PRMT1 is the predominant type I protein arginine methyltransferase in mammalian cells

Promiscuous modification of the nuclear poly(A)-binding protein by multiple protein-arginine methyltransferases does not affect the aggregation behavior

The mammalian nuclear poly(A)-binding protein, PABPN1, carries 13 asymmetrically dimethylated arginine residues in its C-terminal domain. By fractionation of cell extracts, we found that protein-arginine methyltransferases (PRMTs)-1, -3, and -6 are responsible for the modification of PABPN1. Recombinant PRMT1, -3, and -6 also methylated PABPN1. Our data suggest that these enzymes act on their own, and additional polypeptides are not involved in recognizing PABPN1 as a substrate. PRMT1 is the predominant methyltransferase acting on PABPN1. Nevertheless, PABPN1 was almost fully methylated in a Prmt1(-/-) cell line; thus, PRMT3 and -6 suffice for methylation. In contrast to PABPN1, the heterogeneous nuclear ribonucleoprotein (hnRNP) K is selectively methylated only by PRMT1. Efficient methylation of synthetic peptides derived from PABPN1 or hnRNP K suggested that PRMT1, -3, and -6 recognize their substrates by interacting with local amino acid sequences and not with additional domains of the substrates. However, the use of fusion proteins suggested that the inability of PRMT3 and -6 to modify hnRNP K is because of structural masking of the methyl-accepting amino acid sequences by neighboring domains. Mutations leading to intracellular aggregation of PABPN1 cause the disease oculopharyngeal muscular dystrophy. The C-terminal domain containing the methylated arginine residues is known to promote PAPBN1 self-association, and arginine methylation has been reported to inhibit self-association of an orthologous protein. Thus, arginine methylation might be relevant for oculopharyngeal muscular dystrophy. However, in two different types of assays we have been unable to detect any effect of arginine methylation on the aggregation of bovine PABPN1.

Methylation of RUNX1 by PRMT1 abrogates SIN3A binding and potentiates its transcriptional activity

RUNX1/AML1 is required for the development of definitive hematopoiesis, and its activity is altered by mutations, deletions, and chromosome translocations in human acute leukemia. RUNX1 function can be regulated by post-translational modifications and protein-protein interactions. We show that RUNX1 is arginine-methylated in vivo by the arginine methyltransferase PRMT1, and that PRMT1 serves as a transcriptional coactivator for RUNX1 function. Using mass spectrometry, and a methyl-arginine-specific antibody, we identified two arginine residues (R206 and R210) within the region of RUNX1 that interact with the corepressor SIN3A and are methylated by PRMT1. PRMT1- dependent methylation of RUNX1 at these arginine residues abrogates its association with SIN3A, whereas shRNA against PRMT1 (or use of a methyltransferase inhibitor) enhances this association. We find arginine-methylated RUNX1 on the promoters of two bona fide RUNX1 target genes, CD41 and PU.1 and show that shRNA against PRMT1 or RUNX1 down-regulates their expression. These arginine methylation sites and the dynamic regulation of corepressor binding are lost in the leukemia-associated RUNX1-ETO fusion protein, which likely contributes to its dominant inhibitory activity.

Histone modifications and chromatin dynamics: a focus on filamentous fungi

The readout of the genetic information of eukaryotic organisms is significantly regulated by modifications of DNA and chromatin proteins. Chromatin alterations induce genome-wide and local changes in gene expression and affect a variety of processes in response to internal and external signals during growth, differentiation, development, in metabolic processes, diseases, and abiotic and biotic stresses. This review aims at summarizing the roles of histone H1 and the acetylation and methylation of histones in filamentous fungi and links this knowledge to the huge body of data from other systems. Filamentous fungi show a wide range of morphologies and have developed a complex network of genes that enables them to use a great variety of substrates. This fact, together with the possibility of simple and quick genetic manipulation, highlights these organisms as model systems for the investigation of gene regulation. However, little is still known about regulation at the chromatin level in filamentous fungi. Understanding the role of chromatin in transcriptional regulation would be of utmost importance with respect to the impact of filamentous fungi in human diseases and agriculture. The synthesis of compounds (antibiotics, immunosuppressants, toxins, and compounds with adverse effects) is also likely to be regulated at the chromatin level.

Protein arginine methyltransferase 1 coactivates NF-kappaB-dependent gene expression synergistically with CARM1 and PARP1

Nuclear factor kappa B (NF-kappaB) plays an important role in the transcriptional regulation of genes involved in inflammation and cell survival. Transcriptional coactivators that methylate histones become increasingly important. Recently, we provided evidence that coactivator-associated arginine methyltransferase 1 (CARM1) is a transcriptional coactivator of NF-kappaB and functions as a promoter-specific regulator of NF-kappaB recruitment to chromatin. Here, we show that protein arginine methyltransferase 1 (PRMT1) synergistically coactivates NF-kappaB-dependent gene expression at the macrophage inflammatory protein 2 and human immunodeficiency virus 1 long terminal repeat promoters in concert with the transcriptional coactivators p300/CREB binding protein, CARM1, and poly(ADP-ribose) polymerase 1. PRMT1 formed a complex with poly(ADP-ribose) polymerase 1 and NF-kappaB in vivo and interacted directly with the NF-kappaB subunit p65 in vitro. The methyltransferase activity of PRMT1 appeared essential for its coactivator function in context with CARM1 and p300/CREB binding protein. These results suggest that the cooperative action between PRMT1 and CARM1 is required for NF-kappaB-dependent gene expression.

Interplay between chromatin remodelers and protein arginine methyltransferases

Chromatin modifying enzymes have emerged as key regulators of all DNA based processes, which control cell growth, development, and differentiation. Recently, it has become clear that different chromatin remodeling and histone-modifying activities are involved in transcriptional activation and repression. Among the enzymes involved in regulating chromatin structure is the family of protein arginine methyltransferases (PRMTs) that specializes in methylating both histones as well as key cellular proteins. There are eleven different PRMT genes (PRMT1-11) whose biological function remains under explored. PRMTs regulate various cellular processes such as DNA repair and transcription, RNA processing, signal transduction, and nucleo-cytoplasmic localization. Like histone lysine methylation, methylation of histone arginine residues can either induce or inhibit transcription depending on the residue being modified and the type of methylation being introduced. In this review, we will focus on the latest findings and biological roles of ATP-dependent chromatin remodeling complexes and PRMT enzymes, and how their aberrant expression is linked to cancer.

Characterization of protein arginine methyltransferases in porcine brain

Protein arginine methylation is a posttranslational modification involved in various cellular functions including cell signaling, protein subcellular localization and transcriptional regulation. We analyze the protein arginine methyltransferases (PRMTs) that catalyze the formation of methylarginines in porcine brain. We fractionated the brain extracts and determined the PRMT activities as well as the distribution of different PRMT proteins in subcellular fractions of porcine brain. The majority of the type I methyltransferase activities that catalyze the formation of asymmetric dimethylarginines was in the cytosolic S3 fraction. High specific activity of the methyltransferase was detected in the S4 fraction (high-salt stripping of the ultracentrifugation precipitant P3 fraction), indicating that part of the PRMT was peripherally associated with membrane and ribosomal fractions. The amount and distribution of PRMT1 are consistent with the catalytic activity. The elution patterns from gel filtration and anion exchange chromatography also indicate that the type I activity in S3 and S4 are mostly from PRMT1. Our results suggest that part of the type I arginine methyltransferases in brains, mainly PRMT1, are sequestered in an inactive form as they associated with membranes or large subcellular complexes. Our biochemical analyses confirmed the complex distribution of different PRMTs and implicate their regulation and catalytic activities in brain.

