Histone modification is well established as a fundamental mechanism driving the regulation of transcription, replication, and DNA repair through the control of chromatin structure. Likewise, it is apparent that incorrect targeting of histone modifications contributes to misregulated gene expression and hence to developmental disorders and diseases of genomic instability such as cancer. The KMT2 family of SET domain methyltransferases, typified by mixed lineage leukemia protein-1 (MLL1), is responsible for histone H3 lysine 4 methylation, a marker of active genes. To ensure that this modification is correctly targeted, a multiprotein complex associates with the methyltransferase and directs activity. We have identified a novel interaction site on the core complex protein WD repeat protein-5 (WDR5), and we mapped the complementary site on its partner retinoblastoma-binding protein-5 (RbBP5). We have characterized this interaction by x-ray crystallography and show how it is fundamental to the assembly of the complex and to the regulation of methyltransferase activity. We show which region of RbBP5 contributes directly to mixed lineage leukemia activation, and we combine our structural and biochemical data to produce a model to show how WDR5 and RbBP5 act cooperatively to stimulate activity.Aberrant regulation of epigenetic networks has been identified as a key driver of many diseases, especially where genomic instability is a factor, such as in developmentally related disorders and many cancers (1, 2). The principal signaling components of these networks are covalent post-translational modifications to the side chains of residues on the histone tails that extend beyond the nucleosome core (3). These modified residues generate specific binding sites for the chromatin-associated proteins and multiprotein complexes responsible for processes such as transcription, replication, and DNA repair and are thought to act in a highly coordinated fashion (4 -6). To create and maintain the correct gene transcription profiles and to facilitate the appropriate response to environmental stimulation, it is essential that the enzymes that deposit or remove these marks be accurately targeted and their activity tightly regulated. The best conserved modification is methylation of the histone H3 lysine 4 (H3K4) 3 residue that is predominantly, although not exclusively, associated with transcriptionally active genes (7,8). Lysine side chains can be mono, di-, or trimethylated, and it is essential that the correct level of H3K4 methylation be deposited on the appropriate nucleosomes, as it has been shown that the different levels of methylation are related with differing outcomes for the associated DNA (9). The Set1/MLL (KMT2) family of methyltransferases is the principal enzyme family responsible for H3K4 methylations, and an associated multiprotein complex has evolved to ensure that this activity is tightly regulated (10 -13).The KMT2 family of H3K4 methyltransferases include six members as follows: Set1A, Set1B, and the four mixed linea...
