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Title: NIPP1 maintains EZH2 phosphorylation and promoter occupancy
Other Titles: NIPP1 onderhoudt de fosforylatie en promoter binding van EZH2
Authors: Minnebo, Nikki
Issue Date: 20-Nov-2012
Abstract: Polycomb Group (PcG) proteins are repressive chromatin modifiers that regulate stem-cell pluripotency and differentiation, and are involved in cancer development. They function in large multimeric protein collections termed Polycomb Repressive complexes (PRC) and a key step in PcG-mediated gene silencing is the trimethylation of histone H3 at lysine 27 (H3K27me3) by the PRC2-component EZH2. Both the chromatin targeting of EZH2 and its affinity for regulatory proteins is controlled by multisite phosphorylation. NIPP1, a regulator of the Ser/Thr protein phosphatase PP1, is a direct interactor of the core PRC2-components EZH2 and EED. Moreover, NIPP1 represses genes in a PcG-mediated manner and associates with a subset of EZH2-target genes. Additionally, the loss of NIPP1 is embryonically lethal and is correlated with a global reduction of H3K27me3 levels.In the first part of this work, we show that NIPP1 is present in a nuclear complex with both PP1 and PRC2 components. Moreover, depleting either NIPP1 or PP1 reduces the association of EZH2 with a subset of its target genes, whereas overexpressing NIPP1 results in a retargeting of EZH2 in a PP1-dependent manner. Moreover, mapping of the genome-wide NIPP1 chromatin distribution using the DamID technique reveals that NIPP1 is associated with multiple PcG target genes. A PP1-binding mutant of NIPP1 shows a deficient association with these loci but still associates normally with PRC2 components. In the second part of this work, we show that the interaction between NIPP1 and EZH2 depends on the CDK-catalyzed phosphorylation of EZH2 at T416. This creates a docking site for the FHA domain of NIPP1 that subsequently prevents PP1 from dephosphorylating EZH2 at mitotic target motifs. Accordingly, a NIPP1-binding mutant of EZH2 is hypophosphorylated and shows a deficient association with proliferation-related target loci, as determined by DamID. In conclusion, we demonstrate that both NIPP1 and PP1 play an essential role in the regulation of EZH2-mediated gene silencing. While PP1 is necessary for dephosphorylating EZH2, NIPP1 functions as a PP1-inhibitor to secure the net phosphorylation of EZH2 by CDKs during mitosis. Possibly, a balanced phosphorylation of EZH2 is necessary to safeguard epigenetic memory during cell division in proliferating cells.
ISBN: 978 94 6165 066 5
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Laboratory of Biosignaling & Therapeutics

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