Methylation status of nuclear proteins are changed by liver regeneration in a hepatectomized rat
hepatectomized rat nuclear proteins Methylation status liver regeneration;
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Protein methylation has been implicated in the signal transduction pathway leading to cell growth. Here we show that a regenerating rat liver following partial hepatectomy exhibited change of methylation status of a subset of endogenous proteins in vitro. The 70-kD, 18-kD, 16-kD and 7-kD proteins were shown to be major nuclear proteins undergoing methylation in regenerating hepatocytes. Methylation of the 70-kD protein peaked at 1 day following partial hepatectomy, which declined to a basal level within the next 3 days. Intriguingly, methylation of a 16-kD protein newly appeared at 1 day following partial hepatectomy, and disappeared within □days. Presence of exogenously added bulk histone in the in vitro methylation assay resulted in inhibition of methylation of the 16-kD protein among other proteins. All the histone subtypes tested, histone □A, □B, 3 or 4, were able to inhibit methylation of the 16-kD protein, while exogenously added histones were not methylated by endogenous nuclear methyltransferases at all. Intriguingly, alcohol dehydrogenase inhibited methylation of both 70-kD and 16-kD proteins. Myelin basic protein and cytochrome C inhibited the 16-kD methylation, while α-lactalbumin, carbonic anhydrase, bovine serum albumin, and Υ globulin minimally affected methylation of the nuclear endogenous methyl acceptors. The [methyl-3H]-groups in the 18-kD, 16-kD and 7-kD proteins were stable at pH 10-11 (37℃ for 30 min), but those in 70-kD protein were not, indicating the reaction is likely carboxyl methyl-esterification on isoaspartyl residue. Methylation of 70-kD, 18-kD, 16-kD and 7-kD was not stimulated by GTPΥS (4 mM), thus the reactions are not carboxyl methyl-esterification on C-terminal farnesylated cysteine. Since methylation of the 16-kD protein preceded proliferation of hepatocytes upon partial hepatectomy, it is tempting to speculate that the methylated 16-kD protein may be involved in an early signal critical for liver regeneration.