Phosphorylation of the HP1β hinge region sequesters KAP1 in heterochromatin and promotes the exit from naïve pluripotency
Authors: Weihua Qin
Sebastian Bultmann
Axel Imhof
Irina Solovei
Christopher B Mulholland
Miha Modic
Michael Wierer
Heinrich Leonhardt
Ignasi Forné
Enes Ugur
M Cristina Cardoso
Martha Smets
Journal: Nucleic Acids Research (2021)
Discipline: BIOLOGY & BIOCHEMISTRY
Keywords: HP1β; chromatin organization; cell differentiation; phosphorylation; CK2
Affiliations: Faculty of Biology, Ludwig-Maximilians-Universität München, Butenandtstraße 1, D-81377 Munich, Germany, The Francis Crick Institute and UCL Queen Square Institute of Neurology, London NW1 1AT, United Kingdom, Department of Proteomics and Signal Transduction, Max Planck Institute for Biochemistry, Am Klopferspitz 18, 82152 Martinsried, Germany, Biomedical Center Munich, Faculty of Medicine, Ludwig-Maximilians-Universität München, Großhaderner Str. 9, 82152 Planegg-Martinsried, Germany, Cell Biology and Epigenetics, Department of Biology, Technical University of Darmstadt, 64287 Darmstadt, Germany

Abstract:  Heterochromatin binding protein HP1β plays an important role in chromatin organization and cell differentiation, however the underlying mechanisms remain unclear. Here, we generated HP1β-/- embryonic stem cells and observed reduced heterochromatin clustering and impaired differentiation. We found that during stem cell differentiation, HP1β is phosphorylated at serine 89 by CK2, which creates a binding site for the pluripotency regulator KAP1. This phosphorylation dependent sequestration of KAP1 in heterochromatin compartments causes a downregulation of pluripotency factors and triggers pluripotency exit. Accordingly, HP1β-/- and phospho-mutant cells exhibited impaired differentiation, while ubiquitination-deficient KAP1-/- cells had the opposite phenotype with enhanced differentiation. These results suggest that KAP1 regulates pluripotency via its ubiquitination activity. We propose that the formation of subnuclear membraneless heterochromatin compartments may serve as a dynamic reservoir to trap or release cellular factors. The sequestration of essential regulators defines a novel and active role of heterochromatin in gene regulation and represents a dynamic mode of remote control to regulate cellular processes like cell fate decisions. 

Retracted: No

PubPeer comments:
  Recent studies testing the capacity of HP1 to induce phase separation revealed that only HP1alpha formed phaseseparated droplets (8,9). This phase separation correlates with the formation of heterochromatin compartments (chromocenters) in the nucleus. Recently, we found that the charge of the hinge IDR (IDR-H) is a distinctive feature of HP1 homologues and plays a decisive role in liquid-liquid phase separation (LLPS) (10,11) and that HP1beta also undergoes phase separation in a histone H3K9me3 dependent manner (11). HP1alpha beta together with other chromatin binding proteins, such as SUV39H1 and KAP1, coalesce around the solid chromatin scaffold (12u201315). A small point, but how to reconcile this with the fact that HPAalpha null mice are normal? See previous comments here: https: pubpeer.com publications E0F04DC414EAF3D4C8B22967A10470 ![file] ,