Coupling of microtubule bundles isolates them from local disruptions to set the structural stability of the anaphase spindle

Involvement of RBP-J interacting and tubulin-associated protein in the distribution of protein regulator of cytokinesis 1 in mitotic spindles

The protein regulator of cytokinesis 1 (PRC1) is a key regulator of microtubule crosslinking and bundling, which is crucial for spindle formation and cytokinesis. RITA, the RBP-J interacting and tubulin-associated protein, is a microtubule associated protein. We have reported that RITA localizes to mitotic spindles modulating microtubule dynamics and stability as well as to spindle poles affecting the activity of Aurora A. As defective chromosome congression and segregation are the most remarkable features of cells depleted of RITA, we aimed to explore further potential related mechanisms, using various cellular and molecular techniques, including clustered regularly interspaced short palindromic repeats technique/deactivated CRISPR-associated protein 9 (CRISPR/dCas9), mass spectrometry, confocal microscopy, immunofluorescence, immunoprecipitation and Western blot analysis. Here, we show that FLAG-RITA precipitates PRC1 and tubulin, and that these two proteins co-localize in the central region of the central spindle. Reduction of RITA enlarges the staining area of PRC1 in mitotic spindles as well as in the central spindle. Its suppression reduces the inter-centromere distance in metaphase cells. Interestingly, microtubule bundles of the central spindle are often less organized in a non-parallel pattern, as evidenced by increased angles, relative to corresponding separating chromosomes. These data suggest a novel role for RITA in mitotic distribution of PRC1 and that its deregulation may contribute to defective chromosome movement during mitosis. As both RITA and PRC1 are closely associated with malignant progression, further work is required to elucidate the detailed molecular mechanisms by which RITA acts as a modulator in central spindle formation and cytokinesis.

Spastin regulates anaphase chromosome separation distance and microtubule-containing nuclear tunnels

Nuclear envelope reassembly during the final stages of each mitosis depends on disassembling spindle microtubules without disrupting chromosome separation. This process involves the transient recruitment of the ESCRT-III complex and spastin, a microtubule-severing AAA (ATPases associated with diverse cellular activities) mechanoenzyme, to late-anaphase chromosomes. However, dissecting mechanisms underlying these rapid processes, which can be completed within minutes, has been difficult. Here, we combine fast-acting chemical inhibitors with live-cell imaging and find that spindle microtubules, along with spastin activity, regulate the number and lifetimes of spastin foci at anaphase chromosomes. Unexpectedly, spastin inhibition impedes chromosome separation, but does not alter the anaphase localization dynamics of CHMP4B, an ESCRT-III protein, or increase γ-H2AX foci, a DNA damage marker. We show spastin inhibition increases the frequency of lamin-lined nuclear microtunnels that can include microtubules penetrating the nucleus. Our findings suggest failure to sever spindle microtubules impedes chromosome separation, yet reforming nuclear envelopes can topologically accommodate persistent microtubules ensuring nuclear DNA is not damaged or exposed to cytoplasm.

Phosphorylation controls spatial and temporal activities of motor-PRC1 complexes to complete mitosis

During mitosis, spindle architecture alters as chromosomes segregate into daughter cells. The microtubule crosslinker protein regulator of cytokinesis 1 (PRC1) is essential for spindle stability, chromosome segregation and completion of cytokinesis, but how it recruits motors to the central spindle to coordinate the segregation of chromosomes is unknown. Here, we combine structural and cell biology approaches to show that the human CENP-E motor, which is essential for chromosome capture and alignment by microtubules, binds to PRC1 through a conserved hydrophobic motif. This binding mechanism is also used by Kinesin-4 Kif4A:PRC1. Using in vitro reconstitution, we demonstrate that CENP-E slides antiparallel PRC1-crosslinked microtubules. We find that the regulation of CENP-E -PRC1 interaction is spatially and temporally coupled with relocalization to overlapping microtubules in anaphase. Finally, we demonstrate that the PRC1-microtubule motor interaction is essential in anaphase to control chromosome partitioning, retain central spindle integrity and ensure cytokinesis. Taken together our findings reveal the molecular basis for the cell cycle regulation of motor-PRC1 complexes to couple chromosome segregation and cytokinesis.