The essential yeast RNA binding protein Np13p is methylated.

Arginine methylation is a prevalent modification found in many RNA binding proteins, yet little is known about its functional consequences. Using a monoclonal antibody, 1E4, we have shown that the yeast NPL3 gene product Np13p, an essential RNA binding protein with repeated RGG motifs, is arginine-methylated in vivo. The 1E4 epitope can be generated by incubating recombinant Np13p with partially purified bovine arginine methyltransferase block this reaction. Np13p methylation requires S-adenosyl-L-methionine and also occurs in yeast extracts. An Np13p deletion mutant lacking the RGG domain is not a substrate for methylation, suggesting that the methylation sites lie within the RGG motifs. The discovery of arginine methylation in a genetically tractable organism provides a powerful entrée to understanding the function of this modification, particularly in view of the many roles postulated for Np13p in RNA processing and transport. The recent discovery of phosphorylated serine residues within the RGG domain suggests a hypothesis in which a molecular switch governed by methylation and phosphorylation regulates the biochemical properties of the Np13p RGG domain.