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The methylation of arginine residues in numerous protein targets is a post-translational modification that has gained increased interest in the scientific community over the past two decades. Arginine methylation is performed by the dedicated family of protein arginine methyltransferases and is known to be involved in a plethora of cellular pathways and biochemical mechanisms in both healthy and disease states. The development of inhibitors for these enzymes for use as biological tools can lead to a more detailed understanding of the functions of the different members of the PRMT family. In addition, a number of recent studies point towards PRMTs as therapeutic targets for a number of diseases and the first clinical trials with compounds inhibiting PRMTs are now underway. We here provide a broad overview of the current status of the inhibitors that have been developed against PRMTs using both high-throughput screening and rational design approaches.

Protein arginine N-methyltransferases (PRMTs) catalyze the post-translational methylation of arginine residues within substrate proteins. Their roles in the epigenetic regulation of gene expression make them viable targets for drug discovery. Peptides containing a single arginine residue substituted at the guanidino nitrogen (N(η)) with an ethyl group bearing zero to three fluorine atoms (R1-1, -2, -3, and -4) have been synthesized and tested for methylation and inhibition activity with PRMT1, PRMT6, and CARM1. Only the nonfluorinated R1-1 peptide is methylated by PRMT1, demonstrating that the N(η)-substituted arginine is accommodated by its active site. The R1-1 ethyl-substituted guanidine N(η) was further identified as the methylation site via mass spectrometry. Although weak inhibitors of CARM1, R1-1, -2, -3, and -4 are potent inhibitors of PRMT1 and PRMT6. These peptides are more potent against PRMT1 than product inhibitor peptides, showing that N(η)-substituted arginyl peptides do not work by a purely product inhibitor mechanism. A trend of increasing potency with an increase in the number of fluorine atoms is observed for PRMT1, which may result from the corresponding change in the guanidino dipole moment. Modeling of the ethyl-arginine moiety of the R1-1 peptide demonstrates that the active site of PRMT1 accommodates such modifications. N(η)-Substituted arginyl peptides represent lead compounds for the further development of inhibitors that target the methyl-acceptor binding site of PRMTs.

This thesis describes the antimicrobial discovery strategy developed in our group, the den Hertog Group at the Hubrecht Institute. It includes a cultivation-based screening approach for novel antimicrobial agents from the source of fungi, and a bacterial time-lapse imaging approach for antimicrobial mechanism of action (MoA) identification. With this strategy, we have discovered several interesting antimicrobial agents and have demonstrated the detailed antimicrobial property of two of them, berkchaetoazaphilone B (BAB) and harzianic acid (HA).

The studies presented in the work show the potential of the integrative use of biophysical data in defining the structural basis of protein interactions. Even if the results obtained hold a degree of ambiguity, this approach allows to iteratively refine and validate the model and interpret its meaning for the molecular basis of protein function. Often all three points at the same time. This dynamic nature makes the use of structural models in the design of therapeutic compounds especially useful since the inhibition of a certain protein function might not require a structure to be accurate down to the last atom but rather highlight key interactions or structural features that can be addressed in context of small molecule or peptide inhibitors. Presented are the use of strucutral biochemistry techniques to investigate the mechanism of how the ubiquitine ligase PSIP1 obtains its target specificity. Furthermore, another epigenetic effector protein PSIP1 is investigated with the aim to develop a workflow for the design of potential peptide-based inhibitors.