MS/MS spectra of the phosphopeptides VLpTPLQVQGEGEGR and LDLIDEEEDpSEEDGDSMR. Low-energy sequence ions (b and y-type ions) produced by collision-induced dissociation allowed for identification of several E1 phosphorylation sites after searching the tandem MS data against the NCBI nr sequence database with the MASCOT algorithm. A. Spectrum for VLpTPLQVQGEGEGR. B. Spectrum for LDLIDEEEDpSEEDGDSMR. The b and y-type ions are indicated on the peptide sequence and on the corresponding spectrum peak. A differential modification of 80 Da for serine and threonine was included in the MASCOT search parameters.