Diversity of methyl acceptor proteins in rat pheochromocytoma (PC12) cells revealed after treatment with adenosine dialdehyde.

Protein N-methylation is a widespread modification whose functions are poorly understood. To overcome the inherent technical difficulties in identification of N-methylated proteins, we cultured PC12 cells with a methylation inhibitor, in expectation that proteins would accumulate in a hypomethylated state. Cell extracts were then incubated with [methyl-3H]S-adenosyl-L-methionine to label methyl-accepting sites via endogenous methyltransferases. Using two-dimensional gel electrophoresis we detected over 50 methyl acceptors, ranging from 18 to 120 kDa. Most had isoelectric points greater than 7.0. NG,NG-Dimethylarginine and NG-monomethylarginine accounted for about 90% of the methyl-3H-amino acids recovered after acid hydrolysis and thin-layer chromatography. The production of hypomethylated proteins should prove useful, not only in the identification of new methyl acceptors, but also in the isolation and characterization of new methyltransferases.