PRMT5 directly dimethylates arginines of BCL6 in lymphoma. (A) IP for BCL6 and symmetric arginine dimethylation (SYM10) in lymphoma cell lines OCI-LY1 and Raji reveals symmetric arginine dimethylation of BCL6. (B) Knockdown of PRMT5 with specific siRNA decreases BCL6 symmetric arginine methylation. Raji cells were transfected with PRMT5 or control siRNAs followed by IP with symmetric arginine dimethylation (SYM10) antibody and immunoblotting with BCL6 antibody. Also shown are western blots with indicated antibodies from the same cells. (C) In vitro methyltransferase assay with recombinant PRMT5, MEP50, and BCL6 or H2A proteins. The proteins were blotted with the indicated antibodies. In addition, the reaction mixture was immunoprecipitated with BCL6 antibody and blotted with antibodies for symmetric arginine dimethylation (SYM10) and BCL6. (D) In vitro thymidine incorporation methyltransferase assay with recombinant PRMT5 and BCL6 or H2A proteins.