Heterogeneous nuclear RNP protein A1-arginine methylation during HCT-48 cell cycle

Protein methylase I (protein-arginine N-methyltransferase) was examined in HCT-48 cells, synchronized by serum deprivation and hydroxyurea treatment. The enzyme activity to methylate the added hnRNP protein A1 increased about 2-fold from G₀ to S phase, and then decreased during G₂/M phase. The enzymatically [methyl-³H]-labeled hnRNP protein A1 was identified by SDS-PAGE/fluorography, and the products were identified as N(G)-monomethylarginine and N(G),N(G)-dimethyl-(asymmetric)arginines by HPLC. Among endogenous proteins, the 20-kDa species in the extract was most intensely [methyl-³H]-labeled. This 20-kDa methylation was markedly inhibited by the addition of exogenous hnRNP protein A1, indicating that these two substrates compete for the same protein methylase. The possible role of this post-translational modification has been discussed.