Ubiquitin ligase component Cul4 associates with Clr4 histone methyltransferase to assemble heterochromatin

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Abstract
In eukaryotes, heterochromatin mediates diverse processes including gene silencing and regulation of long-range chromatin interactions1,2. The formation of heterochromatin involves a conserved array of histone modifications; in particular, methylation of histone H3 at Lys 9 (H3K9me) is essential for recruiting HP1/Swi6 proteins3. In fission yeast, the Clr4 methyltransferase is responsible for H3K9me across all heterochromatic domains4,5. However, the mechanism of Clr4 recruitment to these loci is poorly understood. We show that Clr4 associates with Cul4, a cullin family protein that serves as a scaffold for assembling ubiquitin ligases. Mutations in Cul4 result in defective localization of Clr4 and loss of silencing at heterochromatic loci. This is accompanied by a severe reduction in H3K9me and Swi6 levels, and accumulation of transcripts corresponding to naturally silenced repeat elements within heterochromatic domains. Moreover, heterochromatin defects in Cul4 mutants could not be rescued by expression of Cul4 protein lacking Nedd8 modification, which is essential for its ubiquitin ligase activity. Rik1, a protein related to DNA damage binding protein DDB1 and required for H3K9me4,6, also interacts with Cul4, the association of which might serve to target Clr4 to heterochromatic loci. These analyses uncover a role for Cul4-based protein ubiquitination in regulating H3K9me and heterochromatin formation.