No abstract

DNA topoisomerase 3B (TOP3B) is unique among all mammalian topoisomerases for its dual activities that resolve both DNA and RNA topological entanglements to facilitate transcription and translation. However, the mechanism by which TOP3B activity is regulated is still elusive. Here, we have identified arginine methylation as an important post-translational modification (PTM) for TOP3B activity. Protein arginine methyltransferase (PRMT) 1, PRMT3 and PRMT6 all methylate TOP3B in vitro at its C-terminal arginine (R) and glycine (G)-rich motif. Site-directed mutagenesis analysis identified R833 and R835 as the major methylation sites. Using a methylation-specific antibody, we confirmed that TOP3B is methylated in cells and that mutation of R833 and R835 to lysine (K) significantly reduces TOP3B methylation. The methylation-deficient TOP3B (R833/835K) is less active in resolving negatively supercoiled DNA, which consequently lead to accumulation of co-transcriptionally formed R-loops in vitro and in cells. Additionally, the methylation-deficient TOP3B (R833/835K) shows reduced stress granule localization, indicating that methylation is critical for TOP3B function in translation regulation. Mechanistically, we found that R833/835 methylation is partially involved in the interaction of TOP3B with its auxiliary factor, the Tudor domain-containing protein 3 (TDRD3). Together, our findings provide the first evidence for the regulation of TOP3B activity by PTM.

The N6‐methyladenosine (m6A) RNA modification serves crucial functions in RNA metabolism; however, the molecular mechanisms underlying the regulation of m6A are not well understood. Here, we establish arginine methylation of METTL14, a component of the m6A methyltransferase complex, as a novel pathway that controls m6A deposition in mammalian cells. Specifically, protein arginine methyltransferase 1 (PRMT1) interacts with, and methylates the intrinsically disordered C terminus of METTL14, which promotes its interaction with RNA substrates, enhances its RNA methylation activity, and is crucial for its interaction with RNA polymerase II (RNAPII). Mouse embryonic stem cells (mESCs) expressing arginine methylation‐deficient METTL14 exhibit significantly reduced global m6A levels. Transcriptome‐wide m6A analysis identified 1,701 METTL14 arginine methylation‐dependent m6A sites located in 1,290 genes involved in various cellular processes, including stem cell maintenance and DNA repair. These arginine methylation‐dependent m6A sites are associated with enhanced translation of genes essential for the repair of DNA interstrand crosslinks; thus, METTL14 arginine methylation‐deficient mESCs are hypersensitive to DNA crosslinking agents. Collectively, these findings reveal important aspects of m6A regulation and new functions of arginine methylation in RNA metabolism.