Combination of Bottom-up 2D-LC-MS and Semi-top-down GelFree-LC-MS Enhances Coverage of Proteome and Low Molecular Weight Short Open Reading Frame Encoded Peptides of the Archaeon Methanosarcina mazei.

The recent discovery of an increasing number of small open reading frames (sORF) creates the need for suitable analytical technologies for the comprehensive identification of the corresponding gene products. For biological and functional studies the knowledge of the entire set of proteins and sORF gene products is essential. Consequently in the present study we evaluated analytical approaches that will allow for simultaneous analysis of widest parts of the proteome together with the predicted sORF. We performed a full proteome analysis of the methane producing archaeon Methanosarcina mazei strain Gö1 cytosolic proteome using a high/low pH reversed phase LC-MS bottom-up approach. The second analytical approach was based on semi-top-down strategy, encompassing a separation at intact protein level using a GelFree system, followed by digestion and LC-MS analysis. A high overlap in identified proteins was found for both approaches yielding the most comprehensive coverage of the cytosolic proteome of this organism achieved so far. The application of the second approach in combination with an adjustment of the search criteria for database searches further led to a significant increase of sORF peptide identifications, finally allowing to detect and identify 28 sORF gene products.

Comparative Proteome Analysis in Schizosaccharomyces pombe Identifies Metabolic Targets to Improve Protein Production and Secretion.

Protein secretion in yeast is a complex process and its efficiency depends on a variety of parameters. We performed a comparative proteome analysis of a set of Schizosaccharomyces pombe strains producing the α-glucosidase maltase in increasing amounts to investigate the overall proteomic response of the cell to the burden of protein production along the various steps of protein production and secretion. Proteome analysis of these strains, utilizing an isobaric labeling/two dimensional LC-MALDI MS approach, revealed complex changes, from chaperones and secretory transport machinery to proteins controlling transcription and translation. We also found an unexpectedly high amount of changes in enzyme levels of the central carbon metabolism and a significant up-regulation of several amino acid biosyntheses. These amino acids were partially underrepresented in the cellular protein compared with the composition of the model protein. Additional feeding of these amino acids resulted in a 1.5-fold increase in protein secretion. Membrane fluidity was identified as a second bottleneck for high-level protein secretion and addition of fluconazole to the culture caused a significant decrease in ergosterol levels, whereas protein secretion could be further increased by a factor of 2.1. In summary, we show that high level protein secretion causes global changes of protein expression levels in the cell and that precursor availability and membrane composition limit protein secretion in this yeast. In this respect, comparative proteome analysis is a powerful tool to identify targets for an efficient increase of protein production and secretion in S. pombe Data are available via ProteomeXchange with identifiers PXD002693 and PXD003016.

C-Terminal Charge-Reversal Derivatization and Parallel Use of Multiple Proteases Facilitates Identification of Protein C-Termini by C-Terminomics.

The identification of protein C-termini in complex proteomes is challenging due to the poor ionization efficiency of the carboxyl group. Amidating the negatively charged C-termini with ethanolamine (EA) has been suggested to improve the detection of C-terminal peptides and allows for a directed depletion of internal peptides after proteolysis using carboxyl reactive polymers. In the present study, the derivatization with N,N-dimethylethylenediamine (DMEDA) and (4-aminobutyl)guanidine (AG) leading to a positively charged C-terminus was investigated. C-terminal charge-reversed peptides showed improved coverage of b- and y-ion series in the MS/MS spectra compared to their noncharged counterparts. DMEDA-derivatized peptides resulted in many peptides with charge states of 3+, which benefited from ETD fragmentation. This makes the charge-reversal strategy particularly useful for the analysis of protein C-termini, which may also be post-translationally modified. The labeling strategy and the indirect enrichment of C-termini worked with similar efficiency for both DMEDA and EA, and their applicability was demonstrated on an E. coli proteome. Utilizing two proteases and different MS/MS activation mechanisms allowed for the identification of >400 C-termini, encompassing both canonical and truncated C-termini.

