Identification of highly methylated arginine residues in an endogenous 20-kDa polypeptide in cancer cells

Enzymatic methylation of endogenous proteins in several cancer cell lines was investigated to understand a possible relationship between protein- arginine methylation and cellular proliferation. Cytosolic extracts prepared from several cancer cells (HeLa, HCT-48, A549, and HepG2) and incubated with S-adenosyl-L-[methyl-³H]methionine revealed an intensely [methyl-³H]- labeled 20-kDa polypeptide. On the other hand, cytosolic extracts prepared from normal colon cells did not show any methylation of the 20-kDa protein under identical conditions. To identify nature of the 20-kDa polypeptide, purified histones were methylated with HCT-48 cytosolic extracts and analyzed by SDS-PAGE. However, none of the histones comigrated with the methylated 20- kDa polypeptide, indicating that it is unlikely to be any of the histone subclasses. The [methyl-³H]group in the 20-kDa polypeptide was stable at pH 1011 (37°C for 30 min) and methylation was not stimulated by GTPγS (4 mM), thus the reaction is neither carboxyl methylesterification on isoaspartyl residues, nor on C-terminal farnesylated cysteine: The present study together with the previous identification of N(G)-methylated arginine residues in the HCT-48 cytosol fraction suggests that this novel endogenous 20-kDa arginine- methylation is a cellular proliferation-related posttranslational modification reaction.