Regulating the Activation of Ash1/Ash1L Histone Methyltransferase by Intrinsically Disordered Regions

Abstract

Gene transcription can be regulated by post-translational modification of histones on the nucleosome. For example, methylation of H3 at lysines 4 and 36 transcriptionally activate chromatin. Drosophila Ash1 and its mammalian homolog Ash1L are histone methyltransferases (HMTase) that catalyze methylation of lysine36 on histone H3. The HMTase activity of Ash1/Ash1L is conferred by the SET domain which is regulated by the N-terminal AWS region and the C-terminal postSET region. However, the HMTase activity of Ash1/Ash1L is very weak as an auto-inhibitory (AI) loop blocks the substrate-binding site. It is found that Ash1/Ash1L interacts with other proteins and binding with Mrg15 transfers Ash1/Ash1L from the auto-inhibited state to the activated state. Interactions between Ash1/Ash1L and Mrg15 are primarily mediated by the conserved FxLP motif preceding the Ash1L AWS and SET domains and the MRG domain of Mrg15. However, the molecular mechanism through which Mrg15 activates Ash1/Ash1L remained unresolved. In this work, we performed molecular dynamics simulations to investigate the recognition mechanism of Ash1L with Mrg15. All-atom replica-exchange simulations of the free MRG-binding region of Ash1L showed that the MRG-binding region is highly disordered in the unbound state, although it folds into two alpha-helices upon binding to Mrg15. The N-terminal helical segment is unstable in simulation and samples mainly coil conformations. Although the C-terminal helical segment is more stable, the helical conformations are shorter than those observed in the crystal structures. Simulations of Ash1L/Mrg15 complex reveal that the complex structure is heterogeneous and the N-terminus of Ash1L remains highly dynamic. Taken together, our results suggest that recognition of Ash1L with Mrg15 is mediated by intrinsically disordered regions via fuzzy binding mechanism.