Protein arginine methylation, one of the most abundant and important posttranslational modifications, is involved in a multitude of biological processes in eukaryotes, such as transcriptional regulation and RNA processing. Symmetric arginine dimethylation is required for snRNP biogenesis and is assumed to be essential for pre-mRNA splicing; however, except for in vitro evidence, whether it affects splicing in vivo remains elusive. Mutation in an Arabidopsis symmetric arginine dimethyltransferase, AtPRMT5, causes pleiotropic developmental defects, including late flowering, but the underlying molecular mechanism is largely unknown. Here we show that AtPRMT5 methylates a wide spectrum of substrates, including some RNA binding or processing factors and U snRNP AtSmD1, D3, and AtLSm4 proteins, which are involved in RNA metabolism. RNA-seq analyses reveal that AtPRMT5 deficiency causes splicing defects in hundreds of genes involved in multiple biological processes. The splicing defects are identified in transcripts of several RNA processing factors involved in regulating flowering time. In particular, splicing defects at the flowering regulator FLOWERING LOCUS KH DOMAIN (FLK) in atprmt5 mutants reduce its functional transcript and protein levels, resulting in the up-regulation of a flowering repressor FLOWERING LOCUS C (FLC) and consequently late flowering. Taken together, our findings uncover an essential role for arginine methylation in proper premRNA splicing that impacts diverse developmental processes. P RMT5 is a type II protein arginine methyltransferase that catalyzes the formation of monomethylarginine and symmetric ω-N G , N′ G -dimethylarginine (SDMA) (1). PRMT5 is highly conserved among yeast, animals, and higher plants, and is implicated in diverse cellular and biological processes, including transcriptional regulation, RNA metabolism (1, 2), ribosome biogenesis (3), Golgi apparatus structure maintenance (4), and cell cycle regulation (5). In mouse, fly, and worm, PRMT5 orthologs are involved in germ-cell formation, specification, and maintenance (6-10).In mammalian cells, PRMT5 localizes to both the cytoplasm and the nucleus, and methylates multiple histone and nonhistone proteins (1). In the nucleus, PRMT5 usually functions in repressing target genes by methylating histone H4 Arginine 3 (H4R3), H3R8, as well as transcription factors/regulators (5, 11, 12). In the cytoplasm, PRMT5 mainly exists in a 20S methylosome complex, consisting of spliceosomal U snRNP (uridinerich small nuclear riboucleoprotein particles) Sm proteins (including Sm B/B′, D1, D2, D3, E, F, and G), PRMT5, pICln, and WD repeat protein/MEP50 (13-15). In this complex, U1, 2, 4, 5 snRNP Sm proteins Sm B/B′, D1, D3, and U6 snRNP-specific Sm-like protein LSm4 are symmetrically dimethylated by PRMT5 (13,16). Such methylation can increase the binding affinity of these Sm proteins for the downstream recipient, Survival Motor Neuron, the spinal muscular atrophy disease gene product (17,18). Subsequently, the PRMT5-and Survival Motor Neuroncom...

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Methylation and nitric oxide (NO)-based S-nitrosylation are highly conserved protein posttranslational modifications that regulate diverse biological processes. In higher eukaryotes, PRMT5 catalyzes Arg symmetric dimethylation, including key components of the spliceosome. The Arabidopsis prmt5 mutant shows severe developmental defects and impaired stress responses. However, little is known about the mechanisms regulating the PRMT5 activity. Here, we report that NO positively regulates the PRMT5 activity through S-nitrosylation at Cys-125 during stress responses. In prmt5-1 plants, a PRMT5 transgene, carrying a non-nitrosylatable mutation at Cys-125, fully rescues the developmental defects, but not the stress hypersensitive phenotype and the responsiveness to NO during stress responses. Moreover, the salt-induced Arg symmetric dimethylation is abolished in PRMT5/prmt5-1 plants, correlated to aberrant splicing of pre-mRNA derived from a stress-related gene. These findings define a mechanism by which plants transduce stress-triggered NO signal to protein methylation machinery through S-nitrosylation of PRMT5 in response to environmental alterations.

