Nerve growth factor-mediated increases in protein methylation occur predominantly at type I arginine methylation sites and involve protein arginine methyltransferase 1

Abstract

Nerve growth factor (NGF)-specific signal transduction leads to changes in protein methylation during neuronal differentiation of PC12 cells (Cimato et al. [1997] J. Cell Biol. 138:1089-1103). In the present work, we demonstrate that, among NGF-regulated proteins, arginine methylation is more prevalent than carboxylmethylation. Type I protein arginine methyltransferase (PRMT) activity produces asymmetric dimethylation of the terminal guanidinonitrogen of arginines in substrate proteins, particularly glycine and arginine-rich (GAR) segments of proteins. Several GAR peptides were used to assay for methyltransferase activity and to compete with endogenous cellular proteins for the PRMT activity in PC12 cell extracts. Peptides derived from fibrillarin and nucleolin, as well as a synthetic GAR peptide containing a repetitive GRG motif, are each extremely effective at blocking in vitro methylation of the NGF-regulated PC12 cell methylated proteins. Myelin basic protein, a substrate for type II PRMT, selectively inhibits a 45 kDa protein but is a much less effective inhibitor of total methylation at an equimolar concentration. In addition, the fibrillarin- and nucleolin-derived peptides were used to detect elevated PRMT activity in homogenates of NGF-treated PC12 cells. Finally, immunoprecipitation of PRMT1 from PC12 cells provides the first demonstration of an NGF-activated methyltransferase and implicates PRMT1 in NGF signal transduction.