Protein arginine methylation in health and disease

Protein arginine methylation is a rapidly growing field of biomedical research that holds great promise for extending our understanding of developmental and pathological processes. Less than ten years ago, fewer than two dozen proteins were verified to contain methylarginine. Currently, however, hundreds of methylarginine proteins have been detected and many have been confirmed by mass spectrometry and other proteomic and molecular techniques. Several of these proteins are products of disease genes or are implicated in disease processes by recent experimental or clinical observations. The purpose of this chapter is twofold; (1) to re-examine the role of protein arginine methylation placed within the context of cell growth and differentiation, as well as within the rich variety of cellular metabolic methylation pathways and (2) to review the implications of recent advances in protein methylarginine detection and the analysis of protein methylarginine function for our understanding of human disease.

Protein arginine methyltransferase 1: positively charged residues in substrate peptides distal to the site of methylation are important for substrate binding and catalysis

Protein arginine methyltransferases (PRMTs) are a group of eukaryotic enzymes that catalyze the methylation of Arg residues in a variety of proteins (e.g., histones H3 and H4), and their activities influence a wide range of cellular processes, including cell growth, RNA splicing, differentiation, and transcriptional regulation. Dysregulation of these enzymes has been linked to heart disease and cancer, suggesting this enzyme family as a novel therapeutic target. To aid the development of PRMT inhibitors, we characterized the substrate specificity of both the rat and human PRMT1 orthologues using histone based peptide substrates. N- and C-terminal truncations to identify a minimal peptide substrate indicate that long-range interactions between enzyme and substrate are important for high rates of substrate capture. The importance of these long-range interactions to substrate capture were confirmed by "mutagenesis" experiments on a minimal peptide substrate. Inhibition studies on S-adenosyl-homocysteine, thioadenosine, methylthioadenosine, homocysteine, and sinefungin suggest that potent and selective bisubstrate analogue inhibitor(s) for PRMT1 can be developed by linking a histone based peptide substrate to homocysteine or sinefungin. Additionally, we present evidence that PRMT1 utilizes a partially processive mechanism to dimethylate its substrates.

Alternative Splicing Yields Protein Arginine Methyltransferase 1 Isoforms with Distinct Activity, Substrate Specificity, and Subcellular Localization

PRMT1 is the predominant member of a family of protein arginine methyltransferases (PRMTs) that have been implicated in various cellular processes, including transcription, RNA processing, and signal transduction. It was previously reported that the human PRMT1 pre-mRNA was alternatively spliced to yield three isoforms with distinct N-terminal sequences. Close inspection of the genomic organization in the 5'-end of the PRMT1 gene revealed that it can produce up to seven protein isoforms, all varying in their N-terminal domain. A detailed biochemical characterization of these variants revealed that unique N-terminal sequences can influence catalytic activity as well as substrate specificity. In addition, our results uncovered the presence of a functional nuclear export sequence in PRMT1v2. Finally, we find that the relative balance of PRMT1 isoforms is altered in breast cancer.

Protein arginine methylation regulates insulin signaling in L6 skeletal muscle cells

Protein N-arginine methyltransferase (PRMT)1 catalyzes arginine methylation in a variety of substrates, although the potential role of PRMT1 in insulin action has not been defined. We therefore investigated the effect of PRMT1-mediated methylation on insulin signaling and glucose uptake in skeletal L6 myotubes. Exposure of L6 myotubes to insulin rapidly induced translocation of PRMT1 and increased its catalytic activity in membrane fraction. Several proteins in the membrane fraction were arginine-methylated after insulin treatment, which were inhibited by pretreatment with an inhibitor of methyltransferase, 5'-deoxy-5'-(methylthio)adenosine (MTA), or a small interfering RNA against PRMT1 (PRMT1-siRNA). Inhibition of arginine methylation with MTA or PRMT1-siRNA diminished later phase of insulin-stimulated tyrosine phosphorylation of insulin receptor (IR) beta and IRS-1, association of IRS-1 with p85alpha subunit of PI3-K, and glucose uptake. Our results suggest that PRMT1-mediated methylation serves as a positive modulator of IR/IRS-1/PI3-K pathway and subsequent glucose uptake in skeletal muscle cells.

Regulation of protein arginine methyltransferase 8 (PRMT8) activity by its N-terminal domain

Human protein arginine methyltransferase PRMT8 has been recently described as a type I enzyme in brain that is localized to the plasma membrane by N-terminal myristoylation. The amino acid sequence of human PRMT8 is almost 80% identical to human PRMT1, the major protein arginine methyltransferase activity in mammalian cells. However, the activity of a recombinant PRMT8 GST fusion protein toward methyl-accepting substrates is much lower than that of a GST fusion of PRMT1. We show here that both His-tagged and GST fusion species lacking the initial 60 amino acid residues of PRMT8 have enhanced enzymatic activity, suggesting that the N-terminal domain may regulate PRMT8 activity. This conclusion is supported by limited proteolysis experiments showing an increase in the activity of the digested full-length protein, consistent with the loss of the N-terminal domain. In contrast, the activity of the N-terminal truncated protein was slightly diminished by limited proteolysis. Significantly, we detect automethylation at two sites in the N-terminal domain, as well as binding sites for SH3 domain-containing proteins. We suggest that the N-terminal domain may function as an autoregulator that may be displaced by interaction with one or more physiological inducers.

Direct mass-spectrometric identification of Arg296 and Arg299 as the methylation sites of hnRNP K protein for methyltransferase PRMT1

Protein methylation is one of the most important post-translational modifications that contribute to the diversity and complexity of proteome. Here we report the study of in vitro methylation of heterogeneous nuclear ribonucleoprotein K (hnRNP K) with protein arginine methyltransferase 1 (PRMT1), as an enzyme, and S-adenosyl-L-methionine (SAM), as a methyl donor. The mass analysis of tryptic peptides of hnRNP K before and after methylation reveals the addition of four methyl groups in the residues 288-303. Tandem mass-spectrometric analysis of this peptide shows that both Arg296 and Arg299 are dimethylated. In addition, fragmentation analysis of such methylated arginines illustrate that they are both asymmetric dimethylarginines. Since Arg296 and Arg299 are located near the SH3-binding domains of hnRNP K, such methylation has the potential in regulating the interaction of hnRNP K with Src protein family. Our results provide crucial information for further functional study of hnRNP K methylation.

