Effects of sinefungin and 5'-deoxy-5'-S-isobutyl-adenosine on lipopolysaccharide-induced proliferation and protein N-methylation of arginyl residues in murine splenic lymphocytes

Protein Arginine Methyltransferase 5 in T Lymphocyte Biology

Protein arginine methyltransferase 5 (PRMT5) is the major methyltransferase (MT) catalyzing symmetric dimethylation (SDM). PRMT5 regulates developmental, homeostatic and disease processes in vertebrates and invertebrates, and a carcinogenic role has been observed in mammals. Recently, tools generated for PRMT5 loss of function have allowed researchers to demonstrate essential roles for PRMT5 in mouse and human lymphocyte biology. PRMT5 modulates CD4⁺ and CD8⁺ T cell development in the thymus, peripheral homeostasis, and differentiation into CD4⁺ helper T lymphocyte (Th)17 cell phenotypes. Here, we provide a timely review of the milestones leading to our current understanding of PRMT5 in T cell biology, discuss current tools to modify PRMT5 expression/activity, and highlight mechanistic pathways.

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.

Arginine N-methyltransferase 1 is required for early postimplantation mouse development, but cells deficient in the enzyme are viable

Protein arginine N-methyltransferases have been implicated in a variety of processes, including cell proliferation, signal transduction, and protein trafficking. In this study, we have characterized essentially a null mutation induced by insertion of the U3betaGeo gene trap retrovirus into the second intron of the mouse protein arginine N-methyltransferase 1 gene (Prmt1). cDNAs encoding two forms of Prmt1 were characterized, and the predicted protein sequences were found to be highly conserved among vertebrates. Expression of the Prmt1-betageo fusion gene was greatest along the midline of the neural plate and in the forming head fold from embryonic day 7.5 (E7.5) to E8.5 and in the developing central nervous system from E8.5 to E13.5. Homozygous mutant embryos failed to develop beyond E6.5, a phenotype consistent with a fundamental role in cellular metabolism. However, Prmt1 was not required for cell viability, as the protein was not detected in embryonic stem (ES) cell lines established from mutant blastocysts. Low levels of Prmt1 transcripts (approximately 1% of the wild-type level) were detected as assessed by a quantitative reverse transcription-PCR assay. Total levels of arginine N-methyltransferase activity and asymmetric N(G), N(G)-dimethylarginine were reduced by 85 and 54%, respectively, while levels of hypomethylated substrates were increased 15-fold. Prmt1 appears to be a major type I enzyme in ES cells, and in wild-type cells, most substrates of the enzyme appear to be maintained in a fully methylated state.

RNA and protein interactions modulated by protein arginine methylation

This review summarizes the current status of protein arginine N-methylation reactions. These covalent modifications of proteins are now recognized in a number of eukaryotic proteins and their functional significance is beginning to be understood. Genes that encode those methyltransferases specific for catalyzing the formation of asymmetric dimethylarginine have been identified. The enzyme modifies a number of generally nuclear or nucleolar proteins that interact with nucleic acids, particularly RNA. Postulated roles for these reactions include signal transduction, nuclear transport, or a direct modulation of nucleic acid interactions. A second methyltransferase activity that symmetrically dimethylates an arginine residue in myelin basic protein, a major component of the axon sheath, has also been characterized. However, a gene encoding this activity has not been identified to date and the cellular function for this methylation reaction has not been clearly established. From the analysis of the sequences surrounding known arginine methylation sites, we have determined consensus methyl-accepting sequences that may be useful in identifying novel substrates for these enzymes and may shed further light on their physiological role.