Characterization of a Novel WDR5-binding Site That Recruits RbBP5 through a Conserved Motif to Enhance Methylation of Histone H3 Lysine 4 by Mixed Lineage Leukemia Protein-1*

Abstract

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...

Charge-based Interaction Conserved within Histone H3 Lysine 4 (H3K4) Methyltransferase Complexes Is Needed for Protein Stability, Histone Methylation, and Gene Expression

“…Several biochemical and structural studies have shown that the ␤-propeller-like structure of WDR5 has an arginine binding pocket that can associate with N-terminal residues of histone H3 and MLL by its WDR5 interaction motif (7,11,(13)(14)(15)(16)(17)(18)(19). Unlike WDR5, the C terminus of RBBP5, a region outside of the WD40 domains, is important for protein-protein interactions and can interact with ASH2L and WDR5 (7,20). Other studies have also shown that RBBP5, together with WDR5, ASH2L, and DPY30, significantly facilitates MLL1 methyltransferase activity on histone H3K4, suggesting that RBBP5 forms a stable complex with WDR5, ASH2L, and DPY30 (21,22).…”

Structural analysis of the KANSL1/WDR5/KANSL2 complex reveals that WDR5 is required for efficient assembly and chromatin targeting of the NSL complex

“…Within the MLL complexes, WDR5 recognizes both a WIN (WDR5-interacting) motif of MLL proteins and a short motif of RbBP5. These interactions stimulate the methyltransferase activity of MLL proteins (Patel et al 2008b;Odho et al 2010;Avdic et al 2011). The NSL and MLL/COMPASS complexes were proposed to form a larger assembly that would possess both H3K4 methylation and H4 acetylation activities (Dou et al 2005;Li et al 2009).…”

A phosphorylation switch on RbBP5 regulates histone H3 Lys4 methylation

“…WDR5 is crucial for the structural integrity of the complex and acts as a bridge linking each member of the KMT2 family (Dharmarajan et al 2012;Zhang et al 2012) to the regulatory subunits RbBP5, Ash2L, and DPY-30 (Odho et al 2010;Avdic et al 2011). In RbBP5, a predicted unstructured region binds to Ash2L and WDR5 and is important for the stimulation of MLL1 methyltransferase activity (Cao et al 2010;Avdic et al 2011).…”