Protein arginine methylation in Candida albicans: role in nuclear transport

Protein arginine methylation plays a key role in numerous eukaryotic processes, such as protein transport and signal transduction. In Candida albicans, two candidate protein arginine methyltransferases (PRMTs) have been identified from the genome sequencing project. Based on sequence comparison, C. albicans candidate PRMTs display similarity to Saccharomyces cerevisiae Hmt1 and Rmt2. Here we demonstrate functional homology of Hmt1 between C. albicans and S. cerevisiae: CaHmt1 supports growth of S. cerevisiae strains that require Hmt1, and CaHmt1 methylates Npl3, a major Hmt1 substrate, in S. cerevisiae. In C. albicans strains lacking CaHmt1, asymmetric dimethylarginine and omega-monomethylarginine levels are significantly decreased, indicating that Hmt1 is the major C. albicans type I PRMT1. Given the known effects of type I PRMTs on nuclear transport of RNA-binding proteins, we tested whether Hmt1 affects nuclear transport of a putative Npl3 ortholog in C. albicans. CaNpl3 allows partial growth of S. cerevisiae npl3Delta strains, but its arginine-glycine-rich C terminus can fully substitute for that of ScNpl3 and also directs methylation-sensitive association with ScNpl3. Expression of green fluorescent protein-tagged CaNpl3 proteins in C. albicans strains with and without CaHmt1 provides evidence for CaHmt1 facilitating export of CaNpl3 in this fungus. We have also identified the C. albicans Rmt2, a type IV fungus- and plant-specific PRMT, by amino acid analysis of an rmt2Delta/rmt2Delta strain, as well as biochemical evidence for additional cryptic PRMTs.

Surface-scanning mutational analysis of protein arginine methyltransferase 1: roles of specific amino acids in methyltransferase substrate specificity, oligomerization, and coactivator function

Protein arginine methyltransferase 1 (PRMT1) is an arginine-specific protein methyltransferase that methylates a number of proteins involved in transcription and other aspects of RNA metabolism. Its role as a transcriptional coactivator for nuclear receptors involves its ability to bind to other coactivators, such as glucocorticoid receptor-interacting protein 1 (GRIP1), as well as its ability to methylate histone H4 and coactivators such as peroxisome proliferator-activated receptor gamma coactivator-1alpha. Its ability to form homodimers or higher-order homo-oligomers also is important for its methyltransferase activity. To understand the function of PRMT1 further, 19 surface residues were mutated, based on the crystal structure of PRMT1. Mutants were characterized for their ability to bind and methylate various substrates, form homodimers, bind GRIP1, and function as a coactivator for the androgen receptor in cooperation with GRIP1. We identified specific surface residues that are important for methylation substrate specificity and binding of substrates, for dimerization/oligomerization, and for coactivator function. This analysis also revealed functional relationships between the various activities of PRMT1. Mutants that did not dimerize well had poor methyltransferase activity and coactivator function. However, surprisingly, all dimerization mutants exhibited increased GRIP1 binding, suggesting that the essential PRMT1 coactivator function of binding to GRIP1 may require dissociation of PRMT1 dimers or oligomers. Three different mutants with altered substrate specificity had widely varying coactivator activity levels, suggesting that methylation of specific substrates is important for coactivator function. Finally, identification of several mutants that exhibited reduced coactivator function but appeared normal in all other activities tested, and finding one mutant with very little methyltransferase activity but normal coactivator function, suggested that these mutated surface residues may be involved in currently unknown protein-protein interactions that are important for coactivator function.

Molecular characterisation and inductive expression of a fish protein arginine methyltransferase 1 gene in response to virus infection

Protein arginine methyltransferase 1 (PRMT1) is currently thought as an effector to regulate interferon (IFN) signalling. Here Paralichthys olivaceus PRMT1 (PoPRMT1) gene was identified as a virally induced gene from UV-inactivated Scophthalmus maximus Rhabdovirus (SMRV)-infected flounder embryonic cells (FEC). PoPMRT1 encodes a 341-amino-acid protein that shares the conserved domains including post-I, motif I, II and III. Homology comparisons show that the putative PoPMRT1 protein is the closest to zebrafish PMRT1 and belongs to type I PRMT family (including PRMT1, PRMT2, PRMT3, PRMT4, PRMT6, PRMT8). Expression analyses revealed an extensive distribution of PoPMRT1 in all tested tissues of flounder. In vitro induction of PoPRMT1 was determined in UV-inactivated SMRV-infected FEC cells, and under the same conditions, flounder Mx was also transcriptionally up-regulated, indicating that an IFN response might be triggered. Additionally, live SMRV infection of flounders induced an increased expression of PoPRMT1 mRNA and protein significantly in spleen, and to a lesser extent in head kidney and intestine. Immunofluorescence analysis revealed a major cyptoplasmic distribution of PoPRMT1 in normal FEC but an obvious increase occurred in nucleus in response to UV-inactivated SMRV. This is the first report on in vitro and in vivo expression of fish PRMT1 by virus infection, suggesting that PoPRMT1 might be implicated in flounder antiviral immune response.

Protein arginine methyltransferases: Evolution and assessment of their pharmacological and therapeutic potential

Protein arginine N-methylation is a post-translational modification whose influence on cell function is becoming widely appreciated. Protein arginine methyltransferases (PRMT) catalyze the methylation of terminal nitrogen atoms of guanidinium side chains within arginine residues of proteins. Recently, several new members of the PRMT family have been cloned and their catalytic function determined. In this report, we present a review and phylogenetic analysis of the PRMT found so far in genomes. PRMT are found in nearly all groups of eukaryotes. Many human PRMT originated early in eukaryote evolution. Homologs of PRMT1 and PRMT5 are found in nearly every eukaryote studied. The gene structure of PRMT vary: most introns appear to be inserted randomly into the open reading frame. The change in catalytic specificity of some PRMT occurred with changes in the arginine binding pocket within the active site. Because of the high degree of conservation of sequence among the family throughout evolution, creation of specific PRMT inhibitors in pathogenic organisms may be difficult, but could be very effective if developed. Furthermore, because of the intricate involvement of several PRMT in cellular physiology, their inhibition may be fraught with unwanted side effects. Nevertheless, development of pharmaceutical agents to control PRMT functions could lead to significant new targets.

Methylation of DNA polymerase beta by protein arginine methyltransferase 1 regulates its binding to proliferating cell nuclear antigen

DNA polymerase beta (pol beta) is a key player in DNA base excision repair (BER). Here, we describe the complex formation of pol beta with the protein arginine methyltransferase 1 (PRMT1). PRMT1 specifically methylated pol beta in vitro and in vivo. Arginine 137 was identified in pol beta as an important target for methylation by PRMT1. Neither the polymerase nor the dRP-lyase activities of pol beta were affected by PRMT1 methylation. However, this modification abolished the interaction of pol beta with proliferating cell nuclear antigen (PCNA). Together, our results provide evidence that PRMT1 methylation of pol beta might play a regulatory role in BER by preventing the involvement of pol beta in PCNA-dependent DNA metabolic events.

