Protein phosphorylation is a very important protein post-translational modification that controls many cellular processes including metabolism, transcriptional and translation regulation, degradation of proteins, cellular signaling and communication, proliferation, differentiation, and cell survival (1). Approximately 35% of human proteins are phosphorylated. Phosphoproteins are low in abundance, and, therefore, are challenging to detect and characterize by mass spectrometry. Different enrichment systems have been developed to isolate phosphopeptides. Among these techniques, immobilized metal affinity chromatography (IMAC) using Fe3+ and Ga3+ has been widely used for the enrichment of phosphopeptides.
Typical experimental workflows are tedious and consist of numerous steps, including sample collection and cell lysis. One of the major challenges of the process is to maintain the in vivo phosphorylation state of the proteins throughout the preparation process
To evaluate the effect of sample collection protocols on the global phosphorylation status of the cell, a recent paper by Kashin et al. compared different sample workflows by metabolic labeling and quantitative mass spectrometry on Saccharomyces cerevisiae cell cultures (2).
Three different sample collection workflows were evaluated: two that used denaturating conditions and involved mixing of cell cultures with an excess of either ethanol (EtOH) at −80 °C or trichloroacetic acid (TCA), and a third under nondenaturing conditions and washing the cells in PBS.
Their data suggest that either TCA or EtOH sample collection protocols introduced lower collection bias than the PBS protocol. It was also suggested that similar studies be carried out to determine what effects sample preparation has on other post translation modifications such as acetylation or ubiquitination.
- Thingholm T.E. et al, (2009) Analytical strategies for phosphoproteomics. Proteomics 9,1451–68
- Kanshin, E. et al. (2015) Sample Collection Method Bias Effects in Quantitative Phosphoproteomics. J Proteome Res. 14, 2998-04.
Protein phosphorylation is the most widespread type of post-translational modification. It affects every basic cellular process, including metabolism, growth, division, differentiation, motility, organelle trafficking, membrane transport, muscle contraction, immunity, learning and memory (1,2). Protein kinases catalyse the transfer of the phosphate from ATP to specific amino acids in proteins. In eukaryotes, these are usually Ser, Thr and Tyr residues. Due to the development of specific phosphopeptide enrichment techniques and highly sensitive MS instruments, phosphoproteomics has enabled researchers to gain a comprehensive view on the dynamics of protein phosphorylation and phosphorylation based signaling networks.
Due to its high cleavage specificity, trypsin is the commonly used proteolytic enzyme in MS-based proteomics, cleaving peptides carboxyterminal of the amino acids lysine and arginine. However, various factors such as the tertiary structure of a protein, adjacent basic amino acids or negatively charged residues close to cleavage sites as well as PTMs are known to impair proteolysis.
To gain closer insights into the impact of phosphorylation on tryptic digestion, a recent publication(3) systematically characterized the digestion efficiency of model peptide sequences that are known to be prone to incomplete digestion.
The results indicated that increasing trypsin concentrations up to a trypsin to peptide ratio of 1:10 led to a significant gain (1) in the overall number of phosphorylation sites (up to 9%) and in the intensities of individual phosphopeptides, thereby improving the sensitivity of phosphopeptide quantification.
The effect of organic solvents (ACN, acetonitrile and TFE trifuorethanol was also evaluated). Positive results were noted with TFE when determining the digestion of individual peptides. However TFE interfered with TiO2 phosphopeptide enrichment and therefore was not recommended for use with complex samples.
- Engholm-Keller, K and Larsen, M.R. (2013) Technologies and challenges in large scale phosphoroproteomics. Proteomics 13, 910–31.
- Beausoleil, S. A. et al. (2010) Tissue-specific atlas of mouse protein phosphorylation and expression. Cell 143, 1174–89.
- Dickhut, C. et al. (2014) Impact of Digestion Conditions on phosphoproteomics. J. Proteome Res. 13, 2761–70.