Type I PRMTs and PRMT5 Inversely Regulate Post-Transcriptional Intron Detention

Protein arginine methyltransferases (PRMTs) are required for the regulation of RNA processing factors. Type I enzymes catalyze mono- and asymmetric dimethylation; Type II enzymes catalyze mono- and symmetric dimethylation. To understand the specific mechanisms of PRMT activity in splicing regulation, we inhibited Type I and II PRMTs and probed their transcriptomic consequences. Using the newly developed SKaTER-seq method, analysis of co-transcriptional splicing revealed that PRMT inhibition resulted in slower splicing rates. Surprisingly, altered co-transcriptional splicing kinetics correlated poorly with ultimate changes in alternative splicing of polyadenylated RNA--particularly intron retention (RI). Investigation of RI following inhibition of nascent transcription demonstrated that PRMTs inversely regulate RI post-transcriptionally. Subsequent proteomic analysis of chromatin-associated polyadenylated RNA identified aberrant binding of the Type I substrate, CHTOP, and the Type II substrate, SmB. Targeted mutagenesis of all methylarginine sites in SmD3, SmB, and SmD1 recapitulated splicing changes seen with Type II PRMT inhibition. Conversely, mutagenesis of all methylarginine sites in CHTOP recapitulated the splicing changes seen with Type I PRMT inhibition. Closer examination of subcellular fractions indicated that RI were isolated to the nucleoplasm and chromatin. Together, these data demonstrate that PRMTs regulate the post-transcriptional processing of nuclear, detained introns through Sm and CHTOP arginine methylation.