Poly-GR Impairs\nPRMT1-Mediated Arginine Methylation\nof Disease-Linked RNA-Binding Proteins by Acting as a Substrate Sink

Dipeptide repeat proteins (DPRs) are aberrant protein\nspecies found\nin C9orf72-linked amyotrophic lateral sclerosis (ALS) and frontotemporal\ndementia (FTD), two neurodegenerative diseases characterized by the\ncytoplasmic mislocalization and aggregation of RNA-binding proteins\n(RBPs). In particular, arginine (R)-rich DPRs (poly-GR and poly-PR)\nhave been suggested to promiscuously interact with multiple cellular\nproteins and thereby exert high cytotoxicity. Components of the protein\narginine methylation machinery have been identified as modulators\nof DPR toxicity and/or potential cellular interactors of R-rich DPRs;\nhowever, the molecular details and consequences of such an interaction\nare currently not well understood. Here, we demonstrate that several\nmembers of the family of protein arginine methyltransferases (PRMTs)\ncan directly interact with R-rich DPRs in vitro and in the cytosol.\nIn vitro, R-rich DPRs reduce solubility and promote phase separation\nof PRMT1, the main enzyme responsible for asymmetric arginine-dimethylation\n(ADMA) in mammalian cells, in a concentration- and length-dependent\nmanner. Moreover, we demonstrate that poly-GR interferes more efficiently\nthan poly-PR with PRMT1-mediated arginine methylation of RBPs such\nas hnRNPA3. We additionally show by two alternative approaches that\npoly-GR itself is a substrate for PRMT1-mediated arginine dimethylation.\nWe propose that poly-GR may act as a direct competitor for arginine\nmethylation of cellular PRMT1 targets, such as disease-linked RBPs.