This split personality is reminiscent of the master transcriptional coactivator SRC-3/AIB1, which is also sequestered in stress granules under adverse conditions (Yu et al., 2007). methyltransferases (PRMTs) modify a multitude of proteins, both in the nucleus and in the cytoplasm of the cell (Bedford and Clarke, 2009). These enzymes have been implicated in signal transduction processes, translation, DNA repair and epigenetic regulation. Three distinct types of methylarginine residues are found in mammalian cells: asymmetric dimethylarginine (ADMA), symmetric dimethylarginine (SDMA), and monomethylarginine (MMA). All three of these modifications are present on the N-terminal histone tails of H3 and H4. The coactivator-associated arginine methyltransferase (CARM1/PRMT4) deposits ADMA marks at the H3R17 & H3R26 sites (Schurter et al., 2001). PRMT1 also functions as a transcriptional coactivator and methylates H4R3me2a. PRMT5 and PRMT6 are associated with transcriptional repression, and they deposit the H4R3me2s and H3R2me2a marks, respectively (Guccione et al., 2007;Hyllus et al., 2007;Iberg et al., 2008;Pal et al., 2004). AEBSF HCl Recently, PRMT6 was also demonstrated to possess transcriptional coactivator activity at estrogen-regulated loci (Harrison et al., 2010). How these different methyl-marks are used for the interpretation of active and inactive chromatin states AEBSF HCl is a subject of great interest. In the case of PRMT6 and the H3R2me2a mark, effector molecules that are involved in transcriptional activation, like WDR5, BPTF and JMJD2a, are blocked from binding the H3K4me3 mark (Hyllus et al., 2007;Iberg et al., 2008). Recently, it was found that the PHD domain of DNMT3a binds the H4R3me2s repressive mark, which is laid down by PRMT5 (Zhao et al., 2009). This provides a very elegant link between a histone modification andde novoDNA methylation targeting. These two mechanisms of action one that blocks the recruitment of transcriptional activators, and the other that facilitates the recruitment of a AEBSF HCl transcriptional repressor help explain how certain genomic loci are transcriptionally silenced by the methylarginine marks. However, it remains unclear how the marks deposited by PRMT1 and CARM1 (H4R3me2a, H3R17me2a & H3R26me2a) help support active transcription. In a non-chromatin context, there is precedent for both ADMA and SDMA motifs binding to tudor domain-containing proteins. The tudor domain of SMN, SPF30 and TDRD3 interacts with SDMA motifs present in splicing factors like SmB (Cote and Richard, 2005;Friesen et al., 2001) and with ADMA motifs present in CA150, a transcription elongation factor (Cheng et al., 2007). All these described methylarginine-regulated protein-protein interactions are between tudor domains and glycine/arginine-rich (GAR) motifs, which harbor numerous methylated arginine residues in a patch. However, no methylarginine binding protein has yet been identified that can dock onto the isolated methyl-marks found on histone tails, with the exception of the recently described interaction between the PHD domain of DNMT3a and the H4R3me2s repressive mark (Zhao et al., 2009). In order to identify effector molecules EGFR that can read methylarginine marks associated with transcriptional activation, we screen a protein domain microarray (CADOR array) with a peptide corresponding to the primary histone methylation site of CARM1, H3R17me2a. This screen identified TDRD3 as a H3R17me2a binder. TDRD3 can also bind the H4R3me2a mark, which is catalyzed by PRMT1. Importantly, in transcriptional reporter assays, TDRD3 functions as a coactivator, whereas other known methylarginine-binding proteins, SMN and SPF30, do not. This finding is consistent with the idea that TDRD3 binds marks deposited by the coactivators, CARM1 and PRMT1, and interprets them. In this regard, we show that TDRD3 is associated with the pS2 promoter in AEBSF HCl a CARM1 dependent manner. Using an unbiased approach (ChIP-Seq), we further find that TDRD3 is generally associated with transcriptional start sites. This study describes an effector molecule for methylarginine marks that are associated with transcriptional activation. == Results == == Protein domain microarrays identify TDRD3 as a reader for CARM1/PRMT1 marks == The CADOR microarray has been used to identify novel methyllysine binding domains that interact with histone tail peptides (Kim et al., 2006), methylated nucleosomes (Schotta et al., 2008) and non-histone proteins (Huang et al., 2007). Here, for the first time we used this approach to screen for protein domains that can bind.