Figure 6.

Mass spectrometric identification of serine 41 as a phosphorylation site in CDC20. Phosphorylated CDC20 was purified as described in Figure 5A, excised from the gel, and trypsinated. After determination of a candidate phosphopeptide mass (m/z 580.3) by parent ion scans (see MATERIALS AND METHODS), the peptide corresponding to this mass was sequenced on a quadrupole time-of -flight instrument. The MS/MS spectrum of observed phosphopeptide with m/z 582.3 in positive mode is shown. It resulted in the sequence EAAGPAPS*PMR and localizes the modification site to serine 41 because of the losses of H₃PO₄ (98 Da) in the Y“ ion series.