Identification of N G , N G -dimethylarginine and N G , N G -dimethylarginine in the basic A1 protein from bovine myelin

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.

Studies on NG-methylarginine derivatives in myelin basic protein from developing and mutant mouse brain

The amounts of NG-methylarginine derivatives in myelin basic protein (MBP) purified from dysmyelinating mutant and different stages of normal myelinating mouse brains have been studied by using h.p.l.c. with a highly sensitive post-column o-phthaldialdehyde derivative-formation method. All three naturally occurring derivatives (NG-monomethylarginine (MeArg), NGN'G-dimethylarginine [Me2(sym)Arg] and NGNG-dimethylarginine [Me2(asym)Arg]) were found in MBP; however, their relative concentrations varied significantly with the age of the animal. The amounts of MeArg and Me2(sym)Arg in MBP increased as a function of the age of the brain, whereas that of Me2(asym)Arg decreased. MBP from early-myelinating mouse brain was shown to contain a high proportion of Me2(asym)Arg, which was hardly detectable in older brain MBP. This derivative, Me2(asym)Arg, was also absent from MBP embedded in the most compact multilamellar myelin, but was present in MBP in the least compact myelin (P3B). Comparing the extent of total methylation in vivo (sum of all three arginine derivatives), MBP extracted from less-compact myelin (P3A and P3B) showed a level approx. 40% higher than that from compact myelin. MBPs isolated from dysmyelinating mutant mouse brains, such as jimpy (jp/y) and quaking (qk/qk), contained a much higher level of Me2(asym)Arg relative to the other two methyl derivatives and also in comparison with those levels in the mother brain MBP. SDS/PAGE analysis of MBPs extracted from the mutant (both jp/y and qk/qk) as well as young normal (6-13 days old) mouse brains indicated the presence of a high-molecular-mass isoform of MBP (about 32 kDa), but this isoform was not found in adult brains. These results therefore indicate that structural integrity of myelin membrane in which MBP is embedded appears to play a pivotal role in determining the extent and the kind of Me2Arg formation in MBP at the post-translational level.

Time dependence of the methylation of myelin basic protein from bovine brain; evidence for protein-methylarginine demethylation

In the presence of excess S-adenosylmethionine (AdoMet), the extent of methylation of myelin basic protein (MBP) by partially purified AdoMet:MBP methyltransferase is a non-linear function of time, reaching a limiting value as available MBP is depleted and then decreasing monotonically. This decrease is not caused by proteolytic cleavage of MBP nor by effects related to substrate or product instability under the incubation conditions and is not observed in heat-inactivated samples. S-Adenosylhomocysteine is not required for the demethylation to occur, and with purified enzyme, the decrease is not observed. The data strongly suggest that the decrease in methyl content represents an enzyme-catalyzed demethylation reaction. This would represent the first report of an enzyme which catalyzes protein-methylarginine demethylation.

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.

In vivo methylation of an arginine in chicken myelin basic protein

The amino acid sequence around the sole methylarginine residue in chicken myelin basic protein was determined and was found to be similar to that previously reported for mammalian myelin basic protein. The ratio NG, N'G-dimethylarginine: NG-monomethylarginine:arginine was approximately 1.3:0.9:1.0. No NG, NG-dimethylarginine was detected in the protein. The in vivo incorporation of methyl groups from [methyl-3H]methionine into methylarginines in myelin was found to occur readily in 2-day-old chickens. Radioactively labelled NG,N'G-dimethylarginine and NG-monomethylarginine in myelin were derived solely from myelin basic protein. Radioactivity was also incorporated into NG,NG-dimethylargnine, although this was not derived from myelin basic protein. As NG-monomethylarginine was easily separated from the dimethylarginines, and as it was derived from myelin basic protein, it may be a good marker for myelin basic protein turnover in vivo. A time course study of the incorporation showed that radioactivity was incorporated into NG-monomethylarginine up to 6 h after injection, and decayed slowly, with an apparent half-life of about 40 days.