Protein methylation

Inhibition of growth and p21ras methylation in vascular endothelial cells by homocysteine but not cysteine

Although hyperhomocysteinemia has been recognized recently as a prevalent risk factor for myocardial infarction and stroke, the mechanisms by which it accelerates arteriosclerosis have not been elucidated, mostly because the biological effects of homocysteine can only be demonstrated at very high concentrations and can be mimicked by cysteine, which indicates a lack of specificity. We found that 10-50 microM of homocysteine (a range that overlaps levels observed clinically) but not cysteine inhibited DNA synthesis in vascular endothelial cells (VEC) and arrested their growth at the G1 phase of the cell cycle. Homocysteine in this same range had no effect on the growth of vascular smooth muscle cells (VSMC) or fibroblasts. Homocysteine decreased carboxyl methylation of p21(ras) (a G1 regulator whose activity is regulated by prenylation and methylation in addition to GTP-GDP exchange) by 50% in VEC but not VSMC, a difference that may be explained by the ability of homocysteine to dramatically increase levels of S-adenosylhomocysteine, a potent inhibitor of methyltransferase, in VEC but not VSMC. Moreover, homocysteine-induced hypomethylation in VEC was associated with a 66% reduction in membrane-associated p21(ras) and a 67% reduction in extracellular signal-regulated kinase 1/2, which is a member of the mitogen-activated protein (MAP) kinase family. Because the MAP kinases have been implicated in cell growth, the p21(ras)-MAP kinase pathway may represent one of the mechanisms that mediates homocysteine's effect on VEC growth. VEC damage is a hallmark of arteriosclerosis. Homocysteine-induced inhibition of VEC growth may play an important role in this disease process.

Differential regulation of methionine adenosyltransferase in superantigen and mitogen stimulated human T lymphocytes

Superantigens interact with the T cell receptor for antigen (TCR) and are, therefore, more physiological stimulators of T lymphocytes than nonspecific polyclonal T cell mitogens. The effects of these two classes of T cell stimulators on methionine adenosyltransferase (MAT) and S-adenosylmethionine (AdoMet) levels were investigated. Activation of resting human peripheral blood T lymphocytes by the mitogen phytohemagglutinin (PHA) or the superantigen staphylococcal enterotoxin B (SEB) caused a 3- to 6-fold increase in MAT II specific activity. Although the proliferative response was higher in cultures stimulated with PHA compared with SEB, MAT II activity was comparable in both cultures. Both stimuli caused down-regulation of the MAT 68-kDa lambda subunit expression and induced a comparable increase in the expression of the catalytic alpha2/alpha2' subunit mRNA and protein. However, in superantigen-stimulated cells, the expression of the noncatalytic beta subunit was down-regulated and virtually disappeared by 72 h post-stimulation; whereas, no change in the expression of this subunit was noted in PHA-stimulated cells. Thus, at 72 h following stimulation, PHA-stimulated cells expressed MAT II alpha2/alpha2' and beta subunits while SEB-stimulated cells expressed the alpha2/alpha2' subunits only; the beta subunit was no longer expressed in superantigen-stimulated cells. Kinetic analysis of MAT II in extracts of PHA- and SEB-stimulated cells using reciprocal kinetic plots revealed that in the absence of the beta subunit the Km of the enzyme for L-methionine (L-Met) was 3-fold higher than in the presence of the beta subunit. Furthermore, AdoMet levels were 5-fold higher in cell extracts lacking the beta subunit (SEB-stimulated cell extracts) compared with extracts containing MAT II alpha2/alpha2' and beta subunits. We propose that the increased levels of AdoMet in superantigen-stimulated cells may be attributed to the absence of the beta subunit, which seems to have rendered MAT II less sensitive to product feedback inhibition by (-)AdoMet. The data suggest that the beta subunit of MAT II, which has no catalytic activity, may be a regulatory subunit that imparts a lower Km for L-Met but increases the sensitivity to feedback inhibition by AdoMet. The down-regulation of the beta subunit, which occurred when T cells were stimulated via the TCR, may be an important mechanism to regulate AdoMet levels at different stages of T cell differentiation under physiological conditions.

