Abstract
The methylation of a 23-kDa nuclear protein increased after partial hepatectomy and methylation returned to basal levels after the initial stage of regeneration. The methylating enzyme was partially purified from rat liver by ammonium sulfate precipitation, DEAE-anion exchange chromatography and Butyl-Sepharose chromatography. The 23-kDa protein was purified from a nuclear fraction of liver tissue with SP-Sepharose. When the 23-kDa protein was methylated with the partially purified methyltransferase and analyzed on C(18) high performance liquid chromatography (HPLC), the methylated acceptor amino acid was monomethyl lysine (MML). Previously, only arginine N-methylation of specific substrate proteins has been reported during liver regeneration. However, in this report, we found that lysine N-methylation increased during early hepatic regeneration, suggesting that lysine N-methylation of the 23-kDa nuclear protein may play a functional role in hepatic regeneration. The methyltransferase did not methylate other proteins such as histones, hnRNPA1, or cytochrome C, suggesting the enzyme is a 23-kDa nuclear protein- specific lysine N-methyltransferase.
Highlights
Protein methylation is one of the post-translational modifications that occurs in a wide variety of cell types in eukaryotes, as well as in prokaryotes
We have identified specific changes in protein methylation during hepatic regeneration, and previously reported that the arginine methylation of a 23 kDa protein is decreased during early hepatic regeneration (Lee et al, 1994)
We have previously identified specific changes in protein methylation during hepatic regeneration, and reported that the arginine methylation of a 23-kDa nuclear protein decreases during hepatic regeneration (Lee et al, 1994)
Summary
Protein methylation is one of the post-translational modifications that occurs in a wide variety of cell types in eukaryotes, as well as in prokaryotes. Protein methylation is catalyzed by methyltransferases that transfer the methyl group from S-adenonosyl-L-methionine (AdoMet) to a variety of nucleophilic oxygen and nitrogen atoms on polypeptides (Paik and Kim, 1980). Protein arginine N-methyltransferases (PRMTs) transfer a methyl group to arginine residues forming MMA (ω-NG monomethyl arginine) and asymmetric DMA (ω-NG, NG-dimethyl arginine) or symmetric DMA (ω-NG,N′G dimethyl arginine) depending on the specific PRMT (Paik and Kim, 1968; Nakajima et al,1971). Protein lysine N-methylation modifies the ε-amino-group of lysine residues. The protein lysine N-methyltransferase transfers methyl groups to ε-N-lysine residues from AdoMet forming ε-N-MML (monomethyllysine), ε-N-DML (dimethyllysine) and ε-N-TML (trimethyllysine). Several protein lysine N-methyltransferase enzymes have been characterized and shown to methylate specific lysine residues or histone types. With the exception of histone methylation, little is known about the functional role of methylated lysine residues on other proteins. We found that the lysine N-methylation of this 23-kDa nuclear protein increased during early hepatic regeneration, and we have characterized the protein lysine N-methyltransferase from rat liver
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