Increased Protein Arginine Methylation in Chronic Hypoxia

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthesis. ADMA is generated by catabolism of proteins containing methylated arginine residues, and its levels are correlated with endothelial dysfunction in systemic cardiovascular diseases. Arginine methylation of cellular proteins is catalyzed by protein arginine methyltransferases (PRMT). The expression and localization of PRMT in the lung has not been addressed. Here, we sought to analyze the expression of PRMT isoforms in the lung and to determine whether PRMT expression is altered during exposure to chronic hypoxia (10% oxygen). Adult mice were exposed to hypoxia for up to 3 wk, and lung tissues were harvested and processed for RT-PCR, Western blotting, immunohistochemistry, and determination of tissue ADMA levels. All PRMT isoforms investigated were detected at the mRNA and protein level in mouse lung, and were localized primarily to the bronchial and alveolar epithelium. In lungs of mice subjected to chronic hypoxia, PRMT2 mRNA and protein levels were up-regulated, whereas the expression of all other PRMT isoforms remained unchanged. This was mainly due to increased expression of PRMT2 in alveolar type II cells, which did not express detectable levels of PRMT2 under normoxic conditions. Consistent with these observations, lung ADMA levels and ADMA/l-Arginine ratios were increased under hypoxic conditions. These results demonstrate that PRMTs are expressed and functional in the lung, and that hypoxia is a potent regulator of PRMT2 expression and lung ADMA concentrations. These data suggest that structural and functional changes caused by hypoxia may be linked to ADMA metabolism.

Epigenetics--an epicenter of gene regulation: histones and histone-modifying enzymes

The treatment of cancer through the development of new therapies is one of the most important challenges of our time. The decoding of the human genome has yielded important insights into the molecular basis of physical disorders, and in most cases a connection between failures in specific genes and the resulting clinical symptoms can be made. The modulation of epigenetic mechanisms enables, by definition, the alteration of cellular phenotype without altering the genotype. The information content of a single gene can be crucial or harmful, but the prerequisite for a cellular effect is active gene transcription. To this end, epigenetic mechanisms play a very important role, and the transcription of a given gene is directly influenced by the modification pattern of the surrounding histone proteins as well as the methylation pattern of the DNA. These processes are effected by different enzymes which can be directly influenced through the development of specific modulators. Of course, all genetic information is written as a four-character code in DNA. However, epigenetics describes the art of reading between the lines.

Post-translational modification of delta antigen of hepatitis D virus

The hepatitis delta virus (HDV) genome has only one open reading frame, which encodes the viral small delta antigen. After RNA editing, the same open reading frame is extended 19 amino acids at the carboxyl terminus and encodes the large delta antigen. These two viral proteins escort the HDV genome through different cellular compartments for the complicated phases of replication, transcription and, eventually, the formation of progeny virions. To orchestrate these events, the delta antigens have to take distinct cues to traffic to the right compartments and make correct molecular contacts. In eukaryotes, post-translational modification (PTM) is a major mechanism of dictating the multiple functions of a single protein. Multiple PTMs, including phosphorylation, isoprenylation, acetylation, and methylation, have been identified on hepatitis delta antigens. In this chapter we review these PTMs and discuss their functions in regulating and coordinating the life cycle of HDV.

Protein arginine methylation: Cellular functions and methods of analysis

During the last few years, new members of the growing family of protein arginine methyltransferases (PRMTs) have been identified and the role of arginine methylation in manifold cellular processes like signaling, RNA processing, transcription, and subcellular transport has been extensively investigated. In this review, we describe recent methods and findings that have yielded new insights into the cellular functions of arginine-methylated proteins, and we evaluate the currently used procedures for the detection and analysis of arginine methylation.

Pharmacotherapies and their influence on asymmetric dimethylargine (ADMA)

Elevated plasma concentrations of the endogenous nitric oxide synthase inhibitor asymmetric dimethylarginine (ADMA) are found in various clinical settings, including renal failure, coronary heart disease, hypertension, diabetes and pre-eclampsia. In healthy people acute infusion of ADMA promotes vascular dysfunction, and in mice chronic infusion of ADMA promotes progression of atherosclerosis. Thus, ADMA may not only be a marker but also an active player in cardiovascular disease, which makes it a potential target for therapeutic interventions. This review provides a summary and critical discussion of the presently available data concerning the effects on plasma ADMA levels of cardiovascular drugs, hypoglycemic agents, hormone replacement therapy, antioxidants, and vitamin supplementation. We assess the evidence that the beneficial effects of drug therapies on vascular function can be attributed to modification of ADMA levels. To develop more specific ADMA-lowering therapies, mechanisms leading to elevation of plasma ADMA concentrations in cardiovascular disease need to be better understood. ADMA is formed endogenously by degradation of proteins containing arginine residues that have been methylated by S-adenosylmethionine-dependent methyltransferases (PRMTs). There are two major routes of elimination: renal excretion and enzymatic degradation by the dimethylarginine dimethylaminohydrolases (DDAH-1 and -2). Oxidative stress causing upregulation of PRMT expression and/or attenuation of DDAH activity has been suggested as a mechanism and possible drug target in clinical conditions associated with elevation of ADMA. As impairment of DDAH activity or capacity is associated with substantial increases in plasma ADMA concentrations, DDAH is likely to emerge as a prime target for specific therapeutic interventions.

Ki-1/57 interacts with PRMT1 and is a substrate for arginine methylation

The human 57 kDa Ki-1 antigen (Ki-1/57) is a cytoplasmic and nuclear protein, associated with Ser/Thr protein kinase activity, and phosphorylated at the serine and threonine residues upon cellular activation. We have shown that Ki-1/57 interacts with chromo-helicase DNA-binding domain protein 3 and with the adaptor/signaling protein receptor of activated kinase 1 in the nucleus. Among the identified proteins that interacted with Ki-1/57 in a yeast two-hybrid system was the protein arginine-methyltransferase-1 (PRMT1). Most interestingly, when PRMT1 was used as bait in a yeast two-hybrid system we were able to identify Ki-1/57 as prey among 14 other interacting proteins, the majority of which are involved in RNA metabolism or in the regulation of transcription. We found that Ki-1/57 and its putative paralog CGI-55 have two conserved Gly/Arg-rich motif clusters (RGG/RXR box, where X is any amino acid) that may be substrates for arginine-methylation by PRMT1. We observed that all Ki-1/57 protein fragments containing RGG/RXR box clusters interact with PRMT1 and are targets for methylation in vitro. Furthermore, we found that Ki-1/57 is a target for methylation in vivo. Using immunofluorescence experiments we observed that treatment of HeLa cells with an inhibitor of methylation, adenosine-2',3'-dialdehyde (Adox), led to a reduction in the cytoplasmic immunostaining of Ki-1/57, whereas its paralog CGI-55 was partially redistributed from the nucleus to the cytoplasm upon Adox treatment. In summary, our data show that the yeast two-hybrid assay is an effective system for identifying novel PRMT arginine-methylation substrates and may be successfully applied to other members of the growing family of PRMTs.

CARM1 regulates proliferation of PC12 cells by methylating HuD

HuD is an RNA-binding protein that has been shown to induce neuronal differentiation by stabilizing labile mRNAs carrying AU-rich instability elements. Here, we show a novel mechanism of arginine methylation of HuD by coactivator-associated arginine methyltransferase 1 (CARM1) that affected mRNA turnover of p21cip1/waf1 mRNA in PC12 cells. CARM1 specifically methylated HuD in vitro and in vivo and colocalized with HuD in the cytoplasm. Inhibition of HuD methylation by CARM1 knockdown elongated the p21cip1/waf1 mRNA half-life and resulted in a slow growth rate and robust neuritogenesis in response to nerve growth factor (NGF). Methylation-resistant HuD bound more p21cip1/waf1 mRNA than did the wild type, and its overexpression upregulated p21cip1/waf1 protein expression. These results suggested that CARM1-methylated HuD maintains PC12 cells in the proliferative state by committing p21cip1/waf1 mRNA to its decay system. Since the methylated population of HuD was reduced in NGF-treated PC12 cells, downregulation of HuD methylation is a possible pathway through which NGF induces differentiation of PC12 cells.

