The symmetrical dimethylarginine post-translational modification of the SmD3 protein is not required for snRNP assembly and nuclear transport

Arginine methylation of RNA-binding proteins regulates cell function and differentiation

Arginine methylation is a post-translational modification that regulates protein function. RNA-binding proteins are an important class of cell-function mediators, some of which are methylated on arginine. Early studies of RNA-binding proteins and arginine methylation are briefly introduced, and the enzymes that mediate this post-translational modification are described. We review the most common RNA-binding domains and briefly discuss how they associate with RNAs. We address the following groups of RNA-binding proteins: hnRNP, Sm, Piwi, Vasa, FMRP, and HuD. hnRNPs were the first RNA-binding proteins found to be methylated on arginine. The Sm proteins function in RNA processing and germ cell specification. The Piwi proteins are largely germ cell specific and are also required for germ cell production, as is Vasa. FMRP participates in germ cell formation in Drosophila, but is more widely known for its neuronal function. Similarly, HuD plays a role in nervous system development and function. We review the effects of arginine methylation on the function of each protein, then conclude by addressing remaining questions and future directions of arginine methylation as an important and emerging area of regulation.

Knock-out mutations of Arabidopsis SmD3-b induce pleotropic phenotypes through altered transcript splicing

SmD3 is a core protein of small nuclear ribonucleoprotein (snRNP) essential for splicing of primary transcripts. To elucidate function of SmD3 protein in plants, phenotypes and gene expression of SmD3 knock-out and overexpressing mutants in Arabidopsis have been analyzed. smd3-a knock-out mutant or SmD3-a and SmD3-b overexpressors did not show phenotypic alteration. Knock-out of SmD3-b resulted in the pleotropic phenotypes of delayed flowering time and completion of life cycle, reduced root growth, partially defective leaf venation, abnormal numbers of trichome branches, and changed numbers of floral organs. Microarray data revealed that the smd3-b mutant had altered expression of genes related to the above phenotypes, indirectly suggesting that changed splicing of these genes may cause the observed phenotypes. Splicing of selected genes was either totally blocked or reduced in the smd3-b mutant, indicating the important role of SmD3-b in the process. A double knock-out mutant of smd3-a and smd3-b could not be generated, indicating possible redundant function of these two genes. All data indicate that SmD3-b may be major component of the spliceosomal snRNP in Arabidopsis, but the function of SmD3-a may be redundant.

Distinct regulatory programs establish widespread sex-specific alternative splicing in Drosophila melanogaster

In Drosophila melanogaster, female-specific expression of Sex-lethal (SXL) and Transformer (TRA) proteins controls sex-specific alternative splicing and/or translation of a handful of regulatory genes responsible for sexual differentiation and behavior. Recent findings in 2009 by Telonis-Scott et al. document widespread sex-biased alternative splicing in fruitflies, including instances of tissue-restricted sex-specific splicing. Here we report results arguing that some of these novel sex-specific splicing events are regulated by mechanisms distinct from those established by female-specific expression of SXL and TRA. Bioinformatic analysis of SXL/TRA binding sites, experimental analysis of sex-specific splicing in S2 and Kc cells lines and of the effects of SXL knockdown in Kc cells indicate that SXL-dependent and SXL-independent regulatory mechanisms coexist within the same cell. Additional determinants of sex-specific splicing can be provided by sex-specific differences in the expression of RNA binding proteins, including Hrp40/Squid. We report that sex-specific alternative splicing of the gene hrp40/squid leads to sex-specific differences in the levels of this hnRNP protein. The significant overlap between sex-regulated alternative splicing changes and those induced by knockdown of hrp40/squid and the presence of related sequence motifs enriched near subsets of Hrp40/Squid-regulated and sex-regulated splice sites indicate that this protein contributes to sex-specific splicing regulation. A significant fraction of sex-specific splicing differences are absent in germline-less tudor mutant flies. Intriguingly, these include alternative splicing events that are differentially spliced in tissues distant from the germline. Collectively, our results reveal that distinct genetic programs control widespread sex-specific splicing in Drosophila melanogaster.

Arginine methylation of SmB is required for Drosophila germ cell development

Sm proteins constitute the common core of spliceosomal small nuclear ribonucleoproteins. Although Sm proteins are known to be methylated at specific arginine residues within the C-terminal arginine-glycine dipeptide (RG) repeats, the biological relevance of these modifications remains unknown. In this study, a tissue-specific function of arginine methylation of the SmB protein was identified in Drosophila. Analysis of the distribution of SmB during oogenesis revealed that this protein accumulates at the posterior pole of the oocyte, a cytoplasmic region containing the polar granules, which are necessary for the formation of primordial germ cells. The pole plasm localisation of SmB requires the methylation of arginine residues in its RG repeats by the Capsuléen-Valois methylosome complex. Functional studies showed that the methylation of these arginine residues is essential for distinct processes of the germline life cycle, including germ cell formation, migration and differentiation. In particular, the methylation of a subset of these arginine residues appears essential for the anchoring of the polar granules at the posterior cortex of the oocyte, whereas the methylation of another subset controls germ cell migration during embryogenesis. These results demonstrate a crucial role of arginine methylation in directing the subcellular localisation of SmB and that this modification contributes specifically to the establishment and development of germ cells.

The assembly of a spliceosomal small nuclear ribonucleoprotein particle

The U1, U2, U4, U5 and U6 small nuclear ribonucleoprotein particles (snRNPs) are essential elements of the spliceosome, the enzyme that catalyzes the excision of introns and the ligation of exons to form a mature mRNA. Since their discovery over a quarter century ago, the structure, assembly and function of spliceosomal snRNPs have been extensively studied. Accordingly, the functions of splicing snRNPs and the role of various nuclear organelles, such as Cajal bodies (CBs), in their nuclear maturation phase have already been excellently reviewed elsewhere. The aim of this review is, then, to briefly outline the structure of snRNPs and to synthesize new and exciting developments in the snRNP biogenesis pathways.

Arginine methyltransferase Capsuleen is essential for methylation of spliceosomal Sm proteins and germ cell formation in Drosophila

Although arginine modification has been implicated in a number of cellular processes, the in vivo requirement of protein arginine methyltransferases (PRMTs) in specific biological processes remain to be clarified. In this study we characterize the Drosophila PRMT Capsuléen, homologous to human PRMT5. During Drosophila oogenesis, catalytic activity of Capsuléen is necessary for both the assembly of the nuage surrounding nurse cell nuclei and the formation of the pole plasm at the posterior end of the oocyte. In particular, we show that the nuage and pole plasm localization of Tudor, an essential component for germ cell formation, are abolished in csul mutant germ cells. We identify the spliceosomal Sm proteins as in vivo substrates of Capsuléen and demonstrate that Capsuléen, together with its associated protein Valois, is essential for the synthesis of symmetric di-methylated arginyl residues in Sm proteins. Finally, we show that Tudor can be targeted to the nuage in the absence of Sm methylation by Capsuléen, indicating that Tudor localization and Sm methylation are separate processes. Our results thus reveal the role of a PRMT in protein localization in germ cells.