Postimport methylation of the small subunit of ribulose-1,5-bisphosphate carboxylase in chloroplasts

Electron impact mass spectronomy analysis of the amino-terminal amino acid of the small subunit (SSU) of ribulose-1,5-bisphosphate carboxylase (Rubisco) showed that the amino-terminal methionine residue is post-translationally modified to N-methyl-methionine. Modification of the amino-terminal methionine residue was found in mature SSU proteins from the dicotyledonous plants pea and spinach as well as the monocotyledonous plants barley and corn. SSU methyltransferase is a soluble protein in the chloroplast stroma and accepts heterologously expressed non-methylated SSU as a substrate using S-adenosylmethionine as methyl-group donor. We show that this modification occurs after post-translational uptake of the precursor form of SSU into chloroplasts and processing to its mature size. This reaction represents a new step in the import and assembly pathway of Rubisco holoenzyme.

Thermal unfolding of the DNA-binding protein Sso7d from the hyperthermophile Sulfolobus solfataricus

Thermal unfolding of the small hyperthermophilic DNA-binding protein Sso7d was studied by circular dichroism spectroscopy and differential scanning calorimetry. The unfolding transition can be described by a reversible two state process. Maximum stability was observed in the region between pH 4.5 and 7.0 where Sso7d unfolds with a melting temperature between 370.8 to 371.9 K and an unfolding enthalpy between 62.9 and 65.4 kcal/mol. The heat capacity differences between the native and the heat denatured states obtained by differential scanning calorimetry (620 cal/(molK)) and circular dichroism spectroscopy (580 cal/(mol K)) resulted in comparable values. The thermodynamic reason for the high melting temperature of Sso7d is the shallow stability curve with a broad free energy maximum, corresponding to the relatively small heat capacity change which was obtained. The calculated stability curve shows that Sso7d has, despite of its high melting temperature, an only moderate intrinsic stability, which reaches its maximum (approximately 7 kcal/mol) at 282 K. Sso7d is particularly poorly stabilized (approximately 1 kcal/mol) at the maximum physiological growth temperature of Sulfolobus solfataricus. Sso7d has furthermore untypically low specific enthalpy (0.99 kcal/(mol residue)) and entropy (2.99 cal/(mol K)) values at convergence temperatures. No significant differences in thermal stability of the partially methylated Sso7d from Sulfolobus solfataricus and the cloned non-methylated form of the protein expressed in Escherichia coli were observed.

The essential yeast RNA binding protein Np13p is methylated

Arginine methylation is a prevalent modification found in many RNA binding proteins, yet little is known about its functional consequences. Using a monoclonal antibody, 1E4, we have shown that the yeast NPL3 gene product Np13p, an essential RNA binding protein with repeated RGG motifs, is arginine-methylated in vivo. The 1E4 epitope can be generated by incubating recombinant Np13p with partially purified bovine arginine methyltransferase block this reaction. Np13p methylation requires S-adenosyl-L-methionine and also occurs in yeast extracts. An Np13p deletion mutant lacking the RGG domain is not a substrate for methylation, suggesting that the methylation sites lie within the RGG motifs. The discovery of arginine methylation in a genetically tractable organism provides a powerful entrée to understanding the function of this modification, particularly in view of the many roles postulated for Np13p in RNA processing and transport. The recent discovery of phosphorylated serine residues within the RGG domain suggests a hypothesis in which a molecular switch governed by methylation and phosphorylation regulates the biochemical properties of the Np13p RGG domain.

