Figure 3

The efficiency and reproducibility of the phosphopeptide purification and identification when using ~3 μg of protein kinases per each resin and/or phosphoenrichment method (SIMAC, TiO ₂ and IMAC) coupled to R3/C18 and MSA-LTQ ion Trap mass spectrometer is illustrated. [A] Four triplicate experiments were carried out in order to identify the phosphopeptides. The phospho-site identifications were carried out from pooled and non-pooled assays (inter- and intra-assays) confirming a high reproducibility. The 6 phosphorylated peptides identified were isolated and validated in the four triplicate analyses, not only by Mascot (at least 4 continuously -y and -b ions matched)but also by manual inspection of all the spectra. SIMAC allowed the purification of 3 phosphorylated proteins: HuR RNA binding, p38 MAP Kinase and Trapped Ubiquitin-Like Protein Activation Complex, and 6 phosphorylated peptides related to those previously mentioned proteins. TiO₂ and IMAC allowed the isolation of 2 phoshorylated proteins: HuR RNA binding and p38 MAP Kinase, and 1 phosphopeptide related to the protein kinase HuR RNA binding. [B] SIMAC coupled to MSA allowed the identification of one more phosphopeptide compared to SIMAC coupled to DDNLMS3. Nevertheless, both strategies (SIMAC coupled to MSA and SIMAC coupled to DDNLMS3) allowed the identification of the same number of phosphorylated proteins (3). [C] and [D] Three phosphorylated proteins and six phosphopeptides were identified when using SIMAC coupled to MSA. From those three phosphoproteins identified, six phosphopeptides were identified: (a) TiO₂ coupled to MSA allowed the identification of two equal/same phosphorylated proteins and four equal/same phosphopeptides as SIMAC and (b) IMAC allowed the identification of one equal/same protein and two equal/same phosphopeptides. Thus, SIMAC is more efficient than the other tested resins for this study, while TiO₂ and IMAC corroborate the reproducibility of the phosphorylated proteins and phosphopeptides identified.