Following stimulation of NRK49F rat kidney fibroblast cells with epidermal growth factor, possible preemptive cross-talk between arginine methylation and serine and tyrosine phosphorylation was observed for Rho guanidine nucleotide dissociation inhibitor 1 (RhoGDI-1). Five dimethylation sites (Lys50, Lys52, Arg111, Arg152, Arg180) and two new phosphorylation sites (Tyr144, Ser148) were identified for RhoGDI-1. All presently known phosphorylation sites for RhoGDI-1 lie within the 10 residues immediately prior to the 3 sites for arginine dimethylation, and these dimethylation/phosphorylation modules may constitute functional switches. Consideration of structural data and other literature for RhoGDI-1 suggests that methylation and phosphorylation cooperatively affect formation of complexes with different Rho/Rac family proteins and that methylation may be crucial in partitioning of RhoGDI-1 between different functional roles. On the basis of results presented here, it can be implied that unidentified arginine methyltransferases may exist and that arginine methylation may have a greater role in cellular signaling processes than is currently recognized. The combined use of SILAC labeling of arginine (SILAC = stable isotope labeling by amino acids in cell culture), immobilized metal affinity chromatography based phosphoprotein enrichment, and mass spectrometry is clearly a useful method for this investigation.