DPY30 acts as an ASH2L-specific chaperone to stimulate the enzyme activity of MLL family methyltransferases on different substrates

Mixed lineage leukemia (MLL) family proteins catalyze the methylation of histone H3 lysine 4 (H3K4) and play critical roles in epigenetic regulation of gene expression. The histone lysine methyltransferase (HKMT) activity of MLL family proteins is regulated by four conserved core subunits, WDR5, RBBP5, ASH2L, and DPY30. Although DPY30 is essential for the proliferation and differentiation of embryonic stem cells, its biochemical role in maintaining the HKMT activity of MLL family complexes remains elusive. Here, we demonstrate that DPY30 plays a crucial role in regulating MLL1 activity through two complementary mechanisms: a nucleosome-independent mechanism and a nucleosome-specific mechanism. DPY30 functions as an ASH2L-specific chaperone to increase the stability of ASH2L and enhance ASH2L-mediated interactions. As a result, DPY30 promotes the compaction and stabilization of the MLL1 complex, consequently increasing the HKMT activity of the MLL1 complex on non-nucleosome substrates. DPY30-stabilized ASH2L further acquires additional interfaces with nucleosomal DNA and the H3 tail, thereby boosting the HKMT activity of the MLL1 complex on nucleosomes. The DPY30-dependent regulatory mechanism derived from studies of the MLL1 complex can be applied to other MLL family methyltransferases. These results collectively highlight the crucial and conserved roles of DPY30 in the complex assembly and activity regulation of MLL family complexes.