Protein histidine methylation is a rare post-translational modification of unknown biochemical importance. In vertebrates, only a few methylhistidine-containing proteins have been reported, including β-actin as an essential example. The evolutionary conserved methylation of β-actin H73 is catalyzed by an as yet unknown histidine N-methyltransferase. We report here that the protein SETD3 is the actin-specific histidine N-methyltransferase. In vitro, recombinant rat and human SETD3 methylated β-actin at H73. Knocking-out SETD3 in both human HAP1 cells and in Drosophila melanogaster resulted in the absence of methylation at β-actin H73 in vivo, whereas β-actin from wildtype cells or flies was > 90% methylated. As a consequence, we show that Setd3-deficient HAP1 cells have less cellular F-actin and an increased glycolytic phenotype. In conclusion, by identifying SETD3 as the actin-specific histidine N-methyltransferase, our work pioneers new research into the possible role of this modification in health and disease and questions the substrate specificity of SET-domain-containing enzymes.

G-quadruplexes (G4s) are noncanonical nucleic acid structures involved in the regulation of key cellular processes, such as transcription and replication. Since their discovery, G4s have been mainly investigated for their role in cancer and as targets in anticancer therapy. More recently, exploration of the presence and role of G4s in viral genomes has led to the discovery of G4-regulated key viral pathways. In this context, employment of selective G4 ligands has helped to understand the complexity of G4-mediated mechanisms in the viral life cycle, and highlighted the possibility to target viral G4s as an emerging antiviral approach. Research in this field is growing at a fast pace, providing increasing evidence of the antiviral activity of old and new G4 ligands. This review aims to provide a punctual update on the literature on G4 ligands exploited in virology. Different classes of G4 binders are described, with emphasis on possible antiviral applications in emerging diseases, such as the current COVID-19 pandemic. Strengths and weaknesses of G4 targeting in viruses are discussed.

Protein methylation is one of the most common post-translation modification (PTM) in eukaryotic cells [for review see Clarke, 2013] and histones are probably the best studied substrates of protein Nmethyltransferases. Much is therefore known about methylation of lysine and arginine residues in histones as well as the fundamental role of such modifications in the epigenetic control of mammalian gene expression [Greer and Shi, 2012]. However, an increasing number of studies show methylation of non-histone proteins as a prevalent PTM that regulates diverse biological processes, including protein synthesis and signal transduction [for review see Biggar and Li, 2015]. More interestingly, non-histone proteins may be modified on atypical sites for methylation such as glutamate, glutamine, cysteine, and histidine residues [Clarke, 2013]. Methylation of histidine residues has been long reported just for a few proteins in Nature, including -actin [Johnson et al., 1967], S100A9 protein [Raftery et al., 1996], myosin [Elzinga and Collins, 1977] and its kinase [Meyer i Mayr, 1987], and ribosomal protein RPL3 [Webb et al., 2010], whereas a recent study of Ning and coworkers [2016] suggests that such modification may be a quite common phenomenon, involving dozens if not hundreds of intracellular proteins in mammalian cells.Knowledge of protein histidine N-methyltransferases is rather limited, and yeast YIL110W (Hpm1p) protein, which catalyzes methylation of histidine 243 in ribosomal RPL3 protein, remains the first and only histidine protein methyltransferase molecularly identified up to date [Webb et al, 2010].The enzyme responsible for actin histidine methylation has been partially purified, but no gene encoding this enzyme has been identified yet [Vijayasarathy and Rao, 1987; Raghavan et al., 1992].Actin is one of the most abundant protein in eukaryotic cells and a major component of the cytoskeleton [for review, Pollard and Cooper, 2009]. It is highly evolutionary conserved (90% identity between yeast and human protein), and in vertebrates, three main isoforms of this protein differing by only a few amino acids at their N-terminus have been identified. -Actins are expressed in skeletal, cardiac and smooth muscle, while -and -isoforms are present in nonmuscle and muscle cells. Under physiological conditions, actin exists as a 42-kDa monomeric globular protein (G-actin) that binds ATP and spontaneously polymerize into stable filaments (F-actin). Once filament has been assembled, the terminal phosphate of the bound ATP is hydrolyzed and slowly released from the protein, leading to the sequential depolymerization of the filament. The maintenance of a pool of actin monomers, the initialization of polymerization process as well as the regulation of assembly and turnover of filaments are controlled and tuned by more than 100 interacting partner proteins, making actin to contribute to more protein-protein interactions than any other protein does [Lappalainen, 2016].As actin is essential for the survival of cells, particip...

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