Interplay between chromatin remodelers and protein arginine methyltransferases

PRMT5 is upregulated in malignant and metastatic melanoma and regulates expression of MITF and p27(Kip1.)

Protein arginine methyltransferase-5 (PRMT5) is a Type II arginine methyltransferase that regulates various cellular functions. We hypothesized that PRMT5 plays a role in regulating the growth of human melanoma cells. Immunohistochemical analysis indicated significant upregulation of PRMT5 in human melanocytic nevi, malignant melanomas and metastatic melanomas as compared to normal epidermis. Furthermore, nuclear PRMT5 was significantly decreased in metastatic melanomas as compared to primary cutaneous melanomas. In human metastatic melanoma cell lines, PRMT5 was predominantly cytoplasmic, and associated with its enzymatic cofactor Mep50, but not STAT3 or cyclin D1. However, histologic examination of tumor xenografts from athymic mice revealed heterogeneous nuclear and cytoplasmic PRMT5 expression. Depletion of PRMT5 via siRNA inhibited proliferation in a subset of melanoma cell lines, while it accelerated growth of others. Loss of PRMT5 also led to reduced expression of MITF (microphthalmia-associated transcription factor), a melanocyte-lineage specific oncogene, and increased expression of the cell cycle regulator p27^Kip1. These results are the first to report elevated PRMT5 expression in human melanoma specimens and indicate this protein may regulate MITF and p27^Kip1 expression in human melanoma cells.

Methyltransferase PRMT1 is a binding partner of HBx and a negative regulator of hepatitis B virus transcription

The hepatitis B virus X protein (HBx) is essential for virus replication and has been implicated in the development of liver cancer. HBx is recruited to viral and cellular promoters and activates transcription by interacting with transcription factors and coactivators. Here, we purified HBx-associated factors in nuclear extracts from HepG2 hepatoma cells and identified protein arginine methyltransferase 1 (PRMT1) as a novel HBx-interacting protein. We showed that PRMT1 overexpression reduced the transcription of hepatitis B virus (HBV), and this inhibition was dependent on the methyltransferase function of PRMT1. Conversely, depletion of PRMT1 correlated with increased HBV transcription. Using a quantitative chromatin immunoprecipitation assay, we found that PRMT1 is recruited to HBV DNA, suggesting a direct effect of PRMT1 on the regulation of HBV transcription. Finally, we showed that HBx expression inhibited PRMT1-mediated protein methylation. Downregulation of PRMT1 activity was further observed in HBV-replicating cells in an in vivo animal model. Altogether, our results support the notion that the binding of HBx to PRMT1 might benefit viral replication by relieving the inhibitory activity of PRMT1 on HBV transcription.

Overexpression of PRMT5 promotes tumor cell growth and is associated with poor disease prognosis in epithelial ovarian cancer

PRMT5 has been reported to be involved in the processes of tumor progression at various steps. The aim of this study was to examine the role of PRMT5 in epithelial ovarian cancer (EOC). In this study, PRMT5 and Ki-67 expression were examined by immunohistochemistry (IHC) in cohorts of normal, benign, and cancerous ovarian tissues. PRMT5 overexpression was observed in 83.1% (98/118) of EOCs, and it was significantly associated with serous type, poor differentiation, advanced tumor stage, lymph node invasion, presence of residual tumor, and high expression of Ki-67 (p<0.05, respectively). Moreover, overexpression of PRMT5 was an independent prognostic marker for decreased overall survival and progression-free survival in univariate survival analysis and multivariate Cox regression analysis. In ovarian cancer cell lines A2780 and SKOV3, PRMT5 knockdown by siRNA inhibited cell growth/proliferation and induced apoptosis via upregulation of E2F-1. These results suggest that overexpression of PRMT5 correlates with an aggressive malignant phenotype and may constitute a novel prognostic factor for EOC. Thus, PRMT5 may represent a clinically effective new target for therapy of ovarian cancer.

Nucleosome occupancy and gene regulation during tumorigenesis

Nucleosomes are the basic structural units of eukaryotic chromatin. In recent years, it has become evident that nucleosomes and their position, in concert with other epigenetic mechanisms (such as DNA methylation, histone modifications, changes in histone variants, as well as small noncoding regulatory RNAs) play essential roles in the control of gene expression. Here, we discuss the mechanisms and factors that regulate nucleosome position and gene expression in normal and cancer cells.

Chemical and biochemical approaches in the study of histone methylation and demethylation

Histone methylation represents one of the most critical epigenetic events in DNA function regulation in eukaryotic organisms. Classic molecular biology and genetics tools provide significant knowledge about mechanisms and physiological roles of histone methyltransferases and demethylases in various cellular processes. In addition to this stream line, development and application of chemistry and chemistry-related techniques are increasingly involved in biological study, and offer information otherwise difficult to obtain by standard molecular biology methods. Herein, we review recent achievements and progress in developing and applying chemical and biochemical approaches in the study of histone methylation, including chromatin immunoprecipitation, chemical ligation, mass spectrometry, biochemical methylation and demethylation assays, and inhibitor development. These technological advances allow histone methylation to be studied from genome-wide level to molecular and atomic levels. With ChIP technology, information can be obtained about precise mapping of histone methylation patterns at specific promoters, genes, or other genomic regions. MS is particularly useful in detecting and analyzing methylation marks in histone and nonhistone protein substrates. Chemical approaches that permit site-specific incorporation of methyl groups into histone proteins greatly facilitate the investigation of biological impacts of methylation at individual modification sites. Discovery and design of selective organic inhibitors of histone methyltransferases and demethylases provide chemical probes to interrogate methylation-mediated cellular pathways. Overall, these chemistry-related technological advances have greatly improved our understanding of the biological functions of histone methylation in normal physiology and diseased states, and also are of great potential to translate basic epigenetics research into diagnostic and therapeutic applications in the clinic.

Transcription factor positive regulatory domain 4 (PRDM4) recruits protein arginine methyltransferase 5 (PRMT5) to mediate histone arginine methylation and control neural stem cell proliferation and differentiation

During development of the cerebral cortex, neural stem cells (NSCs) undergo a temporal switch from proliferative (symmetric) to neuron-generating (asymmetric) divisions. We investigated the role of Schwann cell factor 1 (SC1/PRDM4), a member of the PRDM family of transcription factors, in this critical transition. We discovered that SC1 recruits the chromatin modifier PRMT5, an arginine methyltransferase that catalyzes symmetric dimethylation of histone H4 arginine 3 (H4R3me2s) and that this modification is preferentially associated with undifferentiated cortical NSCs. Overexpressing SC1 in embryonic NSCs led to an increase in the number of Nestin-expressing precursors; mutational analysis of SC1 showed that this was dependent on recruitment of PRMT5. We found that SC1 protein levels are down-regulated at the onset of neurogenesis and that experimental knockdown of SC1 in primary NSCs triggers precocious neuronal differentiation. We propose that SC1 and PRMT5 are components of an epigenetic regulatory complex that maintains the “stem-like” cellular state of the NSC by preserving their proliferative capacity and modulating their cell cycle progression. Our findings provide evidence that histone arginine methylation regulates NSC differentiation.

