Fig. 2.

Proteome-wide analysis of lysine monomethylation using an isotopic labeling approach. A, Affinity enrichment efficiency and the number of methylated peptides identified in each fraction. B, MS spectrum of a “Light” and “Heavy” methyl-coded monomethylated peptide of protein EZH2from HeLa cells, showing twin peaks with a mass difference of 4.02 Da induced by isotopic labeling using ¹³CD₃. C, Ion trap CID spectra from the “Light” and “Heavy” methylated peptide YSQADALK_me1YVGIER of protein EZH2 from HeLa cells. The 4-Da mass shift of the b- and y-ions of the “Heavy” peptide relative to its “Light” counterpart was induced by the isotopic methyl group. D, The overlap of the methylated peptides from “Light” and “Heavy” methionine-labeled HeLa cells.