Methylation of myelin basic protein by enzymes from rat brain

N(G)-Methylarginines: Biosynthesis, biochemical function and metabolism

N(G)-Methylarginines (N(G)-monomethylarginine, N(G), N(G)-dimethylarginine and N(G), N'(G)-dimethylarginine) occur widely in nature in either proteinbound or in free states. They are posttranslationally synthesized by a group of enzymes called protein methylase I with S-adenosyl-L-methionine as the methyl donor. The enzymes are highly specific not only towards arginine residues but also towards the protein species. Since transmethylation reaction is energy-dependent in the form of S-adenosyl-L-methionine and is catalyzed a group of highly specific enzymes, it is quite logical to assume that the enzymatic methylation of protein-bound arginine residues play an important role in the regulation of the function and/or metabolism of the protein. When determined with histones asin vitro substrates, protein methylase I activity parallels closely the degree of cell proliferation, and the myelin basic protein (MBP)-specific protein methylase I activity decreases drastically in dysmyelinating mutant mouse brain during myelinating period, suggesting an important role played in the formation and/or maintenance of myelin. When the methylated proteins are degraded by intracellular proteolytic enzymes, free N(G)-methylarginines are generated. Some of these free N(G)-methylarginines, particularly N(G)-monomethylarginine, are extensively metabolized by decarboxylation, hydrolysis, transfer of methylamidine and deimination reaction. Recent experiment demonstrates that some of the N(G)-methylarginines may be involved in the neutralization of activity of nitric oxide (NO) which has attracted a great deal of attention as vascular smooth muscle relaxation factor.

Myelin basic protein-diverse conformational states of an intrinsically unstructured protein and its roles in myelin assembly and multiple sclerosis

The 18.5 kDa isoform of myelin basic protein (MBP) is a major component of the myelin sheath in the central nervous system of higher vertebrates, and a member of a larger family of proteins with a multiplicity of forms and post-translational modifications (PTMs). The 18.5 kDa protein is the exemplar of the family, being most abundant in adult myelin, and thus the most-studied. It is peripherally membrane-associated, but has generally been investigated in isolated form. MBP is an 'intrinsically unstructured' protein with a high proportion (approximately 75%) of random coil, but postulated to have core elements of beta-sheet and alpha-helix. We review here the properties of the MBP family, especially of the 18.5 kDa isoform, and discuss how its three-dimensional (3D) structure may be resolved by direct techniques available to us, viz., X-ray and electron crystallography, and solution and solid-state NMR spectrometry. In particular, we emphasise that creating an appropriate environment in which the protein can adopt a physiologically relevant fold is crucial to such endeavours. By solving the 3D structure of 18.5 kDa MBP and the effects of PTMs, we will attain a better understanding of myelin architecture, and of the molecular mechanisms that transpire in demyelinating diseases such as multiple sclerosis.

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.

Biological methylation of myelin basic protein: enzymology and biological significance

Myelin is a membrane characteristic of the nervous tissue and functions as an insulator to increase the velocity of the stimuli being transmitted between a nerve cell body and its target. Myelin isolated from human and bovine nervous tissue is composed of approximately 80% lipid and 20% protein, and 30% of the protein fraction constitutes myelin basic protein (MBP). MBP has an unusual amino acid at Res-107 as a mixture of NG-monomethylarginine and NG, N'G-dimethylarginine. The formation of these methylarginine derivatives is catalysed by one of the subtypes of protein methylase I, which specifically methylates Res-107 of this protein. Evidence is presented to demonstrate an involvement of this biological methylation in the integrity and maintenance of myelin.

