Arginine methyltransferase that can both catalyze the formation of omega-N monomethylarginine (MMA) and symmetrical dimethylarginine (sDMA). Specifically mediates the symmetrical dimethylation of SF3B2. Involved in the regulation of alternative splicing of pre-mRNA (PubMed:25737013, PubMed:25979344). L-arginyl-[protein] + 2 S-adenosyl-L-methionine = 2 H(+) + N(omega),N(omega)'-dimethyl-L-arginyl-[protein] + 2 S-adenosyl-L-homocysteine Optimum pH is between is between 7.5 and 8.0. Optimum temperature is 37 degrees Celsius. Found in a complex with PRMT9, SF3B2 and SF3B4 (PubMed:25737013). Interacts with SF3B2 (PubMed:25737013). Belongs to the class I-like SAM-binding methyltransferase superfamily. Protein arginine N-methyltransferase family. This protein should not be confused with FBXO11 (AC Q86XK2) that was initially erroneously named PRMT9. Intron retention.