Non-redundant Phosphopeptides Research Articles

A Phos-tag-based micropipette-tip method for rapid and selective enrichment of phosphopeptides.

Phosphorylated peptides are attractive targets in the study of the phosphoproteome. Here, we introduce a simple and convenient micropipette-tip method for the separation of phosphorylated and nonphosphorylated peptides by using a phosphate-binding zinc(II) complex of 1,3-bis(pyridin-2-ylmethylamino)propan-2-olate (Phos-tag). A 200-μL micropipette tip containing 10μL of swollen agarose beads functionalized with Phos-tag moieties was prepared. All steps in the phosphate-affinity separation (binding, washing, and elution) were conducted by using aqueous buffers at neutral pH values. The entire separation protocol required less than 30min per sample. By means of three independent separation experiments, followed by mass spectrometric (MS) analyses, we identified 1,649 non-redundant phosphopeptides from the lysates of human embryonic kidney cells (the peptides sample derived from 25μg proteins per an MS analysis). The average ratio of identified phosphopeptides to total peptides in the respective experiments was >90%, showing a high selectivity. Furthermore, the high correlation between the triplicate analyses was confirmed by scatter plots based on the normalized abundance of each peptide, as calculated by a label-free peptide relative quantification analysis in Progenesis QI. This micropipette-tip method would be thus used preferentially as an alternative to existing tools for the reliable enrichment of phosphopeptides.

Quantitative and Functional Phosphoproteomic Analysis Reveals that Ethylene Regulates Water Transport via the C-Terminal Phosphorylation of Aquaporin PIP2;1 in Arabidopsis

Ethylene participates in the regulation of numerous cellular events and biological processes, including water loss, during leaf and flower petal wilting. The diverse ethylene responses may be regulated via dynamic interplays between protein phosphorylation/dephosphorylation and ubiquitin/26S proteasome-mediated protein degradation and protease cleavage. To address how ethylene alters protein phosphorylation through multi-furcated signaling pathways, we performed a 15N stable isotope labelling-based, differential, and quantitative phosphoproteomics study on air- and ethylene-treated ethylene-insensitive Arabidopsis double loss-of-function mutant ein3-1/eil1-1. Among 535 non-redundant phosphopeptides identified, two and four phosphopeptides were up- and downregulated by ethylene, respectively. Ethylene-regulated phosphorylation of aquaporin PIP2;1 is positively correlated with the water flux rate and water loss in leaf. Genetic studies in combination with quantitative proteomics, immunoblot analysis, protoplast swelling/shrinking experiments, and leaf water loss assays on the transgenic plants expressing both the wild-type and S280A/S283A-mutated PIP2;1 in the both Col-0 and ein3eil1 genetic backgrounds suggest that ethylene increases water transport rate in Arabidopsis cells by enhancing S280/S283 phosphorylation at the C terminus of PIP2;1. Unknown kinase and/or phosphatase activities may participate in the initial up-regulation independent of the cellular functions of EIN3/EIL1. This finding contributes to our understanding of ethylene-regulated leaf wilting that is commonly observed during post-harvest storage of plant organs.

Extended Coverage of Singly and Multiply Phosphorylated Peptides from a Single Titanium Dioxide Microcolumn

We developed a novel approach to enlarge phosphoproteome coverage by selective elution depending on the number of phosphoryl group of peptides from a single titanium dioxide (TiO2) microcolumn using hydrophilic interaction chromatography (HILIC). In this approach, acidic methylphosphonate buffer including organic solvent is used for selective elution of singly phosphorylated peptides from an aliphatic hydroxy acid-modified metal oxide chromatography (HAMMOC) microcolumn and basic elution conditions with phosphate, ammonium hydroxide, and pyrrolidine are then employed for eluting multiply phosphorylated peptides retained by the HAMMOC microcolumn. Finally, we successfully identified 11 300 nonredundant phosphopeptides from triplicate analyses of 100 μg of HeLa cell lysates using this approach. This simple strategy made it possible to accomplish comprehensive and efficient phosphoproteome analysis from limited sample amounts loaded onto a single HAMMOC microcolumn without additional fractionation or enrichment approaches.