Sepantronium bromide (YM155) induces disruption of the ILF3/p54(nrb) complex, which is required for survivin expression

YM155, a small-molecule survivin suppressant, specifically binds to the transcription factor ILF3, which regulates the expression of survivin[1]. In this experiment we have demonstrated that p54(nrb) binds to the survivin promoter and regulates survivin expression. p54(nrb) forms a complex with ILF3, which directly binds to YM155. YM155 induces disruption of the ILF3/p54(nrb) complex, which results in a different subcellular localization between ILF3 and p54(nrb). Thus, identification of molecular targets of YM155 in suppression of the survivin pathway, might lead to development of its use as a novel potential target in cancers.

Protein arginine methyltransferase 7 regulates cellular response to DNA damage by methylating promoter histones H2A and H4 of the polymerase δ catalytic subunit gene, POLD1

Covalent modification of histones by protein arginine methyltransferases (PRMTs) impacts genome organization and gene expression. In this report, we show that PRMT7 interacts with the BRG1-based hSWI/SNF chromatin remodeling complex and specifically methylates histone H2A Arg-3 (H2AR3) and histone H4 Arg-3 (H4R3). To elucidate the biological function of PRMT7, we knocked down its expression in NIH 3T3 cells and analyzed global gene expression. Our findings show that PRMT7 negatively regulates expression of genes involved in DNA repair, including ALKBH5, APEX2, POLD1, and POLD2. Chromatin immunoprecipitation (ChIP) revealed that PRMT7 and dimethylated H2AR3 and H4R3 are enriched at target DNA repair genes in parental cells, whereas PRMT7 knockdown caused a significant decrease in PRMT7 recruitment and H2AR3/H4R3 methylation. Decreased PRMT7 expression also resulted in derepression of target DNA repair genes and enhanced cell resistance to DNA-damaging agents. Furthermore, we show that BRG1 co-localizes with PRMT7 on target promoters and that expression of a catalytically inactive form of BRG1 results in derepression of PRMT7 target DNA repair genes. Remarkably, reducing expression of individual PRMT7 target DNA repair genes showed that only the catalytic subunit of DNA polymerase, POLD1, was able to resensitize PRMT7 knock-down cells to DNA-damaging agents. These results provide evidence for the important role played by PRMT7 in epigenetic regulation of DNA repair genes and cellular response to DNA damage.

Synthesis and evaluation of carbocyanine dyes as PRMT inhibitors and imaging agents

Protein arginine methylation regulates multiple biological processes. Deregulation of protein arginine methyltransferase (PRMT) activities has been observed in many disease phenotypes. Small molecule probes that target PRMTs with strong affinity and selectivity can be used as valuable tools to dissect biological mechanisms of arginine methylation and establish the role of PRMT proteins in a disease process. In this work, we report synthesis and evaluation of a class of carbocyanine compounds containing indolium, benz[e]indolium or benz[c,d]indolium heterocyclic moieties that bind to the predominant arginine methyltransferase PRMT1 and inhibit its methyltransferase activity at low micromolar potencies. In particular, the developed molecules have long wavelength colorimetric and fluorometric photoactivities, which can be used for optical and near-infrared fluorescence imaging in cells or biological tissues. Together, these new chemical probes have potential application in PRMT studies both as enzyme inhibitors and as fluorescent dyes for microscope imaging.

The HPV E6 oncoprotein targets histone methyltransferases for modulating specific gene transcription

Expression of viral proteins causes important epigenetic changes leading to abnormal cell growth. Whether viral proteins directly target histone methyltransferases (HMTs), a key family enzyme for epigenetic regulation, and modulate their enzymatic activities remains elusive. Here we show that the E6 proteins of both low-risk and high-risk human papillomavirus (HPV) interact with three coactivator HMTs, CARM1, PRMT1 and SET7, and downregulate their enzymatic activities in vitro and in HPV-transformed HeLa cells. Furthermore, these three HMTs are required for E6 to attenuate p53 transactivation function. Mechanistically, E6 hampers CARM1- and PRMT1-catalyzed histone methylation at p53-responsive promoters, and suppresses the binding of p53 to chromatinized DNA independently of E6-mediated p53 degradation. p53 pre-methylated at lysine-372 (p53K372 mono-methylation) by SET7 protects p53 from E6-induced degradation. Consistently, E6 downregulates p53K372 mono-methylation and thus reduces p53 protein stability. As a result of the E6-mediated inhibition of HMT activity, expression of p53 downstream genes is suppressed. Together, our results not only reveal a clever approach for the virus to interfere with p53 function, but also demonstrate the modulation of HMT activity as a novel mechanism of epigenetic regulation by a viral oncoprotein.

Histone methylation in myelodysplastic syndromes

Histone methylation is a type of epigenetic modification that is critical for the regulation of gene expression. Numerous studies have demonstrated that abnormalities of this newly characterized epigenetic modification are involved in the development of multiple diseases, including cancer. There is also emerging evidence for a link between histone methylation and the pathogenesis of myeloid neoplasms, including myelodysplastic syndromes (MDS). This article provides an overview of recent progress in the studies of histone methylation in myeloid malignancies, with an emphasis on MDS. We cover each type of histone methylation modification and their regulatory mechanisms, as well as their abnormalities in MDS or potential connections to MDS. We also summarize the recent progress in the development of inhibitors targeting histone methylation and their applications as potential therapeutic agents.

