Fig. 4. Identification of serum-stimulated phosphorylation sites by mass spectrometry. (A) Tandem mass spectrum resulting from the analysis of excised phosphopeptide 1 from the map in Figure 1C. This spectrum contained sequence information for a single phosphopeptide. Two separate ion series were recorded simultaneously; b- and y–ion series represent sequencing inward from the N- and C–termini, respectively. The computer program Sequest (Eng et al., 1994) was utilized to match this tandem mass spectrum to the sequence shown, with the serine residue being phosphorylated. (B) Tandem mass spectrum resulting from the analysis of excised phosphopeptide 3. The position of the phosphorylated serine residue is indicated.