Figure 4.

Matrix-assisted laser desorption ionization (MALDI) mass spectra of R-Ras phosphopeptides. Tyrosine-phosphorylated peptides resulting from digestion with endoproteinase Asp-N alone (A) or endoproteinase Asp-N followed by endoproteinase Glu-C (B) were isolated from the peptide mixtures by using anti-phosphotyrosine antibodies conjugated to agarose and analyzed directly by MALDI mass spectrometry. Peaks representing peptides phosphorylated at Y66 are indicated by their respective masses and sequences. Peaks marked B are from background ions detected in control samples and unmarked peaks are unidentified. The peptide at m/z 1836.16 (Ox) has a mass consistent with its being an oxidized form of that at m/z 1819.21.