Abstract
MLL1 belongs to the SET1 family of histone H3 lysine 4 (H3K4) methyltransferases, composed of MLL1–4 and SETd1A/B. MLL1 translocations are present in acute leukemias, and mutations in several family members are associated with cancer and developmental disorders. MLL1 associates with a subcomplex containing WDR5, RbBP5, ASH2L, and DPY-30 (WRAD), forming the MLL1 core complex required for H3K4 mono- and dimethylation and transcriptional activation. Core complex assembly requires interaction of WDR5 with the MLL1 Win (WDR5 interaction) motif, which is conserved across the SET1 family. Agents that mimic the SET1 family Win motif inhibit the MLL1 core complex and have become an attractive approach for targeting MLL1 in cancers. Like MLL1, other SET1 family members interact with WRAD, but the roles of the Win motif in complex assembly and enzymatic activity remain unexplored. Here, we show that the Win motif is necessary for interaction of WDR5 with all members of the human SET1 family. Mutation of the Win motif-WDR5 interface severely disrupts assembly and activity of MLL1 and SETd1A complexes but only modestly disrupts MLL2/4 and SETd1B complexes without significantly altering enzymatic activity in vitro. Notably, in the absence of WDR5, MLL3 interacts with RAD and shows enhanced activity. To further probe the role of the Win motif-WDR5 interaction, we designed a peptidomimetic that binds WDR5 (Kd ∼3 nm) and selectively inhibits activity of MLL1 and SETd1A core complexes within the SET1 family. Our results reveal that SET1 family complexes with the weakest Win motif-WDR5 interaction are more susceptible to Win motif-based inhibitors.
Highlights
Mixed Lineage Leukemia 1 (MLL1) protein is a member of the SET1 family of histone methyltransferases
We previously found that Win motif peptides derived from the human MLL1 and SETd1A sequences bind WDR5 with significantly weaker affinity (Kd ϭ 2.8 and 0.5 M, respectively) compared with that of peptides derived from human MLL2– 4 and SETd1B sequences (Kd ϭ 0.05– 0.1 M) (Table 1) [49]
Previous studies implicate WDR5 in bridging interactions between SET domain and the RbBP5/ASH2L heterodimer within the MLL1 core complex [18, 24], but other studies suggest that it has additional roles in gene targeting
Summary
H3K4, histone H3 lysine 4; H3K4me0, unmethylated H3K4; H3K4me, H3K4me, and H3K4me, H3K4 mono-, di-, and trimethylation, respectively; AML, acute myeloid leukemia; ITC, isothermal titration calorimetry; LSC, liquid scintillation counting; BisTris, 2-[bis (2-hydroxyethyl)amino]-2-(hydroxymethyl)propane-1,3-diol. Most genetic rearrangements of MLL1 result in translocations that fuse the N-terminal fragment of the MLL1 protein (lacking the SET domain) to one of ϳ70 known fusion partners [39] (Fig. 1B). One such type of MLL1 fusion, MLL1AF9, exhibits a dominant gain-of-function phenotype that contributes to AML leukemogenesis in mice [40]. This work reveals that the contributions of the Win motif-WDR5 interaction to complex assembly differ among the human SET1 family members and that such differences can be exploited to alter the enzymatic activities of a subset of SET1 family core complexes. Our results reveal that the MLL1 and SETd1A complexes that bind WDR5 with the weakest affinity are most sensitive to inhibition by molecules that mimic the Win motif
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