The RGG box motif of the herpes simplex virus ICP27 protein mediates an RNA-binding activity and determines in vivo methylation

ICP27 is an essential herpes simplex virus type 1 nuclear regulatory protein that is required for efficient viral gene expression. Although the mechanism by which ICP27 regulates genes is unknown, a variety of evidence suggests that it functions posttranscriptionally, and recent studies indicate that it is an RNA-binding protein. Previously, we noted that a short arginine- and glycine-rich sequence in ICP27 (residues 138 to 152) is similar to an RGG box motif, a putative RNA-binding determinant found in a number of cellular proteins (W. Mears, V. Lam, and S. Rice, J. Virol. 69:935-947, 1995). In the present study, we have further investigated ICP27's association with RNA and examined the role of the RGG box in RNA binding. We find that ICP27 binds efficiently to RNA homopolymers composed of poly(G) and weakly to poly(U) RNA homopolymers. Poly(G) binding activity maps to the N-terminal 189 residues of ICP27 and requires the RGG box sequence. Using a northwestern blotting assay, we demonstrate that the RGG box alone (residues 140 to 152) can mediate RNA binding when attached to a heterologous protein. As many cellular RGG box proteins are methylated on arginine residues, we also investigated the in vivo methylation status of ICP27. Our results demonstrate that ICP27 is methylated in herpes simplex virus-infected cells. Methylation is dependent on the presence of the RGG box, suggesting that one or more arginine residues in the RGG box sequence are modified. These data demonstrate that ICP27 displays the characteristics of an RGG box-type RNA-binding protein.

Methylation of high molecular weight fibroblast growth factor-2 determines post-translational increases in molecular weight and affects its intracellular distribution

The high molecular weight (HMW) forms (24, 22.5, and 22 kDa) of basic fibroblast growth factor-2 (FGF-2) contain an N-terminal extension responsible for their predominantly nuclear localization. These forms of FGF-2 are post-translationally modified, resulting in a 1- to 2-kDa increase in apparent molecular mass. Here we show that this post-translational modification is inhibited by methionine starvation and by the methyltransferase inhibitors 5'-deoxy-5'-methylthioadenosine (MTA) and 3-deaza-adenosine. Inhibition of the methylation-dependent modification results in a significant decrease in HMW FGF-2 nuclear accumulation, suggesting that methylation is relevant to the intracellular distribution of these forms of FGF-2. Treatment with MTA does not affect either the synthesis or the intracellular fate of another nuclear protein, the SV40 large T antigen, demonstrating that this drug does not have a generalized effect on nuclear protein accumulation. These results link HMW FGF-2 post-translational modification to its intracellular distribution.

The mammalian immediate-early TIS21 protein and the leukemia-associated BTG1 protein interact with a protein-arginine N-methyltransferase

The TIS21 immediate-early gene and leukemia-associated BTG1 gene encode proteins with similar sequences. Two-hybrid analysis identified a protein that interacts with TIS21 and BTG1. Sequence motifs associated with S-adenosyl-L-methionine binding suggested this protein might have methyltransferase activity. A glutathione S-transferase (GST) fusion of the putative methyltransferase modifies arginine residues, in appropriate protein substrates, to form NG-monomethyl and NG,NG-dimethylarginine (asymmetric). We term the protein- arginine N-methyltransferase (EC 2.1.1.23) gene "PRMT1, " for protein-arginine methyltransferase 1. GST-TIS21 and GST-BTG1 fusion proteins qualitatively and quantitatively modulate endogenous PRMT1 activity, using control and hypomethylated RAT1 cell extracts as methyl-accepting substrates. PRMT1 message appears ubiquitous, and is constitutive in mitogen-stimulated cells. Modulation of PRMT1 activity by transiently expressed regulatory subunits may be an additional mode of signal transduction following ligand stimulation.