HOXA9 methylation by PRMT5 is essential for endothelial cell expression of leukocyte adhesion molecules

The induction of proinflammatory proteins in stimulated endothelial cells (EC) requires activation of multiple transcription programs. The homeobox transcription factor HOXA9 has an important regulatory role in cytokine induction of the EC-leukocyte adhesion molecules (ELAM) E-selectin and vascular cell adhesion molecule 1 (VCAM-1). However, the mechanism underlying stimulus-dependent activation of HOXA9 is completely unknown. Here, we elucidate the molecular mechanism of HOXA9 activation by tumor necrosis factor alpha (TNF-α) and show an unexpected requirement for arginine methylation by protein arginine methyltransferase 5 (PRMT5). PRMT5 was identified as a TNF-α-dependent binding partner of HOXA9 by mass spectrometry. Small interfering RNA (siRNA)-mediated depletion of PRMT5 abrogated stimulus-dependent HOXA9 methylation with concomitant loss in E-selectin or VCAM-1 induction. Chromatin immunoprecipitation analysis revealed that PRMT5 is recruited to the E-selectin promoter following transient HOXA9 binding to its cognate recognition sequence. PRMT5 induces symmetric dimethylation of Arg140 on HOXA9, an event essential for E-selectin induction. In summary, PRMT5 is a critical coactivator component in a newly defined, HOXA9-containing transcription complex. Moreover, stimulus-dependent methylation of HOXA9 is essential for ELAM expression during the EC inflammatory response.

Protein arginine methyltransferase 5 (Prmt5) promotes gene expression of peroxisome proliferator-activated receptor γ2 (PPARγ2) and its target genes during adipogenesis

Regulation of adipose tissue formation by adipogenic-regulatory proteins has long been a topic of interest given the ever-increasing health concerns of obesity and type 2 diabetes in the general population. Differentiation of precursor cells into adipocytes involves a complex network of cofactors that facilitate the functions of transcriptional regulators from the CCATT/enhancer binding protein, and the peroxisome proliferator-activated receptor (PPAR) families. Many of these cofactors are enzymes that modulate the structure of chromatin by altering histone-DNA contacts in an ATP-dependent manner or by posttranslationally modifying the histone proteins. Here we report that inhibition of protein arginine methyltransferase 5 (Prmt5) expression in multiple cell culture models for adipogenesis prevented the activation of adipogenic genes. In contrast, overexpression of Prmt5 enhanced adipogenic gene expression and differentiation. Chromatin immunoprecipitation experiments indicated that Prmt5 binds to and dimethylates histones at adipogenic promoters. Furthermore, the presence of Prmt5 promoted the binding of ATP-dependent chromatin-remodeling enzymes and was required for the binding of PPARγ2 at PPARγ2-regulated promoters. The data indicate that Prmt5 acts as a coactivator for the activation of adipogenic gene expression and promotes adipogenic differentiation.

Dietary factors and epigenetic regulation for prostate cancer prevention

The role of epigenetic alterations in various human chronic diseases has gained increasing attention and has resulted in a paradigm shift in our understanding of disease susceptibility. In the field of cancer research, e.g., genetic abnormalities/mutations historically were viewed as primary underlying causes; however, epigenetic mechanisms that alter gene expression without affecting DNA sequence are now recognized as being of equal or greater importance for oncogenesis. Methylation of DNA, modification of histones, and interfering microRNA (miRNA) collectively represent a cadre of epigenetic elements dysregulated in cancer. Targeting the epigenome with compounds that modulate DNA methylation, histone marks, and miRNA profiles represents an evolving strategy for cancer chemoprevention, and these approaches are starting to show promise in human clinical trials. Essential micronutrients such as folate, vitamin B-12, selenium, and zinc as well as the dietary phytochemicals sulforaphane, tea polyphenols, curcumin, and allyl sulfur compounds are among a growing list of agents that affect epigenetic events as novel mechanisms of chemoprevention. To illustrate these concepts, the current review highlights the interactions among nutrients, epigenetics, and prostate cancer susceptibility. In particular, we focus on epigenetic dysregulation and the impact of specific nutrients and food components on DNA methylation and histone modifications that can alter gene expression and influence prostate cancer progression.

The methyltransferases PRMT4/CARM1 and PRMT5 control differentially myogenesis in zebrafish

In vertebrates, skeletal myogenesis involves the sequential activation of myogenic factors to lead ultimately to the differentiation into slow and fast muscle fibers. How transcriptional co-regulators such as arginine methyltransferases PRMT4/CARM1 and PRMT5 control myogenesis in vivo remains poorly understood. Loss-of-function experiments using morpholinos against PRMT4/CARM1 and PRMT5 combined with in situ hybridization, quantitative polymerase chain reaction, as well as immunohistochemistry indicate a positive, but differential, role of these enzymes during myogenesis in vivo. While PRMT5 regulates myod, myf5 and myogenin expression and thereby slow and fast fiber formation, PRMT4/CARM1 regulates myogenin expression, fast fiber formation and does not affect slow fiber formation. However, our results show that PRMT4/CARM1 is required for proper slow myosin heavy chain localization. Altogether, our results reveal a combinatorial role of PRMT4/CARM1 and PRMT5 for proper myogenesis in zebrafish.

Fluorescence-based methods for screening writers and readers of histone methyl marks

The histone methyltransferase (HMT) family of proteins consists of enzymes that methylate lysine or arginine residues on histone tails as well as other proteins. Such modifications affect chromatin structure and play a significant regulatory role in gene expression. Many HMTs have been implicated in tumorigenesis and progression of multiple malignancies and play essential roles in embryonic development and stem cell renewal. Overexpression of some HMTs has been observed and is correlated positively with various types of cancer. Here the authors report development of a continuous fluorescence-based methyltransferase assay in a 384-well format and its application in determining kinetic parameters for EHMT1, G9a, PRMT3, SETD7, and SUV39H2 as well as for screening against libraries of small molecules to identify enzyme inhibitors. They also report the development of a peptide displacement assay using fluorescence polarization in a 384-well format to assay and screen protein peptide interactions such as those of WDR5 and EED, components of MLL and EZH2 methyltransferase complexes. Using these high-throughput screening methods, the authors have identified potent inhibitors and ligands for some of these proteins.

Versatility of PRMT5-induced methylation in growth control and development

Arginine methylation governs important cellular processes that impact growth and proliferation, as well as differentiation and development. Through their ability to catalyze symmetric or asymmetric methylation of histone and non-histone proteins, members of the protein arginine methyltransferase (PRMT) family regulate chromatin structure and expression of a wide spectrum of target genes. Unlike other PRMTs, PRMT5 works in concert with a variety of cellular proteins including ATP-dependent chromatin remodelers and co-repressors to induce epigenetic silencing. Recent work also implicates PRMT5 in the control of growth-promoting and pro-survival pathways, which demonstrates its versatility as an enzyme involved in both epigenetic regulation of anti-cancer target genes and organelle biogenesis. These studies not only provide insight into the molecular mechanisms by which PRMT5 contributes to growth control, but also justify therapeutic targeting of PRMT5.

