Stable transmission of genetic information during cell division requires faithful mitotic spindle assembly and chromosome segregation. The Ran GTPase plays a key role in mitotic spindle assembly. However, how the generation of a chemical gradient of Ran-GTP at the spindle is coupled to mitotic post-translational modifications has never been characterized. Here, we solved the complex structure of Ran with the nucleotide release factor Mog1 and delineated a novel mitosis-specific acetylation-regulated Ran–Mog1 interaction during chromosome segregation. Our structure-guided functional analyses revealed that Mog1 competes with RCC1 for Ran binding in a GTP/GDP-dependent manner. Biochemical characterization demonstrated that Mog1-bound Ran prevents RCC1 binding and subsequent GTP loading. Surprisingly, Ran is a bona fide substrate of TIP60, and the acetylation of Lys134 by TIP60 liberates Mog1 from Ran binding during mitosis. Importantly, this acetylation-elicited switch of Ran binding to RCC1 promotes high level of Ran-GTP, which is essential for chromosome alignment. These results establish a previously uncharacterized regulatory mechanism in which TIP60 provides a homeostatic control of Ran-GTP level by tuning Ran effector binding for chromosome segregation in mitosis.

Background: IQGAP1 is a scaffold protein essential for cellular signaling in response to external cues. Results: Perturbation of the IQGAP1-SKAP interaction results in inhibition of directional cell migration. Conclusion: The IQGAP1-SKAP interaction links to dynamic microtubules at the leading edge to steer cell migration. Significance: IQGAP1 serves as a signaling hub for dynamic interaction between microtubule plus-ends and the cell cortex.

Heterochromatin protein 1α (HP1α) regulates chromatin specification and plasticity during cell fate decision. Different structural determinants account for HP1α localization and function during cell division cycle. Our earlier study showed that centromeric localization of HP1α depends on the epigenetic mark H3K9me3 in interphase, while its centromeric location in mitosis relies on uncharacterized PXVXL-containing factors. Here, we identified a PXVXL-containing protein, ligand-dependent nuclear receptor-interacting factor 1 (LRIF1), which recruits HP1α to the centromere of mitotic chromosomes and its interaction with HP1α is essential for accurate chromosome segregation during mitosis. LRIF1 interacts directly with HP1α chromoshadow domain via an evolutionarily conserved PXVXL motif within its C-terminal. Importantly, the LRIF1-HP1α interaction is critical for Aurora B activity in the inner centromere. Mutation of PXVXL motif of LRIF1 leads to defects in HP1α centromere targeting and aberrant chromosome segregation. These findings reveal a previously unrecognized direct link between LRIF1 and HP1α in centromere plasticity control and illustrate the critical role of LRIF1‒HP1α interaction in orchestrating accurate cell division.

During cell division, accurate chromosome segregation is tightly regulated by Polo-like kinase 1 (PLK1) and opposing activities of Aurora B kinase and protein phosphatase 1 (PP1). However, the regulatory mechanisms underlying the aforementioned hierarchical signaling cascade during mitotic chromosome segregation have remained elusive. Sds22 is a conserved regulator of PP1 activity, but how it regulates PP1 activity in space and time during mitosis remains elusive. Here we show that Sds22 is a novel and cognate substrate of PLK1 in mitosis, and the phosphorylation of Sds22 by PLK1 elicited an inhibition of PP1-mediated dephosphorylation of Aurora B at threonine 232 (Thr 232 ) in a dose-dependent manner. Overexpression of a phosphomimetic mutant of Sds22 causes a dramatic increase in mitotic delay, whereas overexpression of a non-phosphorylatable mutant of Sds22 results in mitotic arrest. Mechanistically, the phosphorylation of Sds22 by PLK1 strengthens the binding of Sds22 to PP1 and inhibits the dephosphorylation of Thr 232 of Aurora B to ensure a robust, error-free metaphase-anaphase transition. These findings delineate a conserved signaling hierarchy that orchestrates dynamic protein phosphorylation and dephosphorylation of critical mitotic regulators during chromosome segregation to guard chromosome stability.