The MLL1 trimeric catalytic complex is a dynamic conformational ensemble stabilized by multiple weak interactions.

Lilia Kaustov,Lixin Fan,Ruedi Aebersold,Xianyang Fang,Masoud Vedadi,Yong Wei,Hong Zeng,Alexander Lemak,Hong Wu,Shili Duan,Yunxing Wang,Abdellah Allali-Hassani,Marco Faini,Fengling Li,Cheryl H Arrowsmith,Scott Houliston

doi:10.1093/nar/gkz697

Abstract

Histone H3K4 methylation is an epigenetic mark associated with actively transcribed genes. This modification is catalyzed by the mixed lineage leukaemia (MLL) family of histone methyltransferases including MLL1, MLL2, MLL3, MLL4, SET1A and SET1B. The catalytic activity of this family is dependent on interactions with additional conserved proteins, but the structural basis for subunit assembly and the mechanism of regulation is not well understood. We used a hybrid methods approach to study the assembly and biochemical function of the minimally active MLL1 complex (MLL1, WDR5 and RbBP5). A combination of small angle X-ray scattering, cross-linking mass spectrometry, nuclear magnetic resonance spectroscopy and computational modeling were used to generate a dynamic ensemble model in which subunits are assembled via multiple weak interaction sites. We identified a new interaction site between the MLL1 SET domain and the WD40 β-propeller domain of RbBP5, and demonstrate the susceptibility of the catalytic function of the complex to disruption of individual interaction sites.

Highlights

Post-translational modifications on histone tails are key epigenetic signals for regulation of chromatin structure and gene expression
Normalized Kratky plots of WDR5WD40 and RbBP5NTD exhibit a typical bell-shape with a maximum at (1.73, 1.1) expected for globular proteins and are nearly superimposable in the q range 0
The calculated solution ensembles for each protein taking into account known or predicted disordered regions establish good correspondence between our small angle X-ray scattering (SAXS) measurements and the crystal structures of WDR5 [53], the SET domain of MLL1 [45] and the WD40 domain of RbBP5 [54] (Supplementary Figure S2)

Summary

Introduction

Post-translational modifications on histone tails are key epigenetic signals for regulation of chromatin structure and gene expression. The calculated solution ensembles for each protein taking into account known or predicted disordered regions (see Supplementary Data for details) establish good correspondence between our SAXS measurements and the crystal structures of WDR5 [53], the SET domain of MLL1 [45] and the WD40 domain of RbBP5 [54] (Supplementary Figure S2).

Results

Conclusion