Mechanistic Insights into the Stimulation of the Histone H3K9 Methyltransferase Clr4 by Proximal H3K14 Ubiquitination

H3K9 methylation is an evolutionarily conserved hallmark of heterochromatin and plays crucial roles in chromosome segregation, genome stability, and gene expression regulation. Clr4 is the sole histone methyltransferase responsible for catalyzing H3K9 methylation in Schizosaccharomyces pombe. Clr4 K455/K472 automethylation and histone H3K14 ubiquitination (H3K14Ub) are vital activators of the catalytic activity of Clr4, ensuring appropriate heterochromatin deposition and preventing deleterious gene silencing. While the mechanism by which automethylation activates Clr4 was recently elucidated, the mechanism of the significantly pronounced stimulatory effect of H3K14Ub on Clr4 remains unclear. Here we determined the crystal structures of Clr4 bound to ubiquitinated and unmodified H3 peptides at resolutions of 2.60 and 2.39 angstrom, respectively. Our structures reveal a synergistic mechanism underlying the stronger stimulatory effect by H3K14Ub compared to automethylation: site-specific ubiquitination constrained by the H3K14 linkage increases substrate affinity through multivalent interactions between ubiquitin and Clr4. Additionally, H3K14Ub facilitates the allosteric transition of Clr4 from an inactive apo conformation to a hyperactive "catalyzing state", accompanied by conformational changes in the αC-SET-insertion (SI) region, complete release of the autoregulatory loop (ARL), and retraction of the β9/10 loop. Finally, we propose a structural model for the Clr4 catalytic-regulatory cycle, depicting varying levels of conformational regulation mediated by automethylation and ubiquitination. This work provides structural insights into the interplay between different histone modifications and their collective impact on epigenetic regulation.