Fig. 4.

PTM analysis of recombinant and endogenous β- and α-actin. (A) MS peptide coverage maps of endogenous and recombinant β-actin (Top) and α-actin (Bottom) relative to the protein sequence. Green and magenta lines indicate trypsin and chymotrypsin digestion peptides, respectively, whose masses were identified by MS with FDR <1% (see also Datasets S1 and S2). Red, orange, purple, blue, and cyan circles indicate Nt-acetylation, H73 methylation, H/K/R methylation, K acetylation, and Y phosphorylation, respectively. Only PTMs observed with the highest confidence are shown, i.e., those with b (forward) and y (backward) fragment ions encompassing the PTM in MS2 spectra and occupancy >5% (see also Table 1). MS2 spectra of Nt-acetylated and H73 methylated peptides are shown for endogenous (B) and recombinant (C) β-actin. The spectra show the b (backward, blue) and y (forward, red) fragment ions observed by MS for each peptide. SI Appendix, Figs. S3 and S4 show all the MS2 spectra of PTM-containing β- and α-actin peptides observed with highest confidence.