Physical and Functional Association of a Trimethyl H3K4 Demethylase and Ring6a/MBLR, a Polycomb-like Protein

Histone methylation is a posttranslational modification regulating chromatin structure and gene regulation. BHC110/LSD1 was the first histone demethylase described to reverse dimethyl histone H3 lysine 4 (H3K4). Here we show that JARID1d, a JmjC-domain-containing protein, specifically demethylates trimethyl H3K4. Detailed mapping analysis revealed that besides the JmjC domain, the BRIGHT and zinc-finger-like C₅HC₂ domains are required for maximum catalytic activity. Importantly, isolation of native JARID1d complexes from human cells revealed the association of the demethylase with a polycomb-like protein Ring6a/MBLR. Ring6a/MBLR not only directly interacts with JARID1d but also regulates its enzymatic activity. We show that JARID1d and Ring6a occupy human Engrailed 2 gene and regulate its expression and H3K4 methylation levels. Depletion of JARID1d enhanced recruitment of the chromatin remodeling complex, NURF, and the basal transcription machinery near the transcriptional start site, revealing a role for JARID1d in regulation of transcriptional initiation through H3K4 demethylation.