LIG_WD40_WDR5_WIN_2

Chromatin is packaged DNA in the nuclei of eukaryotic cells made up of a complex of DNA and proteins. The nucleosome units forming the higher-order chromatin structure are composed of an octamer of four highly conserved histones around which the DNA is wound. The N-terminal tails of histones undergo multiple covalent post-translational modifications in order to secure gene regulation, such as acetylation, methylation, phosphorylation, sumoylation and ubiquitylation. Incorrect histone modifications have been associated with developmental defects and different forms of cancer (Bhaumik,2007).
One of the most conserved modifications is methylation of the histone H3 lysine 4 residue (H3K4), which can be mono-, di-, or trimethylated by the KMT2 family of SET domain methyltransferases. Depending on the amount of methylations, different reactions can take place. Hence, a tight regulation of the methyltransferases is essential. Six members belong to the KMT2/SET family (Set1a, Set1B and four mixed lineage leukemia (MLL) proteins) and each possesses a C-terminal conserved catalytic SET domain (PF00856). MLL1 is associated with expression of HOX genes and deregulation of the histone-modifying enzyme MLL1 has been linked to acute myeloid and lymphoblastic leukemia. In mice, a rearrangement of the MLL1 gene leads to defects in hematopoiesis and in skeletal development (Cosgrove,2010).
Full activity of Set1/MLL methyltransferases for H3K4 methylation can only be achieved when these enzymes are assembled in a multi-protein complex. Subunits of this conserved histone methylation core complex include WDR5 (Swd3/Cps30 in yeast), RbBP5 (Swd1/Cps50 in yeast), Ash2L (Bre2/Cps60 in yeast) and Dpy30 (Sdc1/Cps25 in yeast). The WD repeat-protein 5 (WDR5), a WD40 repeat protein forming a seven bladed beta-propeller, is a key protein in this multi-protein complex and is thought to act as a scaffolding protein in the assembly of the histone methylation core complex (Zhang,2012). The catalytic Set1/MLL subunits contain a WDR5-interacting (Win) motif (LIG_WD40_WDR5_WIN_1, LIG_WD40_WDR5_WIN_2, LIG_WD40_WDR5_WIN_3) that binds to an arginine-binding pocket on WDR5 (Dharmarajan,2012). This pocket on WDR5 has also been shown to recognize unmodified, mono-, di-, and trimethylated H3K4 peptides, implying a role for WDR5 in presenting histone H3 tails for modification (Schuetz,2006). In addition, mono- and dimethylated H3K4 peptides were shown to disrupt the WDR5-MLL interaction, suggesting fine-tuned regulation of H3K4 methylation status by a complex interplay between WDR5, MLL and histone H3 (Song,2008).
The WDR5 protein is also involved in recruitment of the accessory Retinoblastoma-binding protein 5 (RbBP5), which binds to the opposite side of WDR5 as the Set1/MLL subunit using a distinct motif (LIG_WD40_WDR5_VDV_1, LIG_WD40_WDR5_VDV_2) (Odho,2010). It has been shown that WDR5 is crucial for full methyltransferase activity, however only in combination with RbBP5 is this activity enhanced. Without the WDR5-RbBP5 interaction, the methyltransferase activity is weakened, leading to the assumption that WDR5 acts as a scaffold protein stabilizing the RbBP5 and MLL1 SET domain interaction with H3K4. RbBP5 is a nuclear protein that, like WDR5, belongs to a conserved family of WD repeat proteins. It contains an N-terminal beta-propeller domain that interacts with the SET domain and an unstructured acidic C-terminal tail containing a WDR5-binding motif. The interaction between RbBP5 and a catalytic SET1 protein family member cooperates with the motif-mediated interactions with WDR5 to assemble a stable and active histone methylation complex.
More recently, WDR5 was found to be involved in acetylation of the histone H4 lysine 16 residue (H4K16), playing a role in the assembly of the NSL (Nonspecific lethal) complex (Dias,2014). This complex contains the catalytic MOF/KAT8, which belongs to the MYST protein family of histone acetyltransferases, and additional subunits, including WDR5, KANSL1 and KANSL2. Similar to its role in histone methylation complex assembly, WDR5 functions as a scaffold protein that binds different subunits of the NSL complex. The KANSL1 subunit conatins a Win motif (LIG_WD40_WDR5_WIN_1, LIG_WD40_WDR5_WIN_2, LIG_WD40_WDR5_WIN_3), while KANSL2 binds WDR5 via a motif similar to the WDR5-binding motif of RbBP5 (LIG_WD40_WDR5_VDV_1, LIG_WD40_WDR5_VDV_2). As the Win motifs in the methyltransferases and KANSL1 bind the same site on WDR5, and similarly, the WDR5-binding motifs in RbBP5 and KANSL2 bind the same site on WDR5, these respective interactions and the assembly of the functionally distinct complexes are mutually exclusive.