Arginine methyltransferase that can both catalyze the formation of omega-N monomethylarginine (MMA) and symmetrical dimethylarginine (sDMA), with a preference for the formation of MMA. Specifically mediates the symmetrical dimethylation of arginine residues in the small nuclear ribonucleoproteins; such methylation being required for the assembly and biogenesis of snRNP core particles. Methylates the arginine in the motif G-R-G-X-G in its substrates histone H2A, H2AX and H4, producing both monomethylated and symmetrically dimethylated 'Arg-3'. Methylates nucleoplasmin at 'Arg-192', producing both monomethylated and symmetrically dimethylated 'Arg-192'. Involved in the DNA replication checkpoint. Promotes entry into mitosis by promoting the proteasomal degradation of wee2-a. May act as a transcriptional corepressor in CRY1-mediated repression of the core circadian component PER1. Involved in spliceosome maturation and mRNA splicing (By similarity). L-arginyl-[protein] + 2 S-adenosyl-L-methionine = 2 H(+) + N(omega),N(omega)'-dimethyl-L-arginyl-[protein] + 2 S-adenosyl-L-homocysteine Heterotetramer; dimer of heterodimer with wdr77. Interacts with wee2-a; this interaction is disrupted upon activation of the DNA replication checkpoint. Enriched on chromatin. Detected in egg (at protein level). Belongs to the class I-like SAM-binding methyltransferase superfamily. Protein arginine N-methyltransferase family.