Targets in epigenetics: inhibiting the methyl writers of the histone code

Growing evidence suggests that protein lysine methyltransferases (PKMTs) and protein arginine methyltransferases (PRMTs) are associated with the development of various human diseases, including cancer, inflammation, and psychiatric disorders. Given the significant role of these proteins in human disease, efforts to discover selective small-molecule inhibitors of these enzymes are quickly gaining momentum. In this review, we focus on the recent progress in the discovery of selective PKMT and PRMT inhibitors. A future perspective on developing methyltransferase inhibitors is also offered.

Brief report: combined chemical treatment enables Oct4-induced reprogramming from mouse embryonic fibroblasts

It has been established that exogenous expression of four transcription factors (Oct4, Klf4, Sox2, and c-Myc) can reprogram mammalian somatic cells to pluripotent states. Further studies demonstrated that such induced pluripotent stem cells (iPSCs) could be generated with fewer exogenous transcription factors, facilitated by endogenous expression of reprogramming factors and/or synthetic small molecules. Here, we reported identification of a new small molecule, a protein arginine methyltransferase inhibitor AMI-5, which enabled Oct4-induced reprogramming of mouse embryonic fibroblasts in combination with transforming growth factor (TGF)-β inhibitor A-83-01. The Oct4-induced iPSCs were shown similar to mouse embryonic stem cells with respect to typical pluripotency criteria. More importantly, they were shown to give rise to liveborn pups through tetraploid complementation assays, demonstrating the high quality of full reprogramming induced by this condition. Furthermore, this study suggests that regulation of protein arginine methylation might be involved in the reprogramming process.

A transient kinetic analysis of PRMT1 catalysis

Post-translational modifications (PTMs) are important strategies used by eukaryotic organisms to modulate their phenotypes. One of the well-studied PTMs, arginine methylation, is catalyzed by protein arginine methyltransferases (PRMTs) with SAM as the methyl donor. The functions of PRMTs have been broadly studied in different biological processes and diseased states, but the molecular basis for arginine methylation is not well-defined. In this study, we report the transient-state kinetic analysis of PRMT1 catalysis. The fast association and dissociation rates suggest that PRMT1 catalysis of histone H4 methylation follows a rapid equilibrium sequential kinetic mechanism. The data give direct evidence that the chemistry of methyl transfer is the major rate-limiting step and that binding of the cofactor SAM or SAH affects the association and dissociation of H4 with PRMT1. Importantly, from the stopped-flow fluorescence measurements, we have identified a critical kinetic step suggesting a precatalytic conformational transition induced by substrate binding. These results provide new insights into the mechanism of arginine methylation and the rational design of PRMT inhibitors.

Bromodomain protein 7 interacts with PRMT5 and PRC2, and is involved in transcriptional repression of their target genes

Histone modification regulates gene expression, and one major regulatory step in this process is the ability of proteins that recognize epigenetic marks to recruit enzymes required to specify transcriptional outcome. Here we show that BRD7 is a component of hSWI–SNF complexes that interacts with PRMT5 and PRC2. Recruitment studies revealed that BRD7 co-localizes with PRMT5 and PRC2 on ‘suppressor of tumorigenecity 7’ (ST7) and retinoblastoma-like protein 2 (RBL2) promoters in patient-derived B cell lines, and that its association with these target genes correlates with hypermethylation of H3R8, H4R3 and H3K27. Furthermore, inhibition of BRD7 expression reduces PRMT5 and PRC2 recruitment to ST7 and RBL2 promoters; however, only ST7 becomes transcriptionally derepressed. Evaluation of the PRMT5- and PRC2-induced epigenetic marks revealed that while H3(Me₂)R8, H4(Me₂)R3 and H3(Me₃)K27 marks are erased from the ST7 promoter, demethylation of RBL2 promoter histones is incomplete. We also show that the arginine demethylase (RDM) JMJD6, which can erase PRMT5-induced H4R3 methylation, and the H3K27-lysine-specific demethylases, KDM6A/UTX and KDM6B/JMJD3, are differentially recruited to ST7 and RBL2. These findings highlight the role played by BRD7 in PRMT5- and PRC2-induced transcriptional silencing, and indicate that recruitment of specific RDMs and KDMs is required for efficient transcriptional derepression.

A germ cell-specific gene, Prmt5, works in somatic cell reprogramming

Germ cells possess the unique ability to acquire totipotency during development in vivo as well as give rise to pluripotent stem cells under the appropriate conditions in vitro. Recent studies in which somatic cells were experimentally converted into pluripotent stem cells revealed that genes expressed in primordial germ cells (PGCs), such as Oct3/4, Sox2, and Lin28, are involved in this reprogramming. These findings suggest that PGCs may be useful for identifying factors that successfully and efficiently reprogram somatic cells into toti- and/or pluripotent stem cells. Here, we show that Blimp-1, Prdm14, and Prmt5, each of which is crucial for PGC development, have the potential to reprogram somatic cells into pluripotent stem cells. Among them, Prmt5 exhibited remarkable reprogramming of mouse embryonic fibroblasts into which Prmt5, Klf4, and Oct3/4 were introduced. The resulting cells exhibited pluripotent gene expression, teratoma formation, and germline transmission in chimeric mice, all of which were indistinguishable from those induced with embryonic stem cells. These data indicate that some of the factors that play essential roles in germ cell development are also active in somatic cell reprogramming.

A role for the arginine methylation of Rad9 in checkpoint control and cellular sensitivity to DNA damage

The genome stability is maintained by coordinated action of DNA repairs and checkpoints, which delay progression through the cell cycle in response to DNA damage. Rad9 is conserved from yeast to human and functions in cell cycle checkpoint controls. Here, a regulatory mechanism for Rad9 function is reported. In this study Rad9 has been found to interact with and be methylated by protein arginine methyltransferase 5 (PRMT5). Arginine methylation of Rad9 plays a critical role in S/M and G2/M cell cycle checkpoints. The activation of the Rad9 downstream checkpoint effector Chk1 is impaired in cells only expressing a mutant Rad9 that cannot be methylated. Additionally, Rad9 methylation is also required for cellular resistance to DNA damaging stresses. In summary, we uncovered that arginine methylation is important for regulation of Rad9 function, and thus is a major element for maintaining genome integrity.

