Structural and biochemical studies on the complex of ASH1L and its catalytic activator MRG15

Abstract

Nucleosome is composed of 146 base pairs of DNA wrapped around an octamer of histone proteins that comprises two copies of each of the core histones H2A, H2B, H3, and H4. Flexible N-terminal tails of histones that protrude from the nucleosome are subject to various post-translational modifications, including methylation, acetylation, phosphorylation or ubiquitination. Modifications occurring on histone H3 are responsible for either transcriptional repression or transcriptional activation in organisms from yeast to human. Trimethylation of lysines 9 and 27, H3K27me3 and H3K27me3 respectively, have been functionally related to transcriptionally repressed chromatin. On the other hand, methylation of H3 at lysines 4 and 36, H3K4me and H3K36me respectively, are critical for transcriptionally active chromatin. ASH1L is the catalytic subunit of the conserved histone methyltransferase (HMTase) complex AMC that dimethylates lysine 36 in histone H3 (H3K36me2) to promote gene transcription in mammals and flies. Unlike AMC, ASH1L alone shows poor catalytic activity. In previous research, it was found that this is due to an autoinhibitory loop (AI loop) from the postSET domain blocking an access to its substrate binding pocket in ASH1L. Here, I report the crystal structure of the minimal catalytic active AMC complex containing ASH1L and its partner subunit MRG15. The structure reveals how binding of the MRG domain of MRG15 to a conserved FxLP motif in ASH1L results in the displacement of the AI loop to permit substrates to access the catalytic pocket of the ASH1L SET domain. Together, ASH1L activation by MRG15 therefore represents a delicate regulatory mechanism for how a cofactor activates an SET domain HMTase by releasing autoinhibition.