Conserved phosphorylation sites in the activation loop of the Arabidopsis phytosulfokine receptor PSKR1 differentially affect kinase and receptor activity.

PSK (phytosulfokine) is a plant peptide hormone perceived by a leucine-rich repeat receptor kinase. Phosphosite mapping of epitope-tagged PSKR1 (phytosulfokine receptor 1) from Arabidopsis thaliana plants identified Ser(696) and Ser(698) in the JM (juxtamembrane) region and probably Ser(886) and/or Ser(893) in the AL (activation loop) as in planta phosphorylation sites. In vitro-expressed kinase was autophosphorylated at Ser(717) in the JM, and at Ser(733), Thr(752), Ser(783), Ser(864), Ser(911), Ser(958) and Thr(998) in the kinase domain. The LC-ESI-MS/MS spectra provided support that up to three sites (Thr(890), Ser(893) and Thr(894)) in the AL were likely to be phosphorylated in vitro. These sites are evolutionarily highly conserved in PSK receptors, indicative of a conserved function. Site-directed mutagenesis of the four conserved residues in the activation segment, Thr(890), Ser(893), Thr(894) and Thr(899), differentially altered kinase activity in vitro and growth-promoting activity in planta. The T899A and the quadruple-mutated TSTT-A (T890A/S893A/T894A/T899A) mutants were both kinase-inactive, but PSKR1(T899A) retained growth-promoting activity. The T890A and S893A/T894A substitutions diminished kinase activity and growth promotion. We hypothesize that phosphorylation within the AL activates kinase activity and receptor function in a gradual and distinctive manner that may be a means to modulate the PSK response.

Identification and relative quantification of phosphopeptides by a combination of multi-protease digestion and isobaric labeling.

RATIONALE: The identification and the determination of the extent of protein phosphorylation are major prerequisites for the comparative analysis of this important posttranslational modification of proteins in different biological situations. High sequence coverages and the availability of straightforward quantification methods are necessary to achieve these goals. METHODS: Phosphoproteins and non-phosphorylated analogues separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) were digested using four different proteases (trypsin, chymotrypsin, elastase and GluC) and the digests were isobarically labeled using eight-plex iTRAQ. The combined labeled digests were subsequently enriched using titanium dioxide and both the phosphorylated and non-phosphorylated fractions were analyzed by liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS). The influence of different data analysis algorithms (Percolator or False Discovery Rate) on the outcome of analysis was investigated. RESULTS: Almost complete sequence coverage could be achieved upon application of a multi-protease approach. The formation of peptides of different lengths and physicochemical properties allowed the identification and the mapping of all phosphorylation sites in the investigated model proteins. The introduction of isobaric labels allowed quantification of different peptides of the same phosphorylation site with more than one peptide, leading to significantly improved statistical confidence. CONCLUSIONS: A workflow for the straightforward comparative analysis of protein phosphorylation in samples of low complexity, e.g. isolated proteins, was developed. The workflow is transferable to other posttranslational modifications.

Systems genomics evaluation of the SH-SY5Y neuroblastoma cell line as a model for Parkinson's disease.

BACKGROUND: The human neuroblastoma cell line, SH-SY5Y, is a commonly used cell line in studies related to neurotoxicity, oxidative stress, and neurodegenerative diseases. Although this cell line is often used as a cellular model for Parkinson's disease, the relevance of this cellular model in the context of Parkinson's disease (PD) and other neurodegenerative diseases has not yet been systematically evaluated. RESULTS: We have used a systems genomics approach to characterize the SH-SY5Y cell line using whole-genome sequencing to determine the genetic content of the cell line and used transcriptomics and proteomics data to determine molecular correlations. Further, we integrated genomic variants using a network analysis approach to evaluate the suitability of the SH-SY5Y cell line for perturbation experiments in the context of neurodegenerative diseases, including PD. CONCLUSIONS: The systems genomics approach showed consistency across different biological levels (DNA, RNA and protein concentrations). Most of the genes belonging to the major Parkinson's disease pathways and modules were intact in the SH-SY5Y genome. Specifically, each analysed gene related to PD has at least one intact copy in SH-SY5Y. The disease-specific network analysis approach ranked the genetic integrity of SH-SY5Y as higher for PD than for Alzheimer's disease but lower than for Huntington's disease and Amyotrophic Lateral Sclerosis for loss of function perturbation experiments.

