# AURKB-driven dissolution of CIZ1–RNA assemblies from the inactive X chromosome in mitosis

## TL;DR

This study shows how CIZ1-RNA assemblies on the inactive X chromosome are regulated during cell division by Aurora Kinase B.

## Contribution

The study identifies a novel regulatory mechanism involving AURKB in dissolving CIZ1–RNA assemblies during mitosis.

## Key findings

- CIZ1 is released from the inactive X chromosome in prometaphase under Aurora Kinase B regulation.
- Phosphomimetic mutations in CIZ1 disrupt RNA interactions but not chromatin or nuclear matrix associations.
- CIZ1's C-terminal domain is crucial for RNA binding and regulated by AURKB phosphorylation.

## Abstract

Cip1-interacting zinc-finger protein 1 (CIZ1) interacts with Xist lncRNA to form large RNA–protein assemblies at the inactive X-chromosome (Xi) in female mammalian nuclei, plus smaller assemblies in both sexes. CIZ1 assemblies influence underlying chromatin, and their disruption alters the expression of autosomal and X-linked gene clusters. Here, we explore the regulated dissolution of CIZ1–Xi assemblies during mitosis and show that, like Xist, CIZ1 is released in prometaphase under the regulation of Aurora Kinase B (AURKB). The part of human/mouse CIZ1 comprising 179/181 C-terminal amino acids encodes a matrin-3 domain that facilitates dimerization to form a compact folded core with disordered C-terminal extensions. Mass spectrometry revealed 56 high-confidence interacting partners of the C-terminal fragment, predominantly chromatin, nuclear matrix, and RNA-binding proteins. Phosphomimetic mutation of three conserved AURKB sites in the C-terminal extensions released CIZ1 from its nuclear anchor points, but did not affect its interaction with chromatin or nuclear matrix proteins. In contrast, the same mutations, or deletion of the C-terminal extensions, abolished interaction with RNAs, including Xist. Together, the data suggest CIZ1 is a regulatable component of the protein–RNA assemblies that preserve epigenetic stability across the nucleus, and that AURKB drives their dissolution in mitosis via dissociation of CIZ1 from RNA.

Graphical Abstract