Arginine methylation of BAF155 regulates interactions with RNA processing machinery
Post-translational modifications (PTMs) of chromatin remodelers are abundant but functionally understudied. Here we investigate the role of asymmetric dimethylation of arginine 1064 (BAF155me2a) on the SWI/SNF core subunit BAF155, a mark deposited by CARM1/PRMT4 that has been linked to tumor progression but whose molecular function remains unclear. Using immunoprecipitation-mass spectrometry with a dimethyl-specific antibody, we found that R1064me2 selectively enhances BAF155 interactions with RNA processing factors, including the anti-termination protein SCAF4, splicing factors, and the transcription factor RFX5. CUT&RUN profiling showed that BAF155me2a, SCAF4, and RFX5 co-occupy promoter regions, and reciprocal immunoprecipitations confirmed that the SCAF4-BAF155 interaction depends on R1064 methylation. To test the functional consequences of this modification, we generated cells expressing either wild-type BAF155 or a methylation-deficient BAF155-R1064K mutant. Loss of methylation did not alter chromatin accessibility, BAF155 genomic occupancy, or SCAF4 recruitment. However, nascent transcription measured by TT-seq revealed a coordinated reduction in 5' sense transcripts and upstream antisense transcripts (PROMPTs) at BAF155-bound promoters, with a quantitatively larger decrease in PROMPTs at SCAF4 co-bound sites. The effect was restricted to the promoter-proximal region and resolved toward the gene end, consistent with a defect in productive elongation downstream of RNA polymerase II recruitment. These data support a model in which BAF155 dimethylation provides a co-transcriptional interface coupling SWI/SNF to RNA processing machinery, and identify regulation of nascent transcription as a non-canonical function of SWI/SNF PTMs.