Accumulation of galactosylsphingosine (psychosine) does not interfere with phosphorylation and methylation of myelin basic protein in the twitcher mouse

In attempts to elucidate mechanisms of demyelination in the twitcher mouse (Twi), phosphorylation and methylation of myelin basic protein (MBP) were examined in the brainstem and spinal cord of this species. Phosphorylation of MBP in isolated myelin by an endogenous kinase and an exogenous [32P]ATP was not impaired and protein kinase C activity in the brain cytosol was not reduced. When the methylation of an arginine residue of MBP was examined in slices of the brainstem and spinal cord, using [3H]methionine as a donor of the methyl groups, no difference was found between Twi and the controls. Radioactivity of the [3H] methionine residue of MBP of Twi was also similar to that of the controls. Thus, accumulation of psychosine in Twi does not interfere with the activity of endogenous kinase, methylation of MBP, and the synthesis and transport of MBP into myelin membrane.

Kinetics of carboxylmethylation of the charge isoforms of myelin basic protein by protein methyltransferase II

The charge isoforms (C1-C5) of bovine myelin basic protein (MBP) were used as substrates for the rat brain enzyme protein carboxylmethyltransferase (PM II). The objective of these experiments was to ascertain whether the kinetic behavior of the MBP isoforms reflected differences in the structures of this molecular family. Initial velocity plots as a function of the MBP-isoform concentration showed significant differences (p less than 0.05) among the assayed isoforms except for isoforms C2 and C4. Under the conditions of our experiment all the curves exhibited a consistent sigmoidicity. The kinetic data were best fitted by a model, previously described for the enzyme D-beta-hydroxybutyrate dehydrogenase, in which two independent sites must be randomly occupied before any catalytic activity can occur. This mechanism is substantially different from that proposed by other investigators for similar PM II enzymes and other substrates. The differences in the rates of isoform carboxylmethylation are largely accounted for by the different apparent dissociation constants Ks and is explained on the basis of inherent structural differences among the charge isoforms.

Purification and kinetic mechanism of S-adenosylmethionine: myelin basic protein methyltransferase from bovine brain

The enzyme S-adenosylmethionine (AdoMet): myelin basic protein (MBP) methyltransferase was purified 250-fold from bovine brain with an overall yield of 130%, relative to crude supernatant. The purification involves acid-base and (NH4)2SO4 precipitation, chromatography over Sephadex G-100 and DEAE-cellulose, followed by preparative isoelectric focusing. The enzyme has a pI of 5.60 +/- 0.05, and the Mr is estimated to be between 71,000 (from SDS/polyacrylamide-gel electrophoresis) and 74,500 (from gel filtration). The enzyme is stable at 37 degrees C for over 2 h, is stable frozen and does not require metal ions or reductants. The enzyme shows a high specificity for MBP and does not accept polyarginine as a substrate; F1 histone is methylated at 37% of the rate of MBP. Methylation occurs on an arginine residue in a single h.p.l.c.-resolvable peptide from the tryptic cleavage of MBP. Simple saturation kinetics are observed with respect to both substrates, with Km values of 18 microM and 32 microM for MBP and AdoMet respectively. The simplest kinetic mechanism that is consistent with the data requires ordered rapid-equilibrium binding, with AdoMet as the first substrate. The enzyme isolated in this work is different, both physically and kinetically, from the histone-specific arginine methyltransferases described by other workers. A new, simple, assay system for the methylation of MBP is described.

Correlation between inhibition of myelin basic protein (arginine) methyltransferase by sinefungin and lack of compact myelin formation in cultures of cerebral cells from embryonic mice

Sinefungin, a known inhibitor of protein methylation, inhibited the myelin basic protein (arginine) methyltransferase activity in homogenates of cultured cerebral cells from embryonic mice. Fifty percent inhibition was achieved with 25 microM sinefungin. Electron microscopic examination of the myelin fraction, isolated by gradient density centrifugation and obtained from untreated cells, revealed numerous ringlike multilamellar membranous substructures that had a major dense line periodicity, compactness, and the general appearance expected of myelin obtained by the same technique from whole brain. Cells treated with 30 microM sinefungin, which inhibits myelin basic protein methyltransferase in broken cell preparations about 60%, produced ringlike structures that were devoid of multilamellar periodicity and compactness reminiscent of the vacuolated myelin observed in subacute combined degeneration and in nitrous-oxide- or cycloleucine-treated animals in which methyltransferase activity is also inhibited. The sinefungin-induced change in multilamellar periodicity cannot be attributed to a lack of myelin basic protein, since the ratio of myelin basic protein to total protein did not decrease in sinefungin-treated cells. This primary culture system should be useful for further evaluating the hypothesis that the methylation of myelin basic protein is related to the formation of compact myelin.

