Fig. 4.

A, Normalized phosphorylation levels based on MS signals corresponding to all six phosphorylated residues detected in LC-MS/MS experiment. B, Carboxyl group modification extents of residues in activation loop. A total of four modified residues were detected by LC-MS/MS. The distinction between modification of unphosphorylated tryptic peptide and phosphorylated tryptic peptide was based on mass difference and product-ion spectrum of each peptide ion. C, Activation loop in Her4 kinase-domain (crystal structure, PDB id: 3BCE). The loop is labeled in rainbow colors. Three Glu residues and the phosphorylated residue Tyr-850 are highlighted in stick mode. The Asp residue Asp-853 is not seen in the crystal structure. D, Phosphorylation effects on carboxyl group modification. Two synthetic peptides (unphosphorylated versus phosphorylated form) with same sequence as the Tyr-850-containing tryptic peptide were used as a control to check the effects of phosphorylation on carboxyl group modification.