Methylates (mono- and asymmetric dimethylation) the guanidino nitrogens of arginyl residues in several proteins involved in DNA packaging, transcription regulation, pre-mRNA splicing, and mRNA stability. Recruited to promoters upon gene activation together with histone acetyltransferases from EP300/P300 and p160 families, methylates histone H3 at 'Arg-17' (H3R17me), forming mainly asymmetric dimethylarginine (H3R17me2a), leading to activates transcription via chromatin remodeling. During nuclear hormone receptor activation and TCF7L2/TCF4 activation, acts synergically with EP300/P300 and either one of the p160 histone acetyltransferases NCOA1/SRC1, NCOA2/GRIP1 and NCOA3/ACTR or CTNNB1/beta-catenin to activate transcription. During myogenic transcriptional activation, acts together with NCOA3/ACTR as a coactivator for MEF2C. During monocyte inflammatory stimulation, acts together with EP300/P300 as a coactivator for NF-kappa-B. Acts as coactivator for PPARG, promotes adipocyte differentiation and the accumulation of brown fat tissue. Plays a role in the regulation of pre-mRNA alternative splicing by methylation of splicing factors. Also seems to be involved in p53/TP53 transcriptional activation. Methylates EP300/P300, both at 'Arg-2142', which may loosen its interaction with NCOA2/GRIP1, and at 'Arg-580' and 'Arg-604' in the KIX domain, which impairs its interaction with CREB and inhibits CREB-dependent transcriptional activation. Also methylates arginine residues in RNA-binding proteins PABPC1, ELAVL1 and ELAV4, which may affect their mRNA-stabilizing properties and the half-life of their target mRNAs. Acts as a transcriptional coactivator of ACACA/acetyl-CoA carboxylase by enriching H3R17 methylation at its promoter, thereby positively regulating fatty acid synthesis (By similarity). Isoform 3 specifically affects pre-mRNA splicing. This activity is independent from methyltransferase activity. L-arginyl-[protein] + 2 S-adenosyl-L-methionine = 2 H(+) + N(omega),N(omega)-dimethyl-L-arginyl-[protein] + 2 S-adenosyl-L-homocysteine Methylation of H3R17 (H3R17me) by CARM1 is stimulated by preacetylation of H3 'Lys-18' (H3K18ac) H3 'Lys-23' (H3K23ac) by EP300 and blocked by citrullination of H3 'Arg-17' (H3R17ci) by PADI4. Homodimer (Probable). Interacts with NR1H4. Interacts with the C-terminus of NCOA2/GRIP1, NCO3/ACTR and NCOA1/SRC1. Part of a complex consisting of CARM1, EP300/P300 and NCOA2/GRIP1. Interacts with FLII, TP53, myogenic factor MEF2, EP300/P300, TRIM24, CREBBP and CTNNB1. Identified in a complex containing CARM1, TRIM24 and NCOA2/GRIP1. Interacts with NCOA3/SRC3. Interacts with RELA (By similarity). Interacts with SNRPC. Interacts with SKP2 (By similarity). Interacts (via PH domain-like fold) with C9orf72 (By similarity). Mainly nuclear during the G1, S and G2 phases of the cell cycle (By similarity). Cytoplasmic during mitosis, after breakup of the nuclear membrane (By similarity). Named isoforms=2. Isoform 1 is expressed at low levels in brain, liver and testis. Isoform 2 is highly expressed in brain, liver, skeletal muscle and testis. Isoform 3 is highly expressed in spleen, liver and kidney. Isoform 4 is expressed in spleen, liver and kidney. Isoforms 2 and 3 are expressed in fetal brain. Auto-methylated on Arg-551. Methylation enhances transcription coactivator activity. Methylation is required for its role in the regulation of pre-mRNA alternative splicing (By similarity). Phosphorylation at Ser-217 interferes with S-adenosyl-L-methionine binding and strongly reduces methyltransferase activity. Phosphorylation at Ser-217 is strongly increased during mitosis, and decreases rapidly to a very low, basal level after entry into the G1 phase of the cell cycle. Phosphorylation at Ser-217 may promote location in the cytosol (By similarity). Belongs to the class I-like SAM-binding methyltransferase superfamily. Protein arginine N-methyltransferase family.