FIGURE 5.

MS2 spectra corresponding to p85α phosphorylation at Thr-86. Panel A shows the MS2 spectra of a tryptic peptide (identified as ISPPT*PK) from gel-purified p85 isolated from BAEC. Spectra shown is from a 1+ precursor with 819.2 m/z. This precursor displayed a prominent neutral ion loss at 721.2 m/z. (* indicates the phosphorylated residue Thr-86). The phosphopeptide sequence was assigned using Sequest. Panel B shows the MS2 spectra of 2+ ion at 410.2 m/z, which co-elutes with 819.2. The MS2 of this 2+ precursor provided 1+ and 2+ ions associated with ISPPT*PK fragmentation. These spectra and sequence assignments were confirmed in three separate experiments. Panels C and D show MS2 spectra of synthetic peptide ISPPT*PK as a 1+ and 2+ precursor, respectively.