The purified CARM1 was enzymatically active, and capable of dose-dependent methylation of PABP1 fragment (E. around the N-terminus by 1050 amino acids, most likely due to endogenous proteolytic activity. In contrast, substantial quantities of soluble full-length CARM1 were purified from transiently transfected HEK293T cells. The CARM1 from HEK293T cells was isolated alongside a number of co-purifying interacting proteins. The covalent bond formed between the HaloTag and the HaloLink resin allowed the use of stringent wash conditions without risk of eluting the CARM1 protein. The results also illustrate a highly effective approach for purifying and enriching both CARM1-associated proteins as well as substrates for CARM1s methyltransferase activity. Keywords:CARM1, PRMT family, methyl transferase, HaloTag, HaloLink resin, affinity purification == Introduction == Protein arginine methyl transferases (PRMT) are a diverse family of proteins with at least 10 mammalian associates [1,2]. These proteins catalyze the transfer of a methyl group from S-adenosylmethionine (SAM) to diverse protein targets [3]. In turn, substrate methylation affects many different cellular processes such as RNA splicing, DNA damage repair, and transcriptional regulation [3]. Most PRMT family Platycodin D members methylate the glycine-arginine rich repeats (GAR) found in substrate proteins [4]. During the methylation reaction, the PRMT enzyme transfers one or more methyl groups from SAM to a terminal amino group(s) in the arginine sidechain around the substrate protein [4]. PRMT4, also known as coactivator – associated arginine methyltransferase 1 (CARM1), was recognized for its function as a coactivator of transcriptional activation [5]. It is the largest known PRMT enzyme (608 amino acids); CARM1 forms homodimers necessary for catalytic activity [6]. CARM1 is unique among PRMT family members in that it does not methylate the GAR motif on its substrates [7], but methylates a diverse quantity of dissimilar sequences. Its catalytic core, comprised of amino acids 150470, is highly conserved among all of the other PRMTs and is essential for the methyltransferase enzymatic activity [8]. The structure of CARM1 core, spanning amino acids 28508, has been solved [9]. Although not necessary for catalytic activity, amino acids 1130 and 480608 of CARM1 interact with coactivators and are important for transcriptional activation [10]. CARM1 is known to interact with p160 family members and assemble into large complexes composed of diverse proteins that affect transcriptional activation [11,12]. It is localized primarily in the nucleus where it methylates a large number of various substrates involved in chromatin remodeling and RNA processing [11,13,14]. Rabbit Polyclonal to PPM1L Intracellular CARM1 levels are important for estrogen receptor and androgen receptor signaling; CARM1 expression levels are altered in breast malignancy and prostate malignancy tissue [15,16]. Several PRMT family members, including CARM1, are able to transfer the methyl group from SAM onto themselves; however, the mechanism or the physiological role of automethylation is not well comprehended [1719]. In order to identify and characterize the automethylation activity of CARM1 it is necessary to express and purify the full-length protein. Previous attempts to effectively characterize the post-translational modifications on CARM1 have been hampered by Platycodin D the lack of effective purification methodologies for full-length protein. Additionally, tags such as GST [17] and Flag tag [our unpublished results] have been shown to be methylatable an artifactual finding that complicates additional analysis. The full-length CARM1 protein expressed inE. coliforms mostly insoluble inclusion body that are very hard to refold. The C-terminus, comprising the last 100 amino acids, is largely unstructured and prone to proteolysis during protein purification [9]. This region of CARM1 is likely the cause of the difficulty in purifying the full-length protein. The C-terminus has been implicated in the transcriptional coactivation of different hormone signaling pathways [10,20], and contains the putative site of automethylation [21]. Therefore it is Platycodin D important to purify full-length CARM1 protein in order to identify post-translational modifications and potential binding partners. HaloTag is usually a proprietary 34 kDa protein tag developed by the Promega Corporation (Madison, WI) for use in protein purification. This tag is based on the dehalogenase enzyme present inRhodococcusbacteria [22]. The HaloTag functions by forming a covalent bond Platycodin D between itself and a chloroalkane substrate. The high-affinity, irreversible, covalent conversation between HaloTag and chloroalkane-derivatized resin allows for quick purification of HaloTag-tagged.