HETEROGENEOUS NUCLEAR RNP PROTEIN A1 ARGININE METHYLATION DURING HCT-48 CELL-CYCLE

Protein methylase I (protein-arginine N-methyltransferase) was examine d in HCT-48 cells, synchronized by serum deprivation and hydroxyurea t reatment. The enzyme activity to methylate the added hnRNP protein A1 increased about 2-fold from G(0) to S phase, and then decreased during G(2)/M phase. The enzymatically [methyl-H-3]-labeled hnRNP protein Al was identified by SDS-PAGE/fluorography, and the products were identi fied as N-G-monomethylarginine and N-G,N-G-dimethyl(asymmetric)arginin es by HPLC. Among endogenous proteins, the 20-kDa species in the extra ct was most intensely [methyl-H-3]-labeled. This 20-kDa methylation wa s markedly inhibited by the addition of exogenous hnRNP protein A1, in dicating that these two substrates compete for the same protein methyl ase. The possible role of this post-translational modification has bee n discussed.