Dynamics of epigenetic modifications in leukemia

Chromatin modifications at both histones and DNA are critical for regulating gene expression. Mis-regulation of such epigenetic marks can lead to pathological states; indeed, cancer affecting the hematopoietic system is frequently linked to epigenetic abnormalities. Here, we discuss the different types of modifications and their general impact on transcription, as well as the polycomb group of proteins, which effect transcriptional repression and are often mis-regulated. Further, we discuss how chromosomal translocations leading to fusion proteins can aberrantly regulate gene transcription through chromatin modifications within the hematopoietic system. PML-RARa, AML1-ETO and MLL-fusions are examples of fusion proteins that mis-regulate epigenetic modifications (either directly or indirectly), which can lead to acute myeloblastic leukemia (AML). An in-depth understanding of the mechanisms behind the mis-regulation of epigenetic modifications that lead to the development and progression of AMLs could be critical for designing effective treatments.

Natural history of the eukaryotic chromatin protein methylation system

In eukaryotes, methylation of nucleosomal histones and other nuclear proteins is a central aspect of chromatin structure and dynamics. The past 15 years have seen an enormous advance in our understanding of the biochemistry of these modifications, and of their role in establishing the epigenetic code. We provide a synthetic overview, from an evolutionary perspective, of the main players in the eukaryotic chromatin protein methylation system, with an emphasis on catalytic domains. Several components of the eukaryotic protein methylation system had their origins in bacteria. In particular, the Rossmann fold protein methylases (PRMTs and DOT1), and the LSD1 and jumonji-related demethylases and oxidases, appear to have emerged in the context of bacterial peptide methylation and hydroxylation systems. These systems were originally involved in synthesis of peptide secondary metabolites, such as antibiotics, toxins, and siderophores. The peptidylarginine deiminases appear to have been acquired by animals from bacterial enzymes that modify cell-surface proteins. SET domain methylases, which display the β-clip fold, apparently first emerged in prokaryotes from the SAF superfamily of carbohydrate-binding domains. However, even in bacteria, a subset of the SET domains might have evolved a chromatin-related role in conjunction with a BAF60a/b-like SWIB domain protein and topoisomerases. By the time of the last eukaryotic common ancestor, multiple SET and PRMT methylases were already in place and are likely to have mediated methylation at the H3K4, H3K9, H3K36, and H4K20 positions, and carried out both asymmetric and symmetric arginine dimethylation. Inference of H3K27 methylation in the ancestral eukaryote appears uncertain, though it was certainly in place a little later in eukaryotic evolution. Current data suggest that unlike SET methylases, which are universally present in eukaryotes, demethylases are not. They appear to be absent in the earliest-branching eukaryotic lineages, and emerged later along with several other chromatin proteins, such as the Dot1-methylase, prior to divergence of the kinetoplastid-heterolobosean lineage from the remaining eukaryotes. This period also corresponds to the point of origin of DNA cytosine methylation by DNMT1. Origin of major lineages of SET domains such as the Trithorax, Su(var)3-9, Ash1, SMYD, and TTLL12 and E(Z) might have played the initial role in the establishment of multiple distinct heterochromatic and euchromatic states that are likely to have been present, in some form, through much of eukaryotic evolution. Elaboration of these chromatin states might have gone hand-in-hand with acquisition of multiple jumonji-related and LSD1-like demethylases, and functional linkages with the DNA methylation and RNAi systems. Throughout eukaryotic evolution, there were several lineage-specific expansions of SET domain proteins, which might be related to a special transcription regulation process in trypanosomes, acquisition of new meiotic recombination hotspots in animals, and methylation and associated modifications of the diatom silaffin proteins involved in silica biomineralization. The use of specific domains to "read" the methylation marks appears to have been present in the ancestral eukaryote itself. Of these the chromo-like domains appear to have been acquired from bacterial secreted proteins that might have a role in binding cell-surface peptides or peptidoglycan. Domain architectures of the primary enzymes involved in the eukaryotic protein methylation system indicate key features relating to interactions with each other and other modifications in chromatin, such as acetylation. They also emphasize the profound functional distinction between the role of demethylation and deacetylation in regulation of chromatin dynamics.

Nuclear receptor coactivators: structural and functional biochemistry

Transcription of eukaryotic cell is a multistep process tightly controlled by concerted action of macromolecules. Nuclear receptors are ligand-activated sequence-specific transcription factors that bind DNA and activate (or repress) transcription of specific sets of nuclear target genes. Successful activation of transcription by nuclear receptors and most other transcription factors requires "coregulators" of transcription. Coregulators make up a diverse family of proteins that physically interact with and modulate the activity of transcription factors and other components of the gene expression machinery via multiple biochemical mechanisms. The coregulators include coactivators that accomplish reactions required for activation of transcription and corepressors that suppress transcription. This review summarizes our current knowledge of nuclear receptor coactivators with an emphasis on their biochemical mechanisms of action and means of regulation.

Nuclear cyclin D1/CDK4 kinase regulates CUL4 expression and triggers neoplastic growth via activation of the PRMT5 methyltransferase

Cyclin D1 elicits transcriptional effects through inactivation of the retinoblastoma protein and direct association with transcriptional regulators. The current work reveals a molecular relationship between cyclin D1/CDK4 kinase and protein arginine methyltransferase 5 (PRMT5), an enzyme associated with histone methylation and transcriptional repression. Primary tumors of a mouse lymphoma model exhibit increased PRMT5 methyltransferase activity and histone arginine methylation. Analyses demonstrate that MEP50, a PRMT5 coregulatory factor, is a CDK4 substrate, and phosphorylation increases PRMT5/MEP50 activity. Increased PRMT5 activity mediates key events associated with cyclin D1-dependent neoplastic growth, including CUL4 repression, CDT1 overexpression, and DNA rereplication. Importantly, human cancers harboring mutations in Fbx4, the cyclin D1 E3 ligase, exhibit nuclear cyclin D1 accumulation and increased PRMT5 activity.

A Bayesian network model of proteins' association with promyelocytic leukemia (PML) nuclear bodies

The modularity that nuclear organization brings has the potential to explain the function of aggregates of proteins and RNA. Promyelocytic leukemia nuclear bodies are implicated in important regulatory processes. To understand the complement of proteins associated with these intra-nuclear bodies, we construct a Bayesian network model that integrates sequence and protein-protein interaction data. The model predicts association with promyelocytic leukemia nuclear bodies accurately when interaction data is available. At a false positive rate of 10%, the true positive rate is almost 50%, indicated by an independent nuclear proteome reference set. The model provides strong support for further expanding the protein complement with several important regulators and a richer functional repertoire. Using special support vector machine (SVM)-nodes (equipped with string kernels), the Bayesian network is also able to produce predictions on the basis of sequence only, with an accuracy superior to that of baseline models. Supplementary Material is available online at www.liebertonline.com.