Comparative phosphoproteome analysis reveals more ERK activation in MDA-MB-231 than in MCF-7

Protein phosphorylation forms the foundation of many intra-cellular signaling networks. Enrichment is very important to analyze phosphoproteins due to their low abundance in cells. We used TiO 2 enrichment and LC–MS/MS to identify phosphopeptides from two breast cancer cells, MCF-7 and MDA-MB-231. The dataset generated were searched using Sequest algorithm with 1% false discovery rate. It was observed that 234 phosphoproteins and 280 phosphopeptides were common from a total of 590 and 455 phosphoproteins and 802 and 599 non-redundant phosphopeptides identified from MCF-7 and MDA-MB-231, respectively. Besides, a total of 843 and 578 non-redundant phosphorylation sites were also identified from these two cell lines. ERK1/ERK2 kinase substrate motif (PX[S/T]#P) was identified in MDA-MB-231 but not in MCF-7, showing that this kinase signal transduction pathway is a key factor during breast cancer progression. Activation of ERK1/ERK2 in MDA-MB-231 was further confirmed by Western blot. Quantitative analysis by spectral counting revealed 31 phosphoproteins were statistically in different levels between the two cells lines. This difference is due to the differences in kinase activities as well as protein expression levels. Since breast cancer research makes extensive use of both of these cell lines, our study's findings will be of value in understanding phosphorylation signal of breast cancer.

In-depth Qualitative and Quantitative Profiling of Tyrosine Phosphorylation Using a Combination of Phosphopeptide Immunoaffinity Purification and Stable Isotope Dimethyl Labeling

Several mass spectrometry-based assays have emerged for the quantitative profiling of cellular tyrosine phosphorylation. Ideally, these methods should reveal the exact sites of tyrosine phosphorylation, be quantitative, and not be cost-prohibitive. The latter is often an issue as typically several milligrams of (stable isotope-labeled) starting protein material are required to enable the detection of low abundance phosphotyrosine peptides. Here, we adopted and refined a peptidecentric immunoaffinity purification approach for the quantitative analysis of tyrosine phosphorylation by combining it with a cost-effective stable isotope dimethyl labeling method. We were able to identify by mass spectrometry, using just two LC-MS/MS runs, more than 1100 unique non-redundant phosphopeptides in HeLa cells from about 4 mg of starting material without requiring any further affinity enrichment as close to 80% of the identified peptides were tyrosine phosphorylated peptides. Stable isotope dimethyl labeling could be incorporated prior to the immunoaffinity purification, even for the large quantities (mg) of peptide material used, enabling the quantification of differences in tyrosine phosphorylation upon pervanadate treatment or epidermal growth factor stimulation. Analysis of the epidermal growth factor-stimulated HeLa cells, a frequently used model system for tyrosine phosphorylation, resulted in the quantification of 73 regulated unique phosphotyrosine peptides. The quantitative data were found to be exceptionally consistent with the literature, evidencing that such a targeted quantitative phosphoproteomics approach can provide reproducible results. In general, the combination of immunoaffinity purification of tyrosine phosphorylated peptides with large scale stable isotope dimethyl labeling provides a cost-effective approach that can alleviate variation in sample preparation and analysis as samples can be combined early on. Using this approach, a rather complete qualitative and quantitative picture of tyrosine phosphorylation signaling events can be generated.

Lys-N and Trypsin Cover Complementary Parts of the Phosphoproteome in a Refined SCX-Based Approach

The analysis of proteome-wide phosphorylation events is still a major analytical challenge because of the enormous complexity of protein phosphorylation networks. In this work, we evaluate the complementarity of Lys-N, Lys-C, and trypsin with regard to their ability to contribute to the global analysis of the phosphoproteome. A refined version of low-pH strong cation exchange was used to efficiently separate N-terminally acetylated, phosphorylated, and nonmodified peptides. A total of 5036 nonredundant phosphopeptides could be identified with a false discovery rate of <1% from 1 mg of protein using a combination of the three enzymes. Our data revealed that the overlap between the phosphopeptide data sets generated with different proteases was marginal, whereas the overlap between two similarly generated tryptic data sets was found to be at least 4 times higher. In this way, the parallel use of Lys-N and trypsin enabled a 72% increase in the number of detected phosphopeptides as compared to trypsin alone, whereas a trypsin replicate experiment only led to a 25% increase. Thus, when focusing solely on the trypsin and Lys-N data, we identified 4671 nonredundant phosphopeptides. Further analysis of the detected sites showed that the Lys-N and trypsin data sets were enriched in significantly different phosphorylation motifs, further evidencing that multiprotease approaches are very valuable in phosphoproteome analyses.

