Coactivator associated arginine methyltransferase 1 (CARM1) is a member of the protein arginine methyltransferase (PRMT) family and methylates a range of proteins in eukaryotic cells. Overexpression of CARM1 is implicated in a number of cancers, and it is therefore seen as a potential therapeutic target. Peptide sequences derived from the well-defined CARM1 substrate poly(A)-binding protein 1 (PABP1) were covalently linked to an adenosine moiety as in the AdoMet cofactor to generate transition state mimics. These constructs were found to be potent CARM1 inhibitors and also formed stable complexes with the enzyme. High-resolution crystal structures of CARM1 in complex with these compounds confirm a mode of binding that is indeed reflective of the transition state at the CARM1 active site. Given the transient nature of PRMT–substrate complexes, such transition state mimics represent valuable chemical tools for structural studies aimed at deciphering the regulation of arginine methylation mediated by the family of arginine methyltransferases

DNA, RNA and histone methylation is implicated in various human diseases such as cancer or viral infections, playing a major role in cell process regulation, especially in modulation of gene expression. Here we developed a convergent synthetic pathway starting from a protected bromomethylcytosine derivative to synthesize transition state analogues of the DNA methyltransferases. This approach led to seven 5-methylcytosine-adenosine compounds that were, surprisingly, inactive against hDNMT1, hDNMT3Acat, TRDMT1 and other RNA human and viral methyltransferases. Interestingly, compound and its derivative showed an inhibitory activity against PRMT4 in the micromolar range. Crystal structures showed that compound binds to the PRMT4 active site, displacing strongly the-adenosyl-l-methionine cofactor, occupying its binding site, and interacting with the arginine substrate site through the cytosine moiety that probes the space filled by a substrate peptide methylation intermediate. Furthermore, the binding of the compounds induces important structural switches. These findings open new routes for the conception of new potent PRMT4 inhibitors based on the 5-methylcytosine-adenosine scaffold.This article is part of a discussion meeting issue 'Frontiers in epigenetic chemical biology'.