Summary Polycomb Group (PcG) proteins play an essential role in the epigenetic maintenance of repressive chromatin states. The gene silencing activity of the Polycomb Repressive Complex 2 (PRC2) depends on its ability to tri-methylate lysine 27 of histone H3 (H3K27) via the catalytic SET domain of the EZH2 subunit, and at least two other subunits of the complex: Suz12 and Eed. We show that the C-terminal domain of Eed specifically binds to histone tails carrying tri-methyl lysine residues associated with repressive chromatin marks and that this leads to the allosteric activation of the methyltransferase activity of PRC2. Mutations in Eed that prevent it from recognising repressive trimethyl-lysine marks abolish activation of PRC2 in vitro and, in Drosophila, reduces global methylation and disrupts development. These findings suggest a model for the propagation of the H3K27me3 mark that accounts for the maintenance of repressive chromatin domains and for the transmission of a histone modification from mother to daughter cells.

Polycomb repressive complex 2 (PRC2) silences gene expression through trimethylation of K27 of histone H3 (H3K27me3) via its catalytic SET domain. A missense mutation in the substrate of PRC2, histone H3K27M, is associated with certain pediatric brain cancers and is linked to a global decrease of H3K27me3 in the affected cells thought to be mediated by inhibition of PRC2 activity. We present here the crystal structure of human PRC2 in complex with the inhibitory H3K27M peptide bound to the active site of the SET domain, with the methionine residue located in the pocket that normally accommodates the target lysine residue. The structure and binding studies suggest a mechanism for the oncogenic inhibition of H3K27M. The structure also reveals how binding of repressive marks, like H3K27me3, to the EED subunit of the complex leads to enhancement of the catalytic efficiency of the SET domain and thus the propagation of this repressive histone modification.

SummaryThe mechanisms by which the major Polycomb group (PcG) complexes PRC1 and PRC2 are recruited to target sites in vertebrate cells are not well understood. Building on recent studies that determined a reciprocal relationship between DNA methylation and Polycomb activity, we demonstrate that, in methylation-deficient embryonic stem cells (ESCs), CpG density combined with antagonistic effects of H3K9me3 and H3K36me3 redirects PcG complexes to pericentric heterochromatin and gene-rich domains. Surprisingly, we find that PRC1-linked H2A monoubiquitylation is sufficient to recruit PRC2 to chromatin in vivo, suggesting a mechanism through which recognition of unmethylated CpG determines the localization of both PRC1 and PRC2 at canonical and atypical target sites. We discuss our data in light of emerging evidence suggesting that PcG recruitment is a default state at licensed chromatin sites, mediated by interplay between CpG hypomethylation and counteracting H3 tail modifications.

The kinetochore localization of MPS1 is necessary for mitotic checkpoint activity and requires the microtubule-binding domain of HEC1 and the Aurora B–dependent regulation of the TPR domain.

All organisms need to ensure that no DNA segments are rereplicated in a single cell cycle. Eukaryotes achieve this through a process called origin licensing, which involves tight spatiotemporal control of the assembly of prereplicative complexes (pre-RCs) onto chromatin. Cdt1 is a key component and crucial regulator of pre-RC assembly. In higher eukaryotes, timely inhibition of Cdt1 by Geminin is essential to prevent DNA rereplication. Here, we address the mechanism of DNA licensing inhibition by Geminin, by combining X-ray crystallography, small-angle X-ray scattering, and functional studies in Xenopus and mammalian cells. Our findings show that the Cdt1:Geminin complex can exist in two distinct forms, a ''permissive'' heterotrimer and an ''inhibitory'' heterohexamer. Specific Cdt1 residues, buried in the heterohexamer, are important for licensing. We postulate that the transition between the heterotrimer and the heterohexamer represents a molecular switch between licensing-competent and licensing-defective states.solution structure ͉ X-ray structure ͉ pre-RC ͉ cell cycle