Successive and Selective Release of Phosphorylated Peptides Captured by Hydroxy Acid-Modified Metal Oxide Chromatography

We developed a novel approach to enlarge phosphoproteome coverage using successive elution of phosphopeptides with various buffers in series from a single microcolumn packed with hydroxy acid-modified metal oxides, such as titania and zirconia. Elution conditions were investigated to maximize the recovery of phosphopeptides from three standard phosphoproteins. Secondary amines, such as piperidine and pyrrolidine, provided better efficiency than the conventional conditions using ammonium hydroxide and phosphate buffers. Furthermore, elution with these secondary amines provided unique phosphopeptides that were not eluted under the conventional conditions in the analysis of HeLa cell lysates. On the basis of these results, we fractionated phosphopeptides captured by a single metal oxide microcolumn using successive elution with 5% ammonium hydroxide solution, 5% piperidine solution and 5% pyrrolidine solution in series. We identified 1,803 nonredundant phosphopeptides from 100 microg of HeLa cells, which represented a 1.6-fold increase in phosphopeptide number and a 1.9-fold increase in total peak area of phosphopeptides in comparison with the results obtained under the conventional conditions. Since this approach is applicable to any single tip-based protocol without coupling with other enrichment methods, this simple strategy can be easily incorporated as an option into existing protocols for phosphopeptide enrichment, and would be suitable for high-throughput analysis in a parallel format.

Phosphoproteomic Analysis of the Mouse Brain Cytosol Reveals a Predominance of Protein Phosphorylation in Regions of Intrinsic Sequence Disorder

We analyzed the mouse forebrain cytosolic phosphoproteome using sequential (protein and peptide) IMAC purifications, enzymatic dephosphorylation, and targeted tandem mass spectrometry analysis strategies. In total, using complementary phosphoenrichment and LC-MS/MS strategies, 512 phosphorylation sites on 540 non-redundant phosphopeptides from 162 cytosolic phosphoproteins were characterized. Analysis of protein domains and amino acid sequence composition of this data set of cytosolic phosphoproteins revealed that it is significantly enriched in intrinsic sequence disorder, and this enrichment is associated with both cellular location and phosphorylation status. The majority of phosphorylation sites found by MS were located outside of structural protein domains (97%) but were mostly located in regions of intrinsic sequence disorder (86%). 368 phosphorylation sites were located in long regions of disorder (over 40 amino acids long), and 94% of proteins contained at least one such long region of disorder. In addition, we found that 58 phosphorylation sites in this data set occur in 14-3-3 binding consensus motifs, linear motifs that are associated with unstructured regions in proteins. These results demonstrate that in this data set protein phosphorylation is significantly depleted in protein domains and significantly enriched in disordered protein sequences and that enrichment of intrinsic sequence disorder may be a common feature of phosphoproteomes. This supports the hypothesis that disordered regions in proteins allow kinases, phosphatases, and phosphorylation-dependent binding proteins to gain access to target sequences to regulate local protein conformation and activity.

Automated Phosphoproteome Analysis for Cultured Cancer Cells by Two-Dimensional NanoLC-MS Using a Calcined Titania/C18 Biphasic Column

We have developed an on-line automated system for phosphoproteome analysis using titania-based phosphopeptide enrichment followed by nanoLC-MS/MS. Titania beads were prepared by calcination of commercial chromatographic titania beads at 800 degrees C to convert the crystalline structure. The obtained rutile-form titania exhibited higher selectivity in phosphopeptide enrichment than commercial titania, even in the absence of a competitive chelating reagent for non-phosphopeptides. For phosphoproteome analysis of human cervical cancer HeLa cells, tryptic digests of the cell extracts were directly injected into this on-line system, and 696 non-redundant phosphopeptides with 671 unambiguously determined phosphorylation sites, derived from 512 phosphoproteins, were successfully identified. This is the first successful application of an on-line automated phosphoproteome analysis system to complex biological samples.