Identification of a PRMT5-dependent repressor complex linked to silencing of human fetal globin gene expression

Defining the molecular mechanisms underpinning fetal (gamma) globin gene silencing may provide strategies for reactivation of gamma-gene expression, a major therapeutic objective in patients with beta-thalassemia and sickle cell disease (SCD). We have previously demonstrated that symmetric methylation of histone H4 Arginine 3 (H4R3me2s) by the protein arginine methyltransferase PRMT5 is required for recruitment of the DNA methyltransferase DNMT3A to the gamma-promoter, and subsequent DNA methylation and gene silencing. Here we show in an erythroid cell line, and in primary adult erythroid progenitors that PRMT5 induces additional repressive epigenetic marks at the gamma-promoter through the assembly of a multiprotein repressor complex containing the histone modifying enzymes SUV4-20h1, casein kinase 2alpha (CK2alpha), and components of the nucleosome remodeling and histone deacetylation complex. Expression of a mutant form of PRMT5 lacking methyltransferase activity or shRNA-mediated knockdown of SUV4-20h1 resulted in loss of complex binding to the gamma-promoter, reversal of both histone and DNA repressive epigenetic marks, and increased gamma-gene expression. The repressive H4K20me3 mark induced by SUV4-20h1 is enriched on the gamma-promoter in erythroid progenitors from adult bone marrow compared with cord blood, suggesting developmental specificity. These studies define coordinated epigenetic events linked to fetal globin gene silencing, and provide potential therapeutic targets for the treatment of beta-thalassemia and SCD.

Chromatin remodeling regulation by small molecules and metabolites

The eukaryotic genome is a highly organized nucleoprotein structure comprising of DNA, histones, non-histone proteins, and RNAs, referred to as chromatin. The chromatin exists as a dynamic entity, shuttling between the open and closed forms at specific nuclear regions and loci based on the requirement of the cell. This dynamicity is essential for the various DNA-templated phenomena like transcription, replication, and repair and is achieved through the activity of ATP-dependent chromatin remodeling complexes and covalent modifiers of chromatin. A growing body of data indicates that chromatin enzymatic activities are finely and specifically regulated by a variety of small molecules derived from the intermediary metabolism. This review tries to summarize the work conducted in many laboratories and on different model organisms showing how ATP-dependent chromatin remodeling complexes are regulated by small molecules and metabolites such as adenosine triphosphate (ATP), acetyl coenzyme A (AcCoA), S-adenosyl methionine (SAM), nicotinamide adenine dinucleotide (NAD), and inositol polyphosphates (IPs).

Methylation of histone H3 and H4 by PRMT5 regulates ribosomal RNA gene transcription

In an effort to understand the epigenetic regulation of ribosomal RNA gene (rDNA) expression we have previously demonstrated the role of DNA methyltransferases and methyl CpG binding proteins in rRNA synthesis. Here, we studied the role of protein arginine methyltransferase PRMT5 and the two methylated histones H3R8Me2 and H4R3Me2, in rDNA expression in Epstein Barr virus- transformed primary B-cells (LCLs) and in HeLa cells responding to serum-regulated growth. Chromatin immunoprecipitation assay showed that histones H3 and H4 associated with rRNA promoters were differentially methylated at arginine residues 8 and 3, respectively, depending on its transcriptional activity. Association of PRMT5 and methylated H3 with the unmethylated promoters in resting B-cells was significantly reduced in rapidly growing LCLs. Unlike PRMT5 and H3R8Me2, histone H4 associated with both methylated and unmethylated rRNA promoters in resting B-cells was methylated at the R3 residue. However, a dramatic decrease in R3 methylation of H4 recruited to the unmethylated rRNA promoters was observed in LCLs while it remained unaltered in the fraction bound to the methylated promoters. Differential interaction of PRMT5 and methylation of H3 and H4 associated with the rRNA promoters was also observed when serum starved HeLa cells were allowed to grow in serum replenished media. Ectopic expression of PRMT5 suppressed activity of both unmethylated and methylated rRNA promoter in transient transfection assay whereas siRNA mediated knockdown of PRMT5 increased rRNA synthesis in HeLa cells. These data suggest a key role of PRMT5 and the two methylated histones in regulating rRNA promoter activity.

BTG1 regulates glucocorticoid receptor autoinduction in acute lymphoblastic leukemia

Resistance to glucocorticoids (GCs) is a major clinical problem in the treatment of acute lymphoblastic leukemia (ALL), but the underlying mechanisms are not well understood. Although mutations in the glucocorticoid receptor (GR) gene can give rise to therapy resistance in vitro, acquired somatic mutations in the GR are rarely encountered in patients. Here we report that the protein encoded by the BTG1 gene, which is frequently deleted in (pediatric) ALL, is a key determinant of GC responsiveness. Using RNA interference, we show that loss of BTG1 expression causes GC resistance both by decimating GR expression and by controlling GR-mediated transcription. Conversely, reexpression of BTG1 restores GC sensitivity by potentiating GC-induced GR expression, a phenomenon known as GR autoinduction. In addition, the arginine methyltransferase PRMT1, a BTG1-binding partner and transcriptional coactivator, is recruited to the GR gene promoter in a BTG1-dependent manner. These results implicate the BTG1/PRMT1 complex in GR-mediated gene expression and reveal that deregulation of a nuclear receptor coactivator complex can give rise to GC resistance. Further characterization of this complex as part of the GR regulatory circuitry could offer novel opportunities for improving the efficacy of GC-based therapies in ALL and other hematologic malignancies.

Protein arginine methylation in estrogen signaling and estrogen-related cancers

Estrogen signaling pathways regulate multiple cellular processes including proliferation and differentiation, and dysregulation of these pathways underlies several human pathologies. Post-translational modifications (PTMs) play an important role in estrogen signaling. This review focuses on recent findings pertinent to arginine methylation of non-histone proteins and their implications in estrogen signaling. We describe protein arginine methyltransferases and demethylases, the role of methylarginine proteins in estrogen action and crosstalk with other PTMs such as phosphorylation and lysine methylation. The relationships between various PTMs form a specific code that is likely to play an important role in hormone signaling. In addition, dysregulation of arginine methylation or of enzymes responsible for these modifications could be key events in estrogen-dependent cancers such as breast cancer.

The role of histone modifications and variants in regulating gene expression in breast cancer

The role of epigenetic phenomena in cancer biology is increasingly being recognized. Here we focus on the mechanisms and enzymes involved in regulating histone methylation and acetylation, and the modulation of histone variant expression and deposition. Implications of these epigenetic marks for tumor development, progression and invasiveness are discussed with a particular emphasis on breast cancer progression.

