Identification of the phosphorylated Gag-p6 site. (A) Schematic overview of the experimental workflow. (B) MS peptide map of phosphorylated GST-Gag after trypsin digestion. The red arrow indicates the 1881.95 Da peak which is unique to the sample treated with active aPKC. This is consistent with the mass of the theoretical phosphorylated peptides and the map shows a neutral loss peak at 1783.95. (C) The resulting phosphopeptide was analyzed using MALDI-TOF/TOF MS. Fragment ions of the b and y-series identified in the tandem mass spectra from the peptide are shown. Tandem mass spectra of m/z 1881.95 signals revealed that each sequence corresponded to the phosphopeptide (QEPIDKELYPLTpSLR) of Gag-P6.