Different methylation characteristics of protein arginine methyltransferase 1 and 3 toward the Ewing Sarcoma protein and a peptide

The multifunctional Ewing Sarcoma (EWS) protein, a member of a large family of RNA-binding proteins, is extensively asymmetrically dimethylated at arginine residues within RGG consensus sequences. Using recombinant proteins we examined whether type I protein arginine methyltransferase (PRMT)1 or 3 is responsible for asymmetric dimethylations of the EWS protein. After in vitro methylation of the EWS protein by GST-PRMT1, we identified 27 dimethylated arginine residues out of 30 potential methylation sites by mass spectrometry-based techniques (MALDI-TOF MS and MS/MS). Thus, PRMT1 recognizes most if not all methylation sites of the EWS protein. With GST-PRMT3, however, only nine dimethylated arginines, located mainly in the C-terminal region of EWS protein, could be assigned, indicating that structural determinants prevent complete methylation. In contrary to previous reports this study also revealed that trypsin is able to cleave after methylated arginines. Pull-down experiments showed that endogenous EWS protein binds efficiently to GST-PRMT1 but less to GST-PRMT3, which is in accordance to the in vitro methylation results. Furthermore, methylation of a peptide containing different methylation sites revealed differences in the site selectivity as well as in the kinetic properties of GST-PRMT1 and GST-PRMT3. Kinetic differences due to an inhibition effect of the methylation inhibitor S-adenosyl-L-homocysteine could be excluded by determining the corresponding K(i) values of the two enzymes and the K(d) values for the methyl donor S-adenosyl-L-methionine. The study demonstrates the strength of MS-based methods for a qualitative and quantitative analysis of enzymic arginine methylation, a posttranslational modification that becomes more and more the object of investigations.

Dynamics of Human Protein Arginine Methyltransferase 1(PRMT1) in Vivo

Arginine methylation is a posttranslational protein modification catalyzed by a family of protein arginine methyltransferases (PRMT), the predominant member of which is PRMT1. Despite its major role in arginine methylation of nuclear proteins, surprisingly little is known about the subcellular localization and dynamics of PRMT1. We show here that only a fraction of PRMT1 is located in the nucleus, but the protein is predominantly cytoplasmic. Fluorescence recovery after photobleaching experiments reveal that PRMT1 is highly mobile both in the cytoplasm and the nucleus. However, inhibition of methylation leads to a significant nuclear accumulation of PRMT1, concomitant with the appearance of an immobile fraction of the protein in the nucleus, but not the cytoplasm. Both the accumulation and immobility of PRMT1 is reversed when re-methylation is allowed, suggesting a mechanism where PRMT1 is trapped by unmethylated substrates such as core histones and heterogeneous nuclear ribonucleoprotein proteins until it has executed the methylation reaction.

Genome-wide localization of histone 4 arginine 3 methylation in a differentiation primed myeloid leukemia cell line

Methylation of arginine residues in proteins is involved in modulation of various protein-protein interactions. At the chromatin level H4R3 methylation provides a signal integration step during myeloid differentiation. In order to globally characterize the role of arginine methylation in signal integration and developmental processes we decided to map genomic loci marked by protein arginine methyl transferase 1 (PRMT1) via histone H4 arginine 3 methylation. For this, we used the myeloid leukemia cell line, HL60, which is known to differentiate along the monocyte/macrophage or granulocyte lineage. We used chromatin immunoprecipitation with an antibody specific for the H4 arginine 3 methyl epitope followed by cloning to isolate genomic loci marked by this modification. After sequencing and in silico analysis we found that all of the genomic hits identified were intronic or within 5 kb of 5' ends of specific genes. The locations identified were enriched in conserved transcription factor binding sites of POU2F1, MEF-2 and FOXL1 factors. A significant number of the genes in the proximity of the identified genomic loci are involved in signaling pathways and developmental processes including immune response of myeloid cells.

The Ca2+-induced methyltransferase xPRMT1b controls neural fate in amphibian embryo

We have previously shown that an increase in intracellular Ca2+ is both necessary and sufficient to commit ectoderm to a neural fate in Xenopus embryos. However, the relationship between this Ca2+ increase and the expression of early neural genes has yet to be defined. Using a subtractive cDNA library between untreated and caffeine-treated animal caps, i.e., control ectoderm and ectoderm induced toward a neural fate by a release of Ca2+, we have isolated the arginine N-methyltransferase, xPRMT1b, a Ca2+-induced target gene, which plays a pivotal role in this process. First, we show in embryo and in animal cap that xPRMT1b expression is Ca2+-regulated. Second, overexpression of xPRMT1b induces the expression of early neural genes such as Zic3. Finally, in the whole embryo, antisense approach with morpholino oligonucleotide against xPRMT1b impairs neural development and in animal caps blocks the expression of neural markers induced by a release of internal Ca2+. Our results implicate an instructive role of an enzyme, an arginine methyltransferase protein, in the embryonic choice of determination between epidermal and neural fate. The results presented provide insights by which a Ca2+ increase induces neural fate.

Activation of nuclear receptor coactivator PGC-1alpha by arginine methylation

Peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha), a tissue-specific and inducible transcriptional coactivator for several nuclear receptors, plays a key role in energy metabolism. We report here that PGC-1alpha coactivator activity is potentiated by arginine methylation by protein arginine methyltransferase 1 (PRMT1), another nuclear receptor coactivator. Mutation of three substrate arginines in the C-terminal region of PGC-1alpha abolished the cooperative coactivator function of PGC-1alpha and PRMT1, and compromised the ability of PGC-1alpha to induce endogenous target genes. Finally, endogenous PRMT1 contributes to PGC-1alpha coactivator activity, and to the induction of genes important for mitochondrial biogenesis.

Arginine methylation provides epigenetic transcription memory for retinoid-induced differentiation in myeloid cells

Cellular differentiation is governed by changes in gene expression, but at the same time, a cell's identity needs to be maintained through multiple cell divisions during maturation. In myeloid cell lines, retinoids induce gene expression and a well-characterized two-step lineage-specific differentiation. To identify mechanisms that contribute to cellular transcriptional memory, we analyzed the epigenetic changes taking place on regulatory regions of tissue transglutaminase, a gene whose expression is tightly linked to retinoid-induced differentiation. Here we report that the induction of an intermediary or "primed" state of myeloid differentiation is associated with increased H4 arginine 3 and decreased H3 lysine 4 methylation. These modifications occur before transcription and appear to prime the chromatin for subsequent hormone-regulated transcription. Moreover, inhibition of methyltransferase activity, pre-acetylation, or activation of the enzyme PAD4 attenuated retinoid-regulated gene expression, while overexpression of PRMT1, a methyltransferase, enhanced retinoid responsiveness. Taken together, our results suggest that H4 arginine 3 methylation is a bona fide positive epigenetic marker and regulator of transcriptional responsiveness as well as a signal integration mechanism during cell differentiation and, as such, may provide epigenetic memory.