Specificity of the histone lysine methyltransferases from rat brain chromatin

The histone lysine methyltransferases catalyze the transfer of methyl groups from S-adenosylmethionine to specific epsilon-N-lysine residues in the N-terminal regions of histones H3 and H4. These enzymes are located exclusively within the nucleus and are firmly bound to chromatin. The chromosomal bound enzymes do not methylate free or nonspecifically associated histones, while histones H3 and H4 within newly synthesized chromatin are methylated. These enzymes can be solubilized by limited digestion (10-16%) of chromosomal DNA from rapidly proliferating rat brain chromatin with micrococcal nuclease. Histone H3 lysine methyltransferase remained associated with a short DNA fragment throughout purification. Dissociation of the enzyme from the DNA fragment with DNAase digestion resulted in complete loss of enzyme activity; however, when this enzyme remained associated with DNA it was quite stable. Activity of the dissociated enzyme could not be restored upon the addition of sheared calf thymus or Escherichia coli DNA. Histone H3 lysine methyltransferase was found to methylate lysine residues in chromosomal bound or soluble histone H3, while H3 associated with mature nucleosomes was not methylated. The histone H4 lysine methyltransferase which was detectable in the crude nuclease digest was extremely labile, losing all activity upon further purification. We isolated a methyltransferase by DEAE-cellulose chromatography, which would transfer methyl groups to arginine residues in soluble histone H4. However, this enzyme would not methylate nucleosomal or chromosomal bound histone H4, nor were methylated arginine nucleosomal or chromosomal bound histone H4, nor were methylated arginine residues detectable upon incubating intact nuclei or chromatin with S-adenosylmethionine.

Studies on myelin basic protein-specific protein methylase I in various dysmyelinating mutant mice

Jimpy mice are dysmyelinating mutants characterized by producing near normal levels of myelin basic protein (MBP) in the brain but failing to incorporate these proteins into the myelin sheath. In this study, the activity of MBP-specific protein-arginine N-methyltransferase (protein methylase I) was studied in the brains of normal and jimpy mice of different ages. The enzyme activity varied little with age in normal mice but in 18 and 21 days-old homozygous jimpy mice the activity was reduced by 50% and 75% respectively from the level of their normal littermates. Interestingly, however, heterozygous jimpy mice who are phenotypically normal and quaking mice (a similar dysmyelinating mutant) showed unaltered enzyme levels.

Regulation of myelin basic protein (arginine) methyltransferase by thyroid hormone in myelinogenic cultures of cells dissociated from embryonic mouse brain

The ontogenetic expression of myelin basic protein (arginine) methyltransferase in myelinogenic cultures of cells dissociated from embryonic mouse brain is highly dependent on the presence of thyroid hormone. Restoration of myelin basic protein methyltransferase to normal activities occurred 16 h after the addition of 100 nM L-3,5,3'-triiodothyronine to hypothyroid medium. These data demonstrate that thyroid hormone can regulate a posttranslational event. On the other hand, histone (arginine) methyltransferase has a different temporal activity pattern, which is not coordinated with myelination, and is not influenced by the lack of thyroid hormone. These data, which suggest the existence of two methyltransferases, were substantiated by demonstrating that the total amount of methylation of added myelin basic protein and histone is the same whether they are incubated together or separately. The requirement of thyroid hormone for the expression of the myelin basic protein methyltransferase and not for histone methyltransferase suggests that thyroid hormone preferentially regulates myelin-associated events in these cultures.

