Figure 1. Protein identification by mass spectrometry.

Figure 2. Structure of the 4‐plex iTRAQ reagent. The reagent consists of an amine‐reactive group, a balance group, and a reporter group. The isobaric tags have a mass of 145 Da, which results in a mass difference of 144.1 Da for all differently labeled peptides. Upon fragmentation, the reporter group and a neutral fragment (balance group) are released. Due to the isotope composition of the reporter and the balance group, the generated reporter ions show different masses for the different iTRAQ reagents.

Figure 3. MS‐based quantification of spliceosomal B and C complexes. (a) MS spectrum of peptides derived from metabolically (SILAC) labeled B and C complexes. C complexes were purified from a light nuclear extract, and B complexes from a heavy nuclear extract. The signal intensities of the differently labeled peptides are used for quantification; (b) MS/MS spectrum of chemically (iTRAQ) labeled peptides derived from B and C complexes. The signal intensities of the iTRAQ reporter ions are used for quantification. Peptides derived from B complexes were labeled with iTRAQ‐115, and peptides derived from C complexes with iTRAQ‐116. The internal standard (containing a mixture of B and C complex peptides) was labeled with iTRAQ‐114.