A systematic investigation into the nature of tryptic HCD spectra

Parallel reaction monitoring using quadrupole-Orbitrap mass spectrometer: Principle and applications

Targeted mass spectrometry-based approaches are nowadays widely used for quantitative proteomics studies and more recently have been implemented on high resolution/accurate mass (HRAM) instruments resulting in a considerable performance improvement. More specifically, the parallel reaction monitoring technique (PRM) performed on quadrupole-Orbitrap mass spectrometers, leveraging the high resolution and trapping capabilities of the instrument, offers a clear advantage over the conventional selected reaction monitoring (SRM) measurements executed on triple quadrupole instruments. Analyses performed in HRAM mode allow for an improved discrimination between signals derived from analytes and those resulting from matrix interferences translating in the reliable quantification of low abundance components. The purpose of the study defines various implementation schemes of PRM, namely: (i) exploratory experiments assessing the detectability of very large sets of peptides (100-1000), (ii) wide-screen analyses using (crude) internal standards to obtain statistically meaningful (relative) quantitative analyses, and (iii) precise/accurate quantification of a limited number of analytes using calibrated internal standards. Each of the three implementation schemes requires specific acquisition methods with defined parameters to appropriately control the acquisition during the actual peptide elution. This tutorial describes the different PRM approaches and discusses their benefits and limitations in terms of quantification performance and confidence in analyte identification.

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.

RePLiCal: A QconCAT Protein for Retention Time Standardization in Proteomics Studies

This study introduces a new reversed-phase liquid chromatography retention time (RT) standard, RePLiCal (Reversed-phase liquid chromatography calibrant), produced using QconCAT technology. The synthetic protein contains 27 lysine-terminating calibrant peptides, meaning that the same complement of standards can be generated using either Lys-C or trypsin-based digestion protocols. RePLiCal was designed such that each constituent peptide is unique with respect to all eukaryotic proteomes, thereby enabling integration into a wide range of proteomic analyses. RePLiCal has been benchmarked against three commercially available peptide RT standard kits and outperforms all in terms of LC gradient coverage. RePLiCal also provides a higher number of calibrant points for chromatographic retention time standardization and normalization. The standard provides stable RTs over long analysis times and can be readily transferred between different LC gradients and nUHPLC instruments. Moreover, RePLiCal can be used to predict RTs for other peptides in a timely manner. Furthermore, it is shown that RePLiCal can be used effectively to evaluate trapping column performance for nUHPLC instruments using trap-elute configurations, to optimize gradients to maximize peptide and protein identification rates, and to recalibrate the m/z scale of mass spectrometry data post-acquisition.

Histone H1 Variants in Arabidopsis Are Subject to Numerous Post-Translational Modifications, Both Conserved and Previously Unknown in Histones, Suggesting Complex Functions of H1 in Plants

Linker histones (H1s) are conserved and ubiquitous structural components of eukaryotic chromatin. Multiple non-allelic variants of H1, which differ in their DNA/nucleosome binding properties, co-exist in animal and plant cells and have been implicated in the control of genetic programs during development and differentiation. Studies in mammals and Drosophila have revealed diverse post-translational modifications of H1s, most of which are of unknown function. So far, it is not known how this pattern compares with that of H1s from other major lineages of multicellular Eukaryotes. Here, we show that the two main H1variants of a model flowering plant Arabidopsis thaliana are subject to a rich and diverse array of post-translational modifications. The distribution of these modifications in the H1 molecule, especially in its globular domain (GH1), resembles that occurring in mammalian H1s, suggesting that their functional significance is likely to be conserved. While the majority of modifications detected in Arabidopsis H1s, including phosphorylation, acetylation, mono- and dimethylation, formylation, crotonylation and propionylation, have also been reported in H1s of other species, some others have not been previously identified in histones.

Identification, Quantification, and Site Localization of Protein Posttranslational Modifications via Mass Spectrometry-Based Proteomics

Posttranslational modifications (PTMs) are important biochemical processes for regulating various signaling pathways and determining specific cell fate. Mass spectrometry (MS)-based proteomics has been developed extensively in the past decade and is becoming the standard approach for systematic characterization of different PTMs on a global scale. In this chapter, we will explain the biological importance of various PTMs, summarize key innovations in PTMs enrichment strategies, high-performance liquid chromatography (HPLC)-based fractionation approaches, mass spectrometry detection methods, and lastly bioinformatic tools for PTMs related data analysis. With great effort in recent years by the proteomics community, highly efficient enriching methods and comprehensive resources have been developed. This chapter will specifically focus on five major types of PTMs; phosphorylation, glycosylation, ubiquitination/sumosylation, acetylation, and methylation.

SuperQuant: A Data Processing Approach to Increase Quantitative Proteome Coverage

SuperQuant is a quantitative proteomics data processing approach that uses complementary fragment ions to identify multiple coisolated peptides in tandem mass spectra allowing for their quantification. This approach can be applied to any shotgun proteomics data set acquired with high mass accuracy for quantification at the MS(1) level. The SuperQuant approach was developed and implemented as a processing node within the Thermo Proteome Discoverer 2.x. The performance of the developed approach was tested using dimethyl-labeled HeLa lysate samples having a ratio between channels of 10(heavy):4(medium):1(light). Peptides were fragmented with collision-induced dissociation using isolation windows of 1, 2, and 4 Th while recording data both with high-resolution and low-resolution. The results obtained using SuperQuant were compared to those using the conventional ion trap-based approach (low mass accuracy MS(2) spectra), which is known to achieve high identification performance. Compared to the common high-resolution approach, the SuperQuant approach identifies up to 70% more peptide-spectrum matches (PSMs), 40% more peptides, and 20% more proteins at the 0.01 FDR level. It identifies more PSMs and peptides than the ion trap-based approach. Improvements in identifications resulted in up to 10% more PSMs, 15% more peptides, and 10% more proteins quantified on the same raw data. The developed approach does not affect the accuracy of the quantification and observed coefficients of variation between replicates of the same proteins were close to the values typical for other precursor ion-based quantification methods. The raw data