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Active Polycomb Repressive Complex 2

Billy O'Neill '18 & Johnathan Cooper '19


I. Introduction

Polycomb Repressive Complex 2 (PRC2) is one of the main polycomb-group (PcG) complexes. Together with PRC1, these PcG complexes are key epigenetic regulators of cell identity determination and maintenance2. Specifically, PRC2 mediates the methylation of histone H3 at lysine 27 -- a process that is indicative of heterochromatin formation and subsequent gene silencing.

PRC2 is composed of 3 core subunits1: enhancer of zeste 2 (Ezh2) , embryonic ectoderm development (Eed) , and suppressor of zeste 12 (Suz12) .

Improperly functioning PRC2 is linked to human diseases, including numerous cancers, Weaver syndrome, and childhood glioblastoma1,3,4.

II. The Regulatory Moiety1

The regulatory moiety of Ezh2 is composed of six discrete domains that form a belt-like structure which surrounds the Eed subunit.


Initial contact by Ezh2 on Eed is made by four regulatory sub-domains of Ezh2 -- EBD, BAM, SBD, and SANT1L -- contact the WD40 domain of Eed. Helix-and-loop regions of EBD are contacted by 7-bladed beta-propellor WD40 domain of Eed  Three beta-strands of BAM associate with the first and seventh blade of the beta-propellor, tightening the connection between Ezh2 and Eed. SBD and SANT1L form a , and touch Eed, tightening the belt-like regulatory moiety of Ezh2 around Eed. Bulky aromatic and hydrophobic residues in the alpha-helix bundle of the binary complex stabilize Eed.

III. The Catalytic Moiety1

The catalytic moiety of Ezh2 is composed of four discrete domains.

The substrate binding pocket and cofactor-binding site are located within the SET-I and post-SET regions respectively.

When PRC2 is in its basal site, the substrate binding pocket is inaccessible and the cofactor-binding pocket is incomplete. During activation, the SET-I region rotates over twenty degrees counterclockwise. This exposes the peptide substrate binding site and results in the completion of the cofactor-binding pocket, with the post-SET region now primed for cofactor SAH binding.

The SET activation loop SAL from the N-terminal portion of Ezh2 sequence extends from the Eed surface toward the SET domain and exits the catalytic moiety along the side of the last beta-strand of SET-N.

Suz12(VEFS) is located in the space behind the catalytic moiety of Ezh2. The N-terminal loop region of Suz12(VEFS) associates Eed with the SAL and SET regions of Ezh2.

The Suz12(VEFS) subunit is bound to Ezh2 by regulatory sub-domains MCSS and SANT2L that form a ten-helix bundle with helical contents of Suz12(VEFS).

IV. PRC2 Function

The methylation of N terminus histone tails by methyltransferases is associated with the formation of compact chromatin and, therefore, transcription inhibition. Specifically, targets for methyltransferase activity on histone 3 (H3) are a number of lysine residues -- K4, K9, K27, and K36. PRC2 and its enzymatic subunit, Ezh2, specifically target K27 on H32.

Silenced Versus Active Chromatin2

The chatacteristic SET domain of Ezh2 functions as a histone-lysine N-methyltransferase -- inducing the formation of di- and tri- methylated lysine 27 (H3K27me)3.

This methylation induces compact or silent chromatin. The chromodomain protein CDYL binds H3K27me marks (created by PRC2) and bridges PRC2 to the chromatin. This interaction mediates PRC2 methytransferase activity and creates a positive feedback loop to maintain the silent state on chromatin4. Additionally, H3K27me marks created by PRC2 can be recognized by subunits of the PRC1 complex2. This recognition again promotes more chromatin silencing as recruitment of PRC1 results in methylation of another H3 lysine — H3K92.

Inhibition by PRC22

PRC2’s role in large scale transcriptional regulation has implications when it comes to proliferation and differentiation of stem cells. Depending on PRC2 recruitment and the subsequent spread of the silent state of chromatin, whole gene lineages can be made accessible or inaccessible to transcriptional machinery.

PRC2 and Cell Differentiation2

V. References

1Jiao, L., & Liu, X. (2015). Structural basis of histone H3K27 trimethylation by an active polycomb repressive complex 2. Science, 350(6258). doi:10.1126/science.aac4383

2Croce, L. D., & Helin, K. (2013). Transcriptional regulation by Polycomb group proteins. Nature Structural & Molecular Biology, 20(10), 1147-1155. doi:10.1038/nsmb.2669

3Yoo, K. H., & Hennighausen, L. (2012). EZH2 Methyltransferase and H3K27 Methylation in Breast Cancer. International Journal of Biological Sciences, 8(1), 59-65. doi:10.7150/ijbs.8.59

4Nichol, J.N.; Dupéré-Richer, D.; Ezponda, T.; Licht, J.D.; Miller, W.H. (2016). H3K27 Mehtylation: A Focal Point of Epigenetic Deregulation in Cancer. Advances in Cancer Research (131), 59-65.

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