Figure EV1. MS/MS spectrum of peptides phosphorylated on S₂₀₉, S₂₁₃, S₂₁₇, and S₂₂₁ .

• A–D
  MS/MS spectrum of peptides phosphorylated on S₂₀₉ (A), S₂₁₃ (B), S₂₁₇ (C), and S₂₂₁ (D). The molecular weight of peptides is indicated on the right. Spectra are an assembly of ions produced by collision‐induced dissociation of the precursor peptides. Fragmentation occurs preferentially at peptide bonds to generate b and y ions, which extend from the amino and carboxy terminus, respectively. The precursor peptide sequence and the different b and y ions identified in the spectrum are shown above. b and y ion peaks are shown in red and blue, respectively. b and y ions with the highest intensity are labeled on the spectrum. Neutral mass losses of H₂O, NH₃, and H₃PO₄ (P) are indicated. Analysis of y and b ion fragmentation patterns showed that S₂₀₉ (A), S₂₁₃ (B), S₂₁₇ (C), and S₂₂₁ (D) were phosphorylated in vivo.