# Phosphorylation of Xenopus M18BP1 governs centromeric localization and CENP-A nucleosome assembly

## TL;DR

This study shows how phosphorylation of a protein called M18BP1 controls when new CENP-A nucleosomes are assembled at centromeres, ensuring proper chromosome segregation during cell division.

## Contribution

The paper identifies a novel phosphorylation-dependent mechanism regulating M18BP1's centromere localization and CENP-A nucleosome assembly.

## Key findings

- Phosphorylation of M18BP1 disrupts its binding to CENP-A nucleosomes during metaphase.
- Phosphomimetic mutations in M18BP1 prevent interphase centromere localization and CENP-A nucleosome assembly.
- Non-phosphorylatable M18BP1 mutations do not drive metaphase binding, indicating additional regulatory factors.

## Abstract

Eukaryotic chromosome segregation requires attachment of chromosomes to microtubules through the kinetochore so that chromosomes can align and move in mitosis. Kinetochores assemble on the centromere, which is epigenetically defined by the histone H3 variant CENtromere Protein A (CENP-A). During DNA replication, CENP-A is equally divided between replicated chromatids, and new CENP-A nucleosomes are re-assembled during the subsequent G1 phase. How cells regulate the cell cycle timing of CENP-A assembly is a central question in the epigenetic maintenance of centromeres. CENP-A nucleosome assembly requires the Mis18 complex (Mis18α, Mis18β, and M18BP1), whose localization to centromeres occurs between metaphase and G1. Here, we define a new regulatory mechanism that works through phosphorylation of Xenopus laevis M18BP1 between metaphase and interphase. Phosphorylation disrupts binding of M18BP1 to CENP-A nucleosomes in metaphase, and when relieved, enables M18BP1 binding to CENP-A nucleosomes in interphase. We show that this phosphorylation-dependent mechanism regulates CENP-A nucleosome assembly. We propose that the phospho-regulated binding of M18BP1 to CENP-A nucleosomes restricts new CENP-A assembly to interphase.

Phosphorylation of a key residue in the CENP-A binding domain of Xenopus M18BP1 controls its centromere localization in interphase and metaphase, such that this phosphoregulatory switch restricts CENP-A nucleosome assembly to interphase.

Phosphomimetic mutations in M18BP1 prevent its interphase localization to the centromere and subsequent CENP-A nucleosome assembly.Non-phosphorylatable mutations in M18BP1 are not sufficient to drive metaphase binding of M18BP1 to CENP-A nucleosomes demonstrating that additional factors restrict M18BP1 binding to interphase centromeres.

Phosphomimetic mutations in M18BP1 prevent its interphase localization to the centromere and subsequent CENP-A nucleosome assembly.

Non-phosphorylatable mutations in M18BP1 are not sufficient to drive metaphase binding of M18BP1 to CENP-A nucleosomes demonstrating that additional factors restrict M18BP1 binding to interphase centromeres.

Phosphorylation of a key residue in the CENP-A binding domain of Xenopus M18BP1 controls its centromere localization in interphase and metaphase, such that this phosphoregulatory switch restricts CENP-A nucleosome assembly to interphase.