Identification of abscission checkpoint bodies as structures that regulate ESCRT factors to control abscission timing

The abscission checkpoint regulates the ESCRT membrane fission machinery and thereby delays cytokinetic abscission to protect genomic integrity in response to residual mitotic errors. The checkpoint is maintained by Aurora B kinase, which phosphorylates multiple targets, including CHMP4C, a regulatory ESCRT-III subunit necessary for this checkpoint. We now report the discovery that cytoplasmic abscission checkpoint bodies (ACBs) containing phospho-Aurora B and tri-phospho-CHMP4C develop during an active checkpoint. ACBs are derived from Mitotic Interchromatin Granules (MIGs), transient mitotic structures whose components are housed in splicing-related nuclear speckles during interphase. ACB formation requires CHMP4C, and the ESCRT factor ALIX also contributes. ACB formation is conserved across cell types and under multiple circumstances that activate the checkpoint. Finally, ACBs retain a population of ALIX, and their presence correlates with delayed abscission and delayed recruitment of ALIX to the midbody where it would normally promote abscission. Thus, a cytoplasmic mechanism helps regulate midbody machinery to delay abscission.

Abscission Checkpoint Bodies Reveal a New Facet of Abscission Checkpoint Control

The abscission checkpoint regulates the ESCRT membrane fission machinery and thereby delays cytokinetic abscission to protect genomic integrity in response to residual mitotic errors. The checkpoint is maintained by Aurora B kinase, which phosphorylates multiple targets, including CHMP4C, a regulatory ESCRT-III subunit necessary for this checkpoint. We now report the discovery that cytoplasmic abscission checkpoint bodies (ACBs) containing phospho-Aurora B and tri-phospho-CHMP4C develop in telophase under an active checkpoint. ACBs are derived from Mitotic Interchromatin Granules (MIGs), transient mitotic structures whose components are housed in splicing-related nuclear speckles during interphase. ACB formation requires CHMP4C, and the ESCRT factor ALIX also contributes. ACB formation is conserved across cell types and under multiple circumstances that activate the checkpoint. Finally, ACBs retain a population of ALIX, and their presence correlates with delayed recruitment of ALIX to the midbody where it would normally promote abscission. Thus, a cytoplasmic mechanism helps regulate midbody machinery to delay abscission.

Tetratricopeptide repeat factor XAB2 mediates the end resection step of homologous recombination

We examined the influence of the tetratricopeptide repeat factor XAB2 on chromosomal break repair, and found that XAB2 promotes end resection that generates the 3′ ssDNA intermediate for homologous recombination (HR). Namely, XAB2 is important for chromosomal double-strand break (DSB) repair via two pathways of HR that require end resection as an intermediate step, end resection of camptothecin (Cpt)-induced DNA damage, and RAD51 recruitment to ionizing radiation induced foci (IRIF), which requires end resection. Furthermore, XAB2 mediates specific aspects of the DNA damage response associated with end resection proficiency: CtIP hyperphosphorylation induced by Cpt and BRCA1 IRIF. XAB2 also promotes histone acetylation events linked to HR proficiency. From truncation mutation analysis, the capacity for XAB2 to promote HR correlates with its ability to form a complex with ISY1 and PRP19, which show a similar influence as XAB2 on HR. This XAB2 complex localizes to punctate structures consistent with interchromatin granules that show a striking adjacent-localization to the DSB marker γH2AX. In summary, we suggest that the XAB2 complex mediates DNA damage response events important for the end resection step of HR, and speculate that its adjacent-localization relative to DSBs marked by γH2AX is important for this function.