Noncoding RNA plays essential roles in transcriptional control and chromatin silencing. AtArabidopsis thaliana FLC,antisense transcription quantitatively influences transcriptional output, but the mechanism by which this occurs is still unclear. Proximal polyadenylation of the antisense transcripts by FCA, an RNA-binding protein that physically interacts with RNA 3′ processing factors, reducesFLCtranscription. This process genetically requires FLD, a homolog of the H3K4 demethylase LSD1. However, the mechanism linking RNA processing to FLD function had not been established. Here, we show that FLD tightly associates with LUMINIDEPENDENS (LD) and SET DOMAIN GROUP 26 (SDG26) in vivo, and, together, they prevent accumulation of monomethylated H3K4 (H3K4me1) over theFLCgene body. SDG26 interacts with the RNA 3′ processing factor FY (WDR33), thus linking activities for proximal polyadenylation of the antisense transcripts to FLD/LD/SDG26-associated H3K4 demethylation. We propose this demethylation antagonizes an active transcription module, thus reducing H3K36me3 accumulation and increasing H3K27me3. Consistent with this view, we show that Polycomb Repressive Complex 2 (PRC2) silencing is genetically required by FCA to repressFLC. Overall, our work provides insights into RNA-mediated chromatin silencing.

Protein arginine methylation, catalyzed by protein arginine methyltransferases (PRMTs), is involved in a multitude of biological processes in eukaryotes. Symmetric arginine dimethylation mediated by PRMT5 modulates constitutive and alternative pre-mRNA splicing of diverse genes to regulate normal growth and development in multiple species; however, the underlying molecular mechanism remains largely unknown. A genetic screen for suppressors of an Arabidopsis symmetric arginine dimethyltransferase mutant, atprmt5, identified two gain-of-function alleles of pre-mRNA processing factor 8 gene (prp8-8 and prp8-9), the highly conserved core component of the U5 small nuclear ribonucleoprotein (snRNP) and the spliceosome. These two atprmt5 prp8 double mutants showed suppression of the developmental and splicing alterations of atprmt5 mutants. In atprmt5 mutants, the NineTeen complex failed to be assembled into the U5 snRNP to form an activated spliceosome; this phenotype was restored in the atprmt5 prp8-8 double mutants. We also found that loss of symmetric arginine dimethylation of Sm proteins prevents recruitment of the NineTeen complex and initiation of spliceosome activation. Together, our findings demonstrate that symmetric arginine dimethylation has important functions in spliceosome assembly and activation, and uncover a key molecular mechanism for arginine methylation in pre-mRNA splicing that impacts diverse developmental processes.arginine methylation | protein arginine methyltransferase | AtPRMT5 | pre-mRNA splicing | Prp19C/NTC P rotein arginine methyltransferase 5 (PRMT5), a highly conserved type II protein arginine methyltransferase, transfers methyl groups to arginine residues, generating monomethylarginine and symmetric ω-N G , N′ G -dimethylarginine (SDMA) (1-3). In mammals, PRMT5 methylates and interacts with diverse proteins, including histones and other proteins, and has long been implicated in the modulation of a variety of processes, such as transcriptional regulation, RNA metabolism (4), apoptosis (5), signal transduction (6), and germ cell development (7). Both loss-of-function and overexpression of PRMT5 are fatal in mammals (8). AtPRMT5, the Arabidopsis homolog of PRMT5, regulates multiple aspects of plant growth and development, such as flowering time, growth rate, leaf morphology, sensitivity to stress conditions, and circadian rhythm, by modulating transcription, constitutive and alternative precursor mRNA (pre-mRNA) splicing of diverse genes (9-13). AtPRMT5 also mediates the symmetric arginine dimethylation of uridine-rich small nuclear ribonucleoproteins (U snRNPs) AtSmD1, D3, and AtLSm4 proteins, thus linking arginine methylation and splicing (10,11,13). However, the exact mechanism remains elusive.Pre-mRNA splicing occurs in the nucleus, removing introns and ligating exons (14). In eukaryotes, most introns are spliced in a series of reactions catalyzed by the spliceosome, which consists of five subcomplexes of U snRNPs and several non-snRNP factors. Each U snRNP contains distinct splicing...