Phosphate groups modifying myelin basic proteins are metabolically labile; methyl groups are stable

Young and adult rats received intracranial injections of [33P]orthophosphoric acid. The time course of the appearance and decay of the radioactive label on basic proteins in isolated myelin was followed for 1 mo. Incorporation was maximal by 1 h, followed by a decay phase with a half-life of approximately 2 wk. However, radioactivity in the acid-soluble precursor pool (which always constituted at least half of the total radioactivity) decayed with a similar half-life, suggesting that the true turnover time of basic protein phosphates might be masked by continued exchange with a long-lived radioactive precursor pool. Calculations based on the rate of incorporation were made to more closely determine the true turnover time; it was found that most of the phosphate groups of basic protein turned over in a matter of minutes. Incorporation was independent of the rate of myelin synthesis but was proportional to the amount of myelin present. Experiments in which myelin was subfractionated to yield fractions differing in degree of compaction suggested that even the basic protein phosphate groups of primarily compacted myelin participated in this rapid exchange. Similar studies were carried out on the metabolism of radioactive amino acids incorporated into the peptide backbone of myelin basic proteins. The metabolism of the methyl groups of methylarginines also was monitored using [methyl-3H]methionine as a precursor. In contrast to the basic protein phosphate groups, both the peptide backbone and the modifying methyl groups had a metabolic half-life of months, which cannot be accounted for by reutilization from a pool of soluble precursor. The demonstration that the phosphate groups of myelin basic protein turn over rapidly suggests that, in contrast to the static morphological picture, basic proteins may be readily accessible to cytoplasm in vivo.

The relationship of myelin basic protein (arginine) methyltransferase to myelination in mouse spinal cord

The relationship between the activity of myelin basic protein (arginine) methyltransferase and myelination in the mouse spinal cord has been examined. The activity of this methylase increases between 8 and 45 days postnatal age and correlates well with other parameters of myelination. A comparison of myelin basic protein methylase with histone methylase activity during development indicates that each is a distinct, specific enzyme activity. Together, these results are considered to establish myelin based protein methylase as a myelination-related enzyme.

S-adenosylmethionine: protein-arginine methyltransferase. Purification and mechanism of the enzyme

Protein methylase I (S-adenosylmethionine: protein-arginine methyltransferase, EC 2.1.1.23) has been purified from calf brain approximately 120-fold with a 14% yield. The final preparation is completely free of any other protein-specific methyltransferases and endogenous substrate protein. The enzyme has an optimum pH of 7.2 and pI value of 5.1. The Km values for S-adenosyl-L-methionine, histone H4, and an ancephalitogenic basic protein are 7.6 X 10(-6), 2.5 X 10(-5), and 7.1 X 10(-5) M, respectively, and the Ki value for S-adenosyl-L-homocysteine is 2.62 X 10(-6) M. The enzyme is highly specific for the arginine residues of protein, and the end products after hydrolysis of the methylated protein are NG,NG-di(asymmetric), NG,N'G-di(symmetric), and NG-monomethylarginine. The ratio of [14C]methyl incorporation into these derivatives by enzyme preparation at varying stages of purification remains unchanged at 40:5:55, strongly indicating that a single enzyme is involved in the synthesis of the three arginine derivatives. The kinetic mechanism of the protein methylase I reaction was studied with the purified enzyme. Initial velocity patterns converging at a point on the extended axis of abscissas were obtained with either histone H4 or S-adenosyl-L-methionine as the varied substrate. Product inhibition by S-adenosyl-L-homocysteine with S-adenosyl-L-methionine as the varied substrate was competitive regardless of whether or not the enzyme was saturated with histone H4. On the other hand, when histone H4 is the variable substrate, noncompetitive inhibition was obtained with S-adenosyl-L-homocysteine under conditions where the enzyme is not saturated with the other substrate, S-adenosyl-L-methionine. These results suggest that the mechanism of the protein methylase I reaction is a Sequential Ordered Bi Bi mechanism with S-adenosyl-L-methionine as the first substrate, histone H4 as the second substrate, methylated histone H4 as the first product, and S-adenosyl-L-homocysteine as the second product released.