Hepatitis C virus-induced up-regulation of protein phosphatase 2A inhibits histone modification and DNA damage repair

The molecular mechanisms underlying hepatocarcinogenesis in chronic viral hepatitis are poorly understood. A potential tumorigenic pathway could involve protein phosphatase 2A (PP2A) and protein arginine methyltransferase 1 (PRMT1), because both enzymes are dysregulated in chronic hepatitis C, and both enzymes have been involved in chromatin remodeling and DNA damage repair. We used cell lines that allow the inducible expression of hepatitis C virus proteins (UHCV57.3) and of the catalytic subunit of PP2A (UPP2A-C8) as well as Huh7.5 cells infected with recombinant cell culture-derived hepatitis C virus (HCVcc) to study epigenetic histone modifications and DNA damage repair. The induction of viral proteins, the overexpression of PP2Ac, or the infection of Huh7.5 cells with HCVcc resulted in an inhibition of histone H4 methylation/acetylation and histone H2AX phosphorylation, in a significantly changed expression of genes important for hepatocarcinogenesis, and inhibited DNA damage repair. Overexpression of PP2Ac in NIH-3T3 cells increased anchorage-independent growth. These changes were partially reversed by the treatment of cells with the methyl-group donor S-adenosyl-L-methionine (SAMe).

CONCLUSION

Hepatitis C virus-induced overexpression of PP2Ac contributes to hepatocarcinogenesis through dysregulation of epigenetic histone modifications. The correction of defective histone modifications by S-adenosyl-L-methionine makes this drug a candidate for chemopreventive therapies in patients with chronic hepatitis C who are at risk for developing hepatocellular carcinoma.

Friend of Prmt1, a novel chromatin target of protein arginine methyltransferases

We describe the isolation and characterization of Friend of Prmt1 (Fop), a novel chromatin target of protein arginine methyltransferases. Human Fop is encoded by C1orf77, a gene of previously unknown function. We show that Fop is tightly associated with chromatin, and that it is modified by both asymmetric and symmetric arginine methylation in vivo. Furthermore, Fop plays an important role in the ligand-dependent activation of estrogen receptor target genes, including TFF1 (pS2). Fop depletion results in an almost complete block of estradiol-induced promoter occupancy by the estrogen receptor. Our data indicate that Fop recruitment to the promoter is an early critical event in the activation of estradiol-dependent transcription.

Targeting epigenetic modifiers: Inhibitors of histone methyltransferases

Reversible histone methylation is an emerging new target in the field of epigenetic regulation. Here, we review histone methyltransferases in light of their role in disease formation and with regard to the state of the art in drug discovery.

Epigenetic mechanisms facilitating oligodendrocyte development, maturation, and aging

The process of oligodendrocyte differentiation is regulated by a dynamic interaction between a genetic and an epigenetic program. Recent studies, addressing nucleosomal histone modifications have considerably increased our knowledge regarding epigenetic regulation of gene expression during oligodendrocyte development and aging. These results have generated new hypotheses regarding the mechanisms underlying the decreased efficiency of endogenous remyelination in response to demyelinating injuries with increasing age. In this review, we present an overview of the epigenetic mechanisms regulating gene expression at specific stages of oligodendrocyte differentiation and maturation as well as the changes that occur with aging.

Poly(ADP-ribose)-dependent regulation of DNA repair by the chromatin remodeling enzyme ALC1

Posttranslational modifications play key roles in regulating chromatin plasticity. Although various chromatin-remodeling enzymes have been described that respond to specific histone modifications, little is known about the role of poly[adenosine 5'-diphosphate (ADP)-ribose] in chromatin remodeling. Here, we identify a chromatin-remodeling enzyme, ALC1 (Amplified in Liver Cancer 1, also known as CHD1L), that interacts with poly(ADP-ribose) and catalyzes PARP1-stimulated nucleosome sliding. Our results define ALC1 as a DNA damage-response protein whose role in this process is sustained by its association with known DNA repair factors and its rapid poly(ADP-ribose)-dependent recruitment to DNA damage sites. Furthermore, we show that depletion or overexpression of ALC1 results in sensitivity to DNA-damaging agents. Collectively, these results provide new insights into the mechanisms by which poly(ADP-ribose) regulates DNA repair.

Arginine methylation regulates telomere length and stability

TRF2, a component of the shelterin complex, functions to protect telomeres. TRF2 contains an N-terminal basic domain rich in glycines and arginines, similar to the GAR motif that is methylated by protein arginine methyltransferases. However, whether arginine methylation regulates TRF2 function has not been determined. Here we report that amino acid substitutions of arginines with lysines in the basic domain of TRF2 induce telomere dysfunction-induced focus formation, leading to induction of cellular senescence. We have demonstrated that cells overexpressing TRF2 lysine mutants accumulate telomere doublets, indicative of telomere instability. We uncovered that TRF2 interacts with PRMT1, and its arginines in the basic domain undergo PRMT1-mediated methylation both in vitro and in vivo. We have shown that loss of PRMT1 induces growth arrest in normal human cells but has no effect on cell proliferation in cancer cells, suggesting that PRMT1 may control cell proliferation in a cell type-specific manner. We found that depletion of PRMT1 in normal human cells results in accumulation of telomere doublets, indistinguishable from overexpression of TRF2 lysine mutants. PRMT1 knockdown in cancer cells upregulates TRF2 association with telomeres, promoting telomere shortening. Taken together, these results suggest that PRMT1 may control telomere length and stability in part through TRF2 methylation.

The distribution and characterization of endogenous protein arginine N-methyltransferase 8 in mouse CNS

Protein arginine N-methyltransferase (PRMT) 8 was first discovered from a database search for genes harboring four conserved methyltransferase motifs, which shares more than 80% homology to PRMT1 in amino acid [Lee J, Sayegh J, Daniel J, Clarke S, Bedford MT (2005) PRMT8, a new membrane-bound tissue-specific member of the protein arginine methyltransferase family. J Biol Chem 280:32890-32896]. Interestingly, its tissue distribution is strikingly restricted to mouse CNS. To characterize the function in the CNS neurons, we raised an antiserum against PRMT8 to perform immunohistochemistry (IHC) and Western blot analysis. By IHC, the immunoreactivity of endogenous PRMT8 was broadly distributed in the CNS neurons with markedly intense signals in the cerebellum, hippocampal formation, and cortex, but was not detected in the cerebellar granular layer. In some subset of the neurons, the immunoreactivity was observed in the dendrites and axon bundles. The subcellular localization of the immunoreactivity was dominantly nuclear, arguing against the original report that exogenously expressed PRMT8 localizes to the plasma membrane via the N-terminal myristoylation. A series of the exogenously expressed proteins with different in-frame translation initiation codons was tested for comparison with the endogenous protein in molecular size. The third initiator codon produced the protein that was equivalent in size to the endogenous and showed a similar localizing pattern in PC12 cells. In conclusion, PRMT8 is a neuron-specific nuclear enzyme and the N-terminus does not contain the glycine end for myristoylation target.