LC-MS based cleavage site profiling of the proteases ADAM10 and ADAM17 using proteome-derived peptide libraries.

A Disintegrin and Metalloproteinase 10 (ADAM10) and ADAM17 catalyze ectodomain shedding of a number of cell surface proteins important for embryonic development and tissue homeostasis. Changes in the expression levels or dysregulated proteolytic activity of ADAM10 and ADAM17 have been shown to play important roles in multiple diseases such as inflammation, cancer, and neurodegenerative disorders. Despite the well documented substrate repertoire of ADAM10 and ADAM17, little is known about their cleavage site specificity. We optimized Q-PICS (Quantitative Proteomics for the Identification of Cleavage Sites) to elucidate the cleavage site specificity of recombinant murine ADAM10 and ADAM17. Two different yeast proteome-derived peptide libraries were used and samples were analyzed by LC-MALDI and LC-ESI MS in parallel. We show that the largest difference in the cleavage site specificities of ADAM10 and ADAM17 is at the P1' site: while both enzymes cleave N-terminal of leucine, only ADAM10 shows additional preference toward aromatic amino acids, whereas ADAM17 exhibits the highest preference for valine. Together with further amino acid preferences more adjacent to the scissile bond, our data is in good agreement with ADAM10/17 cleavage sites previously identified in native substrates. Overall, the precise identification of ADAM10 and ADAM17 cleavage site specificity provides the basis for better substrate identification in vivo and the generation of specific inhibitors or activity based probes.

Paleoproteomic study of the Iceman's brain tissue.

The Tyrolean Iceman, a Copper-age ice mummy, is one of the best-studied human individuals. While the genome of the Iceman has largely been decoded, tissue-specific proteomes have not yet been investigated. We studied the proteome of two distinct brain samples using gel-based and liquid chromatography-mass spectrometry-based proteomics technologies together with a multiple-databases and -search algorithms-driven data-analysis approach. Thereby, we identified a total of 502 different proteins. Of these, 41 proteins are known to be highly abundant in brain tissue and 9 are even specifically expressed in the brain. Furthermore, we found 10 proteins related to blood and coagulation. An enrichment analysis revealed a significant accumulation of proteins related to stress response and wound healing. Together with atomic force microscope scans, indicating clustered blood cells, our data reopens former discussions about a possible injury of the Iceman's head near the site where the tissue samples have been extracted.

Optimized fragmentation conditions for iTRAQ-labeled phosphopeptides.

Protein phosphorylation is an important post-translational modification that plays a regulatory role within numerous biological processes. The simultaneous identification, localization, and quantification of phosphorylated proteins is vital for understanding this dynamic control mechanism. The application of isobaric labeling strategies, for example, iTRAQ, for quantitative phosphopeptide analysis requires simultaneous monitoring of peptide backbone fragmentation, loss of phosphoryl moieties, and the cleavage of isobaric labeling reporter ions. In the present study, we have examined MS/MS fragmentation modes available in the Orbitrap Velos MS (collision induced dissociation (CID), CID plus multistage activation, and higher energy collision dissociation (HCD)), for their ability to generate ions required for simultaneous quantification and identification of iTRAQ labeled phosphopeptides in a semicomplex (12) and a complex (131) phosphopeptide mix. The required normalized collision energies for quantification and identification of iTRAQ-labeled phosphopeptides require a compromise between the optimal parameters for each aspect. Here, we were able to determine an optimized MS/MS measurement protocol that involves CID measurement in ion trap for identification followed by HCD measurement for parallel identification and quantification that satisfies the time requirements for LC-MS/MS experiments.