The methyl transferase PRMT1 functions as co-activator of farnesoid X receptor (FXR)/9-cis retinoid X receptor and regulates transcription of FXR responsive genes

The farnesoid X receptor (FXR) is a nuclear receptor that functions as an endogenous sensor for bile acids (BAs). FXR is bound to and activated by bile acid, and chenodeoxycholic acid (CDCA) is the natural most active ligand. Upon activation, FXR heterodimerizes with the 9-cis retinoic X receptor (RXR) and regulates genes involved in cholesterol and BA homeostasis. 6-Ethyl CDCA (6-ECDCA) is a synthetic BA that binds FXR and induces gene transcription by recruiting coactivators, such as steroid receptor coactivator-1, with histone acetyltransferase activity. In addition to acetylation, histone methylation is critically involved in regulating eukaryotic gene expression. In the present study, we demonstrated that 6-ECDCA activates FXR to interacts with Protein Arginine Methyl-Transferase type I (PRMT1), which induces up-regulation of bile salt export pump (BSEP) and the small heterodimer partner (SHP) mRNA expression and causes a down-regulation of P450 cholesterol 7alpha-hydroxylase and Na(+) taurocholate cotransport peptide genes. Chromatin immunoprecipitation assay suggests that 6-ECDCA induces both the recruitment of PRMT1 and the H4 methylation to the promoter of BSEP and SHP genes. We also provide evidence that a methyltransferase inhibitor blocks the activation of FXR-responsive genes. Our results indicate that histone methylation, similar to acetylation, regulates transcriptional activation of genes involved in cholesterol and BAs homeostasis.

PRMT8, a new membrane-bound tissue-specific member of the protein arginine methyltransferase family

Protein arginine methylation is a common post-translational modification that has been implicated in signal transduction, RNA processing, transcriptional regulation, and DNA repair. A search of the human genome for additional members of the protein arginine N-methyltransferase (PRMT) family of enzymes has identified a gene on chromosome 12 that we have termed PRMT8. This novel enzyme is most closely related to PRMT1, although it has a distinctive N-terminal region. The unique N-terminal end harbors a myristoylation motif, and we have shown here that PRMT8 is indeed modified by the attachment of a myristate to the glycine residue after the initiator methionine. The myristoylation of PRMT8 results in its association with the plasma membrane. The second singular property of PRMT8 is its tissue-specific expression pattern; it is largely expressed in the brain. A glutathione S-transferase fusion protein of PRMT8 has type I PRMT activity, catalyzing the formation of omega-NG-monomethylated and asymmetrically omega-NG,NG-dimethylated arginine residues on a recombinant glycine- and arginine-rich substrate. PRMT8 is thus an active arginine methyltransferase that is membrane-associated and tissue-specific, two firsts for this family of enzymes.

o-Nitrobenzenesulfonamides in Nucleoside Synthesis: Efficient 5‘-Aziridination of Adenosine

5'-Aziridinoadenylates of the form 1 and a related nitrogen mustard variant have been constructed using a novel variation of the Mitsunobu reaction. Such molecules allow conversion of biological methyltransferases into nucleoside transferases, thus providing powerful tools for investigating S-adenosyl-l-methionine (SAM)-dependent methylation. We present here a highly effective synthesis of such molecules that is amenable to aziridine diversification as well as elaboration of the base moiety so as to afford "bumped" cofactor mimics compatible with "hole"-bearing mutant proteins.

NGF promotes copper accumulation required for optimum neurite outgrowth and protein methylation

The role of copper in biological phenomena that involve signal transduction is poorly understood. A well-defined cellular model of neuronal differentiation has been utilized to examine the requirement for copper during nerve growth factor (NGF) signal transduction that results in neurite outgrowth. Experiments demonstrate that NGF increases cellular copper content within 3 days of treatment. Copper chelators reduce the effects of NGF on neurite outgrowth and copper accumulation. The effects of tetraethylene pentamine (TEPA), a copper-specific chelator, are reversible by removal from the culture medium and/or by addition of equimolar copper chloride. Because previous work demonstrated that NGF increases protein methylation in PC12 cells, we examined whether TEPA also inhibits S-adenosylhomocysteine hydrolase (SAHH), an essential copper enzyme involved in all protein methylation reactions. In addition to direct in vitro inhibition of SAHH, we show that TEPA decreases protein arginine methyltransferase 1(PRMT1)-specific enzyme activity in PC12 cells and sympathetic neurons. These data comprise the first biochemical and cellular evidence to address the mechanism of copper involvement in neuronal differentiation.

Multimerization of expressed protein-arginine methyltransferases during the growth and differentiation of rat liver

Protein-arginine methylation is a posttranslational modification which yields monomethyl and dimethyl (asymmetric or symmetric) arginines in proteins. We investigated the expressions of PRMT1 and PRMT5 in relation to their catalytic activities in rat liver during growth and differentiation as well as in the pancreas. Western immunoblot analysis revealed that both PRMT1 and PRMT5 proteins were expressed in the cytosol of liver and pancreas with molecular mass of about 42 kDa and 72 kDa, respectively. However, on molecular sieve chromatography, the enzyme activities were eluted at about 500 kDa for PRMT5 and 440 kDa for PRMT1, indicating that the multimer complex of these expressed monomers were catalytically active. While the 500 kDa complex methylated predominantly myelin basic protein (MBP), the 440 kDa complex methylated hnRNP A1 protein. In fetal rat liver, the amount of expressed 42 kDa PRMT1 protein and the enzyme activity to methylate hnRNPA1 protein were 2- to 3-fold and 4- to 5-fold higher, respectively, than those of post-natal livers. While the 72 kDa PRMT5 protein was consistently expressed, its activity varied only about 2-fold. However, PRMT5 to methylate MBP showed one distinct peak at around the 20th day post-natal. Furthermore, while the PRMT1 enzyme activity increased more than 10-fold after 3 days of 70% partial hepatectomy, the amount of expressed PRMT1 protein was only about 3.2-fold higher than the control livers. In summary, we observed that PRMTs are catalytically active only in the form of multimers, but not as a dimer or tetramer of the expressed subunit. Furthermore, the amount of expressed PRMT protein, determined by Western immunoblot, did not correlate with the amount of their catalytic activity, and thus, some uncharacterized additional factor(s) may multimerize PRMTs to express catalytic activities in vivo.

Signaling pathways in Th2 development

In order for an immune response to be successful, it must be of the appropriate type and magnitude. Intracellular residing pathogens require a cell-mediated immune response, whereas extracellular pathogens evoke a humoral immune response. T-helper (Th) cells orchestrate the immune response and are divided into two subsets, Th1 and Th2 cells. Here, we discuss the mechanisms of Th2 development with a focus on signal transduction pathways that influence Th2 differentiation.

Protein interfaces in signaling regulated by arginine methylation

Posttranslational modifications are well-known effectors of signal transduction. Arginine methylation is a covalent modification that results in the addition of methyl groups to the nitrogen atoms of the arginine side chains. A probable role of arginine methylation in signal transduction is emerging with the identification of new arginine-methylated proteins. However, the functional consequences of arginine methylation and its mode of regulation remain unknown. The identification of the protein arginine methyltransferase family and the development of methylarginine-specific antibodies have raised renewed interest in this modification during the last decade. Arginine methylation was mainly observed on abundant proteins such as RNA-binding proteins and histones, but recent advances have revealed a plethora of arginine-methylated proteins implicated in a variety of cellular processes, including signaling by interferon and cytokines, and in T cell signaling. We discuss these recent advances and the role of arginine methylation in signal transduction.

