Figure 4

LC-MS/MS analysis of synthetic phosphopeptides and tryptic peptides derived from GST–N1 fusion protein phosphorylated by CDK1 in vitro . (A) A mixture of the four synthetic peptides (A–D, 6 pmol each) was separated and detected simultaneously by LC-MS/MS using micrOTOFq. Upper panel: Base peak chromatogram (BPC) and extracted ion chromatogram (EIC) of a single LC run. Lower panel: MS/MS spectra of the four peptides used to assign amino acid sequences. (B) The tryptic peptides derived from the GST–N1 fusion protein (0.2 μg, 6.25 pmol) phosphorylated by CDK1 in vitro were separated and detected by LC-MS/MS using micrOTOFq. Upper panel: BPC and EIC of a single LC run; tryptic peptides corresponding to peptide A-D are highlighted. Lower panel: MS/MS spectra of the four tryptic peptides used to assign amino acid sequences.