‘De novo DNA methyltransferase activity in chromatin: a tale of two N-terminal regions

Host: Giulia Saredi 

Venue: MSI Small Lecture Theatre, SLS

Abstract: 

DNA methylation occurs at upwards of 80% of CpG sites, catalysed by the de novo DNA methyltransferases DNMT3A and DNMT3B. However, not all CpG sites on the genome are equally methylated, which affects gene expression and subsequent cell fate determination. DNA methylation is inextricably linked to the chromatin environment with targeting conferred by both resident histone marks and direct chromatin interaction. However, few molecular details are available to describe how this process is controlled and balanced on chromatin. 

I will present our recent biochemical and structural work characterising DNMT3A and DNMT3B recruitment and activity on chromatin. Using chemical biology approaches we can reconstitute modified chromatin and recapitulate in vivo binding patterns. We have identified the highly divergent N-terminal regions of DNMT3A and DNMT3B as important for chromatin interaction. 

To ensure robust gene repression DNA methyltransferases must be targeted to heterochromatic regions. We have found this is mediated by the N-terminal regions, but with very different mechanisms utilised for each of the paralogs. Tissue-specific recruitment to facultative heterochromatin of DNMT3A is mediated via direct readout of ubiquitylated H2A, expanding the complexity of the epigenetic recognition found within the DNMT3A enzyme. In contrast DNMT3B N-terminal region does not interact directly with heterochromatin, but with a heterochromatic protein HP1, explaining recruitment to factitive heterochromatin. Furthermore, our biochemical reconstitution shows a disconnect between recruitment and catalytic activity, explaining how epigenetic marks are driven to specific genomic loci during development