Hepatitis delta virus antigen is methylated at arginine residues, and methylation regulates subcellular localization and RNA replication

Hepatitis delta virus (HDV) contains a circular RNA which encodes a single protein, hepatitis delta antigen (HDAg). HDAg exists in two forms, a small form (S-HDAg) and a large form (L-HDAg). S-HDAg can transactivate HDV RNA replication. Recent studies have shown that posttranslational modifications, such as phosphorylation and acetylation, of S-HDAg can modulate HDV RNA replication. Here we show that S-HDAg can be methylated by protein arginine methyltransferase (PRMT1) in vitro and in vivo. The major methylation site is at arginine-13 (R13), which is in the RGGR motif of an RNA-binding domain. The methylation of S-HDAg is essential for HDV RNA replication, especially for replication of the antigenomic RNA strand to form the genomic RNA strand. An R13A mutation in S-HDAg inhibited HDV RNA replication. The presence of a methylation inhibitor, S-adenosyl-homocysteine, also inhibited HDV RNA replication. We further found that the methylation of S-HDAg affected its subcellular localization. Methylation-defective HDAg lost the ability to form a speckled structure in the nucleus and also permeated into the cytoplasm. These results thus revealed a novel posttranslational modification of HDAg and indicated its importance for HDV RNA replication. This and other results further showed that, unlike replication of the HDV genomic RNA strand, replication of the antigenomic RNA strand requires multiple types of posttranslational modification, including the phosphorylation and methylation of HDAg.

DAL-1/4.1B tumor suppressor interacts with protein arginine N-methyltransferase 3 (PRMT3) and inhibits its ability to methylate substrates in vitro and in vivo

DAL-1 (differentially expressed in adenocarcinoma of the lung)/4.1B is a tumor suppressor gene on human chromosome 18p11.3 whose expression is lost in >50% of primary non-small-cell lung carcinomas. Based on sequence similarity, DAL-1/4.1B has been assigned to the Protein 4.1 superfamily whose members interact with plasma membrane proteins through their N-terminal FERM (4.1/Ezrin/Radixin/Moesin) domain, and cytoskeletal components via their C-terminal SAB (spectrin-actin binding) region. Using the DAL-1/4.1B FERM domain as bait for yeast two-hybrid interaction cloning, we identified protein arginine N-methyltransferase 3 (PRMT3) as a specific DAL-1/4.1B-interacting protein. PRMT3 catalyses the post-translational transfer of methyl groups from S-adenosyl-L-methionine to arginine residues of proteins. Coimmunoprecipitation experiments using lung and breast cancer cell lines confirmed this interaction in mammalian cells in vivo. In vitro binding assays demonstrated that this was an interaction occurring via the C-terminal catalytic core domain of PRMT3. DAL-1/4.1B was determined not to be a substrate for PRMT3-mediated methylation but its presence inhibits the in vitro methylation of a glycine-rich and arginine-rich methyl-accepting protein, GST (glutathione-S-transferase-GAR (glycine- and arginine-rich), which contains 14 'RGG' consensus methylation sites. In addition, induced expression of DAL-1/4.1B in MCF-7 breast cancer cells showed that the DAL-1/4.1B protein significantly inhibits PRMT3 methylation of cellular substrates. These findings suggest that modulation of post-translational methylation may be an important mechanism through which DAL-1/4.1B affects tumor cell growth.

Identification and phylogenetic analyses of the protein arginine methyltransferase gene family in fish and ascidians

Protein arginine methyltransferases (PRMT) involved in the regulations of signal transduction, protein subcellular localization, and transcription have been mostly studied in mammals and yeast. In this study orthologues of eight human PRMT genes (PRMT1-7 and HRMT1L3) were identified in both puffer fish Fugu rubripes and zebrafish Danio rerio. The fish PRMT genes appear to be conserved with their mammalian orthologues at the levels of amino acid sequences as well as genomic structures. All vertebrate PRMT genes contain 10-16 coding exons except PRMT6 that contains only one coding exon. Western blot analyses of zebrafish tissue extracts confirmed the expression of some PRMT proteins in zebra fish. We further identified six PRMT members (PRMT1, 3-7) in an invertebrate chordate Ciona intestinalis. Genomic structures of the PRMT orthologues are no more conserved in the ascidians, as PRMT3 and PRMT5 contain only one coding exon while PRMT6 contains six exons. PRMT2 and HRMT1L3 that are missing in Ciona appear to be vertebrate-specific. HRMT1L3 is a PRMT1 paralogue with highly conserved sequences and exact exon junctions, whereas the PRMT2 orthologues are very diverged. Different PRMT orthologues are likely to evolve at different rates and the PRMT1 orthologues appear to be most conserved through evolution. Furthermore, phylogenetic analyses using the core regions of various PRMT genes show that PRMT5 with the type II PRMT activity is separated in one branch. All other PRMT genes including PRMT1, 2, 3, 4, 6, 7 and HRMT1L3 clustered in the other branch, probably represent the genes for the type I activity.

Arginine methylation of scaffold attachment factor A by heterogeneous nuclear ribonucleoprotein particle-associated PRMT1

Components of the heterogeneous nuclear ribonucleoprotein (hnRNP) complex and other nucleic acid-binding proteins are subject to methylation on specific arginine residues by the catalytic activity of arginine methyltransferases. The methylation has been implicated in transcriptional regulation and RNA and protein trafficking and signal transduction, but the mechanism by which these functions are achieved has remained undetermined. We show here that the predominant arginine methyltransferase in human cells, protein arginine methyltransferase 1 (PRMT1), is associated with hnRNP complexes, dependent on the methylation status of the cell, and that it methylates its preferred substrates in situ. Binding of PRMT1 occurs through physical interaction with scaffold attachment factor A (SAF-A), also known as hnRNP-U, which is quantitatively methylated by PRMT1 in all investigated cell lines as determined by a novel, highly specific, methylation-sensitive antibody.

Arginine Methylation of NIP45 Modulates Cytokine Gene Expression in Effector T Lymphocytes

Posttranslational modification of proteins within T cell receptor signaling cascades allows T lymphocytes to rapidly initiate an appropriate immune response. Here we report a role for arginine methylation in regulating cytokine gene transcription in the T helper lymphocyte. Inhibition of arginine methylation impaired the expression of several cytokine genes, including the signature type 1 and type 2 helper cytokines, interferon gamma, and interleukin-4. T cell receptor signaling increased expression of the protein arginine methyltransferase PRMT1, which in turn methylated the nuclear factor of activated T cells (NFAT) cofactor protein, NIP45. Arginine methylation of the amino terminus of NIP45 modulated its interaction with NFAT and resulted in augmented cytokine production, while T cells from mice lacking NIP45 had impaired expression of IFNgamma and IL-4. Covalent modification of NIP45 by arginine methylation is an important mechanism of regulating the expression of NFAT-dependent cytokine genes.

