Molecular basis for site-specific read-out of histone H3K4me3 by the BPTF PHD finger of NURF

Abstract

Four papers in this issue tackle the hot topic of chromatin remodelling, specifically, how methyl marks on chromatin are 'read' by the proteins that interact with them. Two report on BPTF (bromodomain and PHD domain transcription factor), a subunit of NURF, the nucleosome remodelling factor. It contains a domain known as a PHD finger, which is shown to bind to histone H3 trimethylated at lysine 4 (H3K4) and to maintain proper activity at developmentally critical HOX genes. The accompanying structural study of the complex explains how the site specificity for H3K4 is achieved. The two other papers reveal that the PHD domain of tumour suppressor ING2 also recognizes trimethylated H3K4, and link the histone mark to repression of transcription. The four papers together establish certain PHD finger domains as previously unrecognized chromatin-binding modules. In a News and Views piece, Peter B. Becker discusses what these papers tell us about the function of the chemical modifications of histone tails. One of four papers establishing certain PHD domains as effectors of trimethylated histone H3K4, a chromatin mark generally associated with active transcription. This paper provides structural insight into the recognition of H3K4me3 by the PHD finger of BPTF, a subunit of the chromatin-remodelling complex NURF. Mono-, di- and trimethylated states of particular histone lysine residues are selectively found in different regions of chromatin, thereby implying specialized biological functions for these marks ranging from heterochromatin formation to X-chromosome inactivation and transcriptional regulation1,2,3. A major challenge in chromatin biology has centred on efforts to define the connection between specific methylation states and distinct biological read-outs impacting on function4. For example, histone H3 trimethylated at lysine 4 (H3K4me3) is associated with transcription start sites of active genes5,6,7, but the molecular ‘effectors’ involved in specific recognition of H3K4me3 tails remain poorly understood. Here we demonstrate the molecular basis for specific recognition of H3(1–15)K4me3 (residues 1–15 of histone H3 trimethylated at K4) by a plant homeodomain (PHD) finger of human BPTF (bromodomain and PHD domain transcription factor), the largest subunit of the ATP-dependent chromatin-remodelling complex, NURF (nucleosome remodelling factor). We report on crystallographic and NMR structures of the bromodomain-proximal PHD finger of BPTF in free and H3(1–15)K4me3-bound states. H3(1–15)K4me3 interacts through anti-parallel β-sheet formation on the surface of the PHD finger, with the long side chains of arginine 2 (R2) and K4me3 fitting snugly in adjacent pre-formed surface pockets, and bracketing an invariant tryptophan. The observed stapling role by non-adjacent R2 and K4me3 provides a molecular explanation for H3K4me3 site specificity. Binding studies establish that the BPTF PHD finger exhibits a modest preference for K4me3- over K4me2-containing H3 peptides, and discriminates against monomethylated and unmodified counterparts. Furthermore, we identified key specificity-determining residues from binding studies of H3(1–15)K4me3 with PHD finger point mutants. Our findings call attention to the PHD finger as a previously uncharacterized chromatin-binding module found in a large number of chromatin-associated proteins.