The arginine methyltransferases CARM1 and PRMT1 associate with the p160 family of nuclear hormone receptor coactivators. This association enhances transcriptional activation by nuclear receptors. We describe a method for identifying arginine N-methyltransferase substrates using arrayed high-density protein membranes to perform solid-phase supported enzyme reactions in the presence of the methyl donor S-adenosyl-l-methionine. Using this screen, we identified distinct substrates for CARM1 and PRMT1. All PRMT1 substrates harbor the expected GGRGG methylation motif, whereas the peptide sequence comparisons of the CARM1 substrates revealed no such motif. The predominant CARM1 substrate identified in this screen was PABP1. We mapped the methylated region of this RNA binding molecule in vitro and demonstrate that PABP1 is indeed methylated in vivo. Prior to these findings, the only known substrate for CARM1 was histone H3. We broaden the number of CARM1 targets and suggest a role for CARM1 in regulating transcription/translation.