C-terminal fragment mediates its dimerization. Perturbation of C-tail attenuates the kinetochore targeting and activity of Mps1, leading to aberrant mitosis due to compromised SAC function. Taken together, our study highlights the importance of Mps1 dimerization and multisite interactions with Ndc80C in enabling responsive SAC signaling.

Mutations in the Drosophila rough deal gene affecting RZZ kinetochore function

BACKGROUND

The RZZ complex, composed of the proteins Rough-Deal (Rod), Zw10 and Zwilch, plays a central role in the spindle assembly checkpoint (SAC), which assures proper sister chromatid segregation during mitosis. RZZ contributes to the regulation of the spindle assembly checkpoint by helping to recruit Mad1-Mad2 and the microtubule motor dynein to unattached kinetochores. It is an important component of the outer kinetochore and specifically the fibrous corona whose expansion is believed to facilitate microtubule capture. How RZZ carries out its diverse activities is only poorly understood. The C-terminal region of the Rod subunit is relatively well-conserved across metazoan phylogeny, but no function has been attributed to it.

RESULTS

To explore the importance of the Rod_C domain in RZZ function in Drosophila, we generated a series of point mutations in a stretch of 200 residues within this domain and we report here their phenotypes. Several of the mutations profoundly disrupt recruitment of RZZ to kinetochores, including one in a temperature-sensitive manner, while still retaining the capacity to assemble into a complex with Zw10 and Zwilch. Others affect aspects of dynein activity or recruitment at the kinetochore.

CONCLUSIONS AND SIGNIFICANCE

These results suggest that the Rod_C domain participates in the protein interactions necessary for RZZ recruitment and functionality at kinetochores.