The protein arginine methyltransferase family: an update about function, new perspectives and the physiological role in humans

Information about the family of protein arginine methyltransferases (PRMTs) has been growing rapidly over the last few years and the emerging role of arginine methylation involved in cellular processes like signaling, RNA processing, gene transcription, and cellular transport function has been investigated. To date, 11 PRMTs gene transcripts have been identified in humans. Almost all PRMTs have been shown to have enzymatic activity and to catalyze arginine methylation. This review will summarize the overall function of human PRMTs and include novel highlights on each family member.

The molecular biology of mixed lineage leukemia

Mixed lineage leukemia is a very aggressive blood cancer that predominantly occurs in pediatric patients. In contrast to other types of childhood acute leukemias, mixed lineage leukemia presents with a dismal prognosis and despite the availability of advanced treatment methods cure rates have stagnated over the last years. Mixed lineage leukemia is characterized by the presence of MLL fusion proteins that are the result of chromosomal translocations affecting the MLL gene at 11q23. These events juxtapose the amino-terminus of the histone methyltransferase MLL with a variety of different fusion partners that destroy normal histone methyltransferase function of MLL and replace it by heterologous functions contributed by the fusion partner. The resulting chimeras are transcriptional regulators that take control of targets normally controlled by MLL with the clustered HOX homeobox genes as prominent examples. Recent studies suggested that MLL fusion partners activate transcription by two different mechanisms. Some of these proteins are themselves chromatin modifiers that introduce histone acetylation whereas other fusion partners can recruit histone methyltransferases. In particular, histone H3 specific methylation at lysine 79 catalyzed by DOT1L has been recognized as a hallmark of chromatin activated by MLL fusion proteins. Interestingly, several frequent MLL fusion partners seem to coordinate DOT1L activity with a protein complex that stimulates the elongation phase of transcription by phosphorylating the carboxy-terminal repeat domain of RNA polymerase II. The discovery of these novel enzymatic activities that are essentially involved in MLL fusion protein function presents potential new targets for a rational drug development.

Caenorhabditis elegans protein arginine methyltransferase PRMT-5 negatively regulates DNA damage-induced apoptosis

Arginine methylation of histone and non-histone proteins is involved in transcription regulation and many other cellular processes. Nevertheless, whether such protein modification plays a regulatory role during apoptosis remains largely unknown. Here we report that the Caenorhabditis elegans homolog of mammalian type II arginine methyltransferase PRMT5 negatively regulates DNA damage-induced apoptosis. We show that inactivation of C. elegans prmt-5 leads to excessive apoptosis in germline following ionizing irradiation, which is due to a CEP-1/p53–dependent up-regulation of the cell death initiator EGL-1. Moreover, we provide evidence that CBP-1, the worm ortholog of human p300/CBP, functions as a cofactor of CEP-1. PRMT-5 forms a complex with both CEP-1 and CBP-1 and can methylate the latter. Importantly, down-regulation of cbp-1 significantly suppresses DNA damage-induced egl-1 expression and apoptosis in prmt-5 mutant worms. These findings suggest that PRMT-5 likely represses CEP-1 transcriptional activity through CBP-1, which represents a novel regulatory mechanism of p53-dependent apoptosis.

Protein arginine methylation is an important posttranslational modification. Aberrant expression of protein arginine methyltransferases (PRMTs) are found in a wide variety of human diseases, especially in many cancers. Given that deregulation of apoptosis is usually related to tumorigenesis, it is not known whether PRMT–mediated protein arginine methylation plays a role in apoptosis. Here we employ the genetic tractable model organism C. elegans to explore the potential regulatory roles of PRMTs in apoptosis. We find that C. elegans PRMTs do not affect developmental cell deaths. However, genetic inactivation of the C. elegans homolog of the mammalian type II protein arginine methyltransferase PRMT5 causes excessive germ cell apoptosis in response to DNA damage. Our genetic analyses indicate that prmt-5–mediated apoptosis is dependent on the C. elegans p53 homolog CEP-1 and requires the core cell death pathway. We further demonstrate that loss of prmt-5 leads to a specific up-regulation of the cell death initiator EGL-1 following DNA damage. Finally, we identify CBP-1, the C. elegans homolog of human p300/CBP, as a CEP-1 cofactor in C. elegans, and we provide genetic and biochemical evidence that PRMT-5 likely functions through CBP-1 to affect CEP-1/p53 transcriptional activity, thereby negatively regulating CEP-1/p53-dependent apoptosis.

Characterization of PRMT1 from Plasmodium falciparum

Arginine methylation is a post-translational modification that affects many cellular processes in eukaryotes. The malaria parasite Plasmodium falciparum encodes three conserved PRMTs (protein arginine N-methyltransferases). We have determined that PfPRMT1 (P. falciparum PRMT1) has authentic type I PRMT activity to form monomethylarginines and asymmetric dimethylarginines. Compared with mammalian PRMT1s, PfPRMT1 possesses a distinctive N-terminal sequence that is approximately 50 amino acids longer and is essential for enzyme activity. Recombinant PfPRMT1 methylated histones H4 and H2A and several conserved substrates involved in RNA metabolism, including fibrillarin, poly(A)-binding protein II, ribosomal protein S2 and a putative splicing factor. Using synthetic peptides and MS, we determined target arginines in several substrates and studied the enzyme kinetics. Whereas the kinetic parameters of recombinant PfPRMT1 on an H4 peptide and S-adenosylmethionine were similar to those of mammalian PRMT1s, PfPRMT1 had much higher substrate-turnover rates. In the histone H4 N-terminus, PfPRMT1 could methylate only Arg3, a mark for transcription activation. Western blotting detected dynamic dimethylation of H4-Arg3 during parasite development, suggesting that histone-arginine methylation may play a conserved role in chromatin-mediated gene regulation. Consistent with the presence of potential substrates in both the cytoplasm and nucleus, green fluorescent protein-tagged PfPRMT1 and untagged PfPRMT1 were localized in both cellular compartments, with the majority in the cytoplasm. in vitro assays showed that PfPRMT1 could be inhibited by several small-molecule inhibitors, with IC50-values in the sub-micromolar range. Most of these compounds also effectively inhibited parasite growth, suggesting that parasite PRMTs are promising targets for developing antiparasitic drugs.