The chromodomain ofDrosophilaPolycomb protein is essential for maintaining the silencing state of homeotic genes during development. Recent studies suggest that Polycomb mediates the assembly of repressive higher-order chromatin structures in conjunction with the methylation of Lys 27 of histone H3 by a Polycomb group repressor complex. A similar mechanism in heterochromatin assembly is mediated by HP1, a chromodomain protein that binds to histone H3 methylated at Lys 9. To understand the molecular mechanism of the methyl-Lys 27 histone code recognition, we have determined a 1.4-Å-resolution structure of the chromodomain of Polycomb in complex with a histone H3 peptide trimethylated at Lys 27. The structure reveals a conserved mode of methyl-lysine binding and identifies Polycomb-specific interactions with histone H3. The structure also reveals a dPC dimer in the crystal lattice that is mediated by residues specifically conserved in the Polycomb family of chromodomains. The dimerization of dPC can effectively account for the histone-binding specificity and provides new mechanistic insights into the function of Polycomb. We propose that self-association is functionally important for Polycomb.

The WD40 domain exhibits a β-propeller architecture, often comprising seven blades. The WD40 domain is one of the most abundant domains and also among the top interacting domains in eukaryotic genomes. In this review, we will discuss the identification, definition and architecture of the WD40 domains. WD40 domain proteins are involved in a large variety of cellular processes, in which WD40 domains function as a protein-protein or protein-DNA interaction platform. WD40 domain mediates molecular recognition events mainly through the smaller top surface, but also through the bottom surface and sides. So far, no WD40 domain has been found to display enzymatic activity. We will also discuss the different binding modes exhibited by the large versatile family of WD40 domain proteins. In the last part of this review, we will discuss how post-translational modifications are recognized by WD40 domain proteins.

N6-methyladenosine (m6A) is the most abundant internal modification in RNA and is specifically recognized by YTH domain containing proteins. Recently we reported that YTHDC1 prefers guanosine and disfavors adenosine at the position preceding the m6A nucleotide in RNA and preferentially binds to the GG(m6A)C sequence. Now we systematically characterized the binding affinities of the YTH domains of three other human proteins and yeast YTH domain protein Pho92, and determined the crystal structures of the YTH domains of human YTHDF1 and yeast Pho92 in complex with a 5-mer m6A RNA, respectively. Our binding and structural data revealed that the YTH domain used a conserved aromatic cage to recognize m6A. Nevertheless, none of these YTH domains, except YTHDC1, display sequence selectivity at the position preceding the m6A modification. Structural comparison of these different YTH domains revealed that among those, only YTHDC1 harbours a distinctly selective binding pocket for the nucleotide preceding the m6A nucleotide.