Protein phosphatase 1γ (PP1γ) is recruited to mitotic chromatin at anaphase by its regulatory subunit Repo-Man (also known as CDCA2), where it promotes dephosphorylation of histone H3 and chromosome decondensation. Vagnarelli et al. now confirm these results and show that Repo-Man actually has two distinct anaphase functions that are mediated by opposite ends of the protein: chromatin remodelling and coordination of nuclear envelope reformation.

The authors previously showed that Repo-Man is phosphorylated in vitro by cyclin-dependent kinase 1 (CDK1)–cyclin B. Here, they identify Thr412 as a key residue for this reaction: mutating Thr412 to Ala led to premature localization of Repo-Man to chromosomes. However, this early chromosomal localization was blocked if the PP1γ-binding site was mutated, suggesting that PP1γ is also required for this localization. Therefore, they conclude that CDK1–cyclin B phosphorylates Repo-Man at multiple sites to prevent PP1γ binding and keep it off chromosomes until anaphase.

They then identified factors that interact with Repo-Man during mitotic exit. Pull-down experiments detected not only the expected PP1γ subunits and histones but also importin-β and nucleoporin 153, factors that are important for nuclear envelope reassembly. Repo-Man partially colocalized with importin-β at the periphery of chromosomes in anaphase. Moreover, importin-β bound directly to an amino-terminal domain of Repo-Man (residues 1–135). This interaction was specific to anaphase, independent of the PP1γ-binding site and negatively controlled by CDK-mediated phosphorylation of Repo-Man.

Repo-Man actually has two distinct anaphase functions

The idea that Repo-Man might regulate not only chromosome organization but also early steps in nuclear envelope reassembly at the chromosome periphery was supported by analysis of its dynamic localization. A carboxy-terminal (403–1023) fragment of Repo-Man localized to bulk chromatin rapidly after anaphase but was not at the periphery of chromosomes, whereas an N-terminal (1–397) region accumulated exclusively at the periphery a few minutes later.

Moreover, Repo-Man was required for both normal chromatin reorganization in G1 and nuclear reassembly. RNA interference-mediated depletion of Repo-Man interfered with normal histone H3 dephosphorylation and reduced heterochromatin protein 1α accumulation on chromatin and subsequent assembly of heterochromatin foci during mitotic exit. It also resulted in irregular nuclear morphology, which the authors attributed to defective importin-β localization.

The authors propose that the N-terminal section of Repo-Man is important for organizing components needed early on for nuclear reassembly, whereas the C-terminal section is required for PP1γ-mediated modification of histone H3 to promote chromatin remodelling. Further work is needed to determine the mechanisms by which the N-terminal and C-terminal domains of Repo-Man control its different anaphase functions. However, this study highlights the importance of having a common regulator coordinating chromosome condensation and nuclear envelope reassembly.