The predominant protein-arginine methyltransferase from Saccharomyces cerevisiae

We have identified the major enzymatic activity responsible for the S-adenosyl-L-methionine-dependent methylation of arginine residues (EC 2.1.1.23) in proteins of the yeast Saccharomyces cerevisiae. The RMT1 (protein-arginine methyltransferase), formerly ODP1, gene product encodes a 348-residue polypeptide of 39.8 kDa that catalyzes both the NG-mono- and NG, NG-asymmetric dimethylation of arginine residues in a variety of endogenous yeast polypeptides. A yeast strain in which the chromosomal RMT1 gene was disrupted is viable, but the level of NG,NG-[3H]dimethylarginine residues detected in intact cells incubated with S-adenosyl-L-[methyl-3H]methionine is reduced to less than 15% of the levels found in the parent strain, while the NG-[3H]monomethylarginine content is reduced to less than 30%. We show that soluble extract from parent cell, but not from mutant rmt1 cells, catalyzes the in vitro methylation of endogenous polypeptides of 55, 41, 38, 34, and 30 kDa. The hypomethylated form of these five polypeptides, as well as that of several others, can be mono- and asymmetrically dimethylated by incubating the mutant rmt1 extract with a purified, bacterially produced, glutathione S-transferase-RMT1 fusion protein and S-adenosyl-L-[methyl-3H]methionine. This glutathione S-transferase-RMT1 fusion protein is also able to methylate a number of mammalian polypeptides including histones, recombinant heterogeneous ribonucleoprotein A1, cytochrome c, and myoglobin, but cannot methylate myelin basic protein. RMT1 appears to be a yeast homolog of a recently characterized mammalian protein-arginine methyltransferase whose activity may be modulated by mitotic stimulation of cells.

Isolation of the murine S100 protein MRP14 (14 kDa migration-inhibitory-factor-related protein) from activated spleen cells: characterization of post-translational modifications and zinc binding

MRP14 (macrophage migration-inhibitory factor-related protein of molecular mass 14 kDa) is an S100 calcium binding protein constitutively expressed in human neutrophils which may be associated with cellular activation/inflammation. Murine MRP14 expression was up-regulated following concanavalin A activation of spleen cells, and the protein was isolated from conditioned medium in high yield (approx. 500 ng/ml). MRP14 had a mass of 12972 +/- 2 Da by electrospray ionization MS, whereas the theoretical mass derived from the cDNA sequence, after removal of the initiator Met, was 12918 Da, suggesting that the protein was post-translationally modified. We identified four post-translational modifications of MRP14: removal of the N-terminal Met, N-terminal acetylation, disulphide bond formation between Cys79 and Cys90, and 1-methylation of His106; the calculated mass was then 12971.8 Da. Methylation of His106 was further characterized after incubation of spleen cells with L-[methyl-3H]Met during concanavalin A stimulation. Sequential analysis of a peptide (obtained by digestion with Lys C) containing methylated His indicated that > 80% of the label in the cycle corresponded to His106, suggesting that the methyl residue was transferred from S-adenosyl-L-methionine. Comparison of the C18 reverse-phase HPLC retention times of phenylthiocarbamoyl derivatives of a hydrolysed digest peptide of MRP14 with those of standards confirmed methyl substitution on the 1-position of the imidazole ring. MRP14 bound more 85Zn2+ than the same amounts of the 10 kDa chemotactic protein (CP10) or S100 beta. Ca2+ decreased Zn2+ binding in S100 beta but it did not influence binding to MRP14, suggesting that the Zn2+ binding site was distinct from and independent of the two Ca2+ binding domains.

Changes in protein methylation associated with the elicitation response in cell cultures of alfalfa (Medicago sativa L.)

The methylation of endogenous proteins increased in alfalfa cell suspension cultures following treatment with a fungal elicitor. Carboxyl methylation, a post-translational modification associated with controlling the localisation and longevity of proteins, was the dominant form of protein methylation in both elicited and unelicited cells. Protein methylation was restricted to a limited number of peptides prior to elicitor treatment but as elicitation progressed the number of endogenous substrates increased. Increases resulted from a combination of an elicitor-dependent increase in the activity of a protein carboxyl methyltransferase and the accumulation of preferred endogenous substrates in the latter stages of elicitation.