Characterization of the Drosophila protein arginine methyltransferases DART1 and DART4

The role of arginine methylation in Drosophila melanogaster is unknown. We identified a family of nine PRMTs (protein arginine methyltransferases) by sequence homology with mammalian arginine methyltransferases, which we have named DART1 to DART9 ( Drosophila arginine methyltransferases 1-9). In keeping with the mammalian PRMT nomenclature, DART1, DART4, DART5 and DART7 are the putative homologues of PRMT1, PRMT4, PRMT5 and PRMT7. Other DART family members have a closer resemblance to PRMT1, but do not have identifiable homologues. All nine genes are expressed in Drosophila at various developmental stages. DART1 and DART4 have arginine methyltransferase activity towards substrates, including histones and RNA-binding proteins. Amino acid analysis of the methylated arginine residues confirmed that both DART1 and DART4 catalyse the formation of asymmetrical dimethylated arginine residues and they are type I arginine methyltransferases. The presence of PRMTs in D. melanogaster suggest that flies are a suitable genetic system to study arginine methylation.

Protein Methylation Activates Reconstituted Ryanodine Receptor-Ca2+ Release Channels from Coronary Artery Myocytes

Ryanodine receptors (RyR) play an important role in the regulation of intracellular Ca(2+) concentration and in the control of vascular tone. However, the mechanism regulating the activity of RyR is poorly understood. The present study determined whether protein methylation participates in the control of RyR activity. Using a planar lipid bilayer clamping system, S-adenosyl-L-methionine (SAM), a methyl donor, significantly increased the activity of a 245-pS reconstituted Ca(2+) release channel from coronary arterial smooth muscle (CASM) in a concentration-dependent manner. Addition of the protein methylation blockers, 3-deazaadenosine, S-adenosylhomocysteine or sinefungin into the cis solution markedly attenuated SAM-induced activation of RyR/Ca(2+) release channels. By Western blot analysis, arginine N-methyltransferase (PRMT1) and FK506 binding protein (FKBP) were detected in the SR used for reconstitution of RyR. In the presence of anti-PRMT1 antibody (1:100), SAM-induced activation of RyR/Ca(2+) channel was completely abolished. In addition, this SAM-induced increase in RyR/Ca(2+) channel activity was blocked by 30 microM ryanodine and by FK506 (100 microM), a ligand for the RyR accessory protein. These results suggest that protein methylation activates RyR/Ca(2+) release channels and may participate in the control of intracellular Ca(2+) mobilization in CASM cells by transferring a methyl group to the arginine moiety of the RyR accessory protein, FKBP 12.

Arginine Methylation of RNA Helicase A Determines Its Subcellular Localization

RNA helicase A (RHA) undergoes nuclear translocation via a classical import mechanism utilizing karyopherin beta. The nuclear transport domain (NTD) of RHA is known to be necessary and sufficient for its bi-directional nuclear trafficking. We report here that arginine methylation is a novel requirement for NTD-mediated nuclear import. Nuclear translocation of glutathione S-transferase (GST)-NTD fusion proteins is abrogated by arginine-methylation inhibitors. However, in vitro arginine-methylation of GST-NTD prior to injection allows the fusion protein to localize to the nucleus in the presence of methylation inhibitors. Removal of the arginine-rich C-terminal region negates the effects of the methylation inhibitors on NTD import, suggesting that methylation of the NTD C terminus the relieves the cytoplasmic retention of RHA. The NTD physically interacts with PRMT1, the major protein arginine methyltransferase. These findings provide evidence for a novel arginine methylation-dependent regulatory pathway controlling the nuclear import of RHA.

Effect of shear stress on asymmetric dimethylarginine release from vascular endothelial cells

We demonstrated recently that plasma concentrations of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide (NO) synthase, are increased by high salt intake concomitantly with a decrease in plasma levels of NO in human hypertension. We investigated the effect of shear stress on ADMA release in 2 types of cells: transformed human umbilical vein endothelial cells (HUVECs; cell line ECV-304) and HUVECs. Exposure of ECV-304 cells and HUVECs to shear stress with the use of a cone-plate viscometer enhanced gene expression of protein arginine methyltransferase (PRMT-1), ADMA synthase. In HUVECs, the ratio of PRMT-1 to glyceraldehyde 3-phosphate dehydrogenase mRNA was increased by 2-fold by a shear stress of > or =15 dyne/cm2. A dominant-negative mutant of IkappaB kinase alpha and troglitazone at 8 micromol/L, an activator of peroxisome proliferator-activated receptor gamma, abolished the shear stress-induced increase in PRMT-1 gene expression in parallel with the blockade of nuclear factor (NF)-kappaB translocation into the nucleus. The activity of dimethylarginine dimethylaminohydrolase, the degradation enzyme of ADMA, was unchanged after shear stress < or =15 dyne/cm2 and was enhanced by 1.48+/-0.06-fold (P<0.05) by shear stress at 25 dyne/cm2. The release of ADMA was increased by 1.64+/-0.10-fold (P<0.05) by shear stress at 15 dyne/cm2 but was not affected by shear stress at 25 dyne/cm2. These results indicate that shear stress enhances gene expression of PRMT-1 and ADMA release via activation of the NF-kappaB pathway. Shear stress at higher magnitudes facilitates the degradation of ADMA, thus returning ADMA release levels to baseline.

Protein arginine methyltransferase I: substrate specificity and role in hnRNP assembly

Prmt1, the major protein arginine methyltransferase in mammalian cells, has been implicated in signal transduction, transcriptional control, and protein trafficking. In the present study, mouse embryonic stem cells homozygous for an essentially null mutation in the Prmt1 gene were used to examine Prmt1 activity and substrate specificity, which by several criteria appeared to be highly specific. First, other methyltransferases did not substitute for the loss of Prmt1 activity. Second, almost all proteins modified by recombinant Prmt1 in vitro were authentic substrates, i.e., proteins rendered hypomethylated by Prmt1 gene disruption. Finally, Prmt1 did not modify the substrates of other methyltransferases from cells treated with methyltransferase inhibitors. Recombinant proteins corresponding to two splice-variants, Prmt1(353) and Prmt1(371), methylated different, proteins in vitro, providing the first evidence for functional differences between the two isoforms. However, the differences in substrate specificity were lost by the addition of an N-terminal His(6) tag. Loss of Prmt1 activity (and hypomethylation of hnRNPs) has no obvious effect on the formation or composition of hnRNP complexes. Finally, methylation of the most abundant Prmt1 substrates appeared to be extensive and constitutive throughout the cell cycle, suggesting the modification does not modulate protein function under normal growth conditions.

Structure of the predominant protein arginine methyltransferase PRMT1 and analysis of its binding to substrate peptides

PRMT1 is the predominant type I protein arginine methyltransferase in mammals and highly conserved among all eukaryotes. It is essential for early postimplantation development in mouse. Here we describe the crystal structure of rat PRMT1 in complex with the reaction product AdoHcy and a 19 residue substrate peptide containing three arginines. The results reveal a two-domain structure-an AdoMet binding domain and a barrel-like domain-with the active site pocket located between the two domains. Mutagenesis studies confirmed that two active site glutamates are essential for enzymatic activity, and that dimerization of PRMT1 is essential for AdoMet binding. Three peptide binding channels are identified: two are between the two domains, and the third is on the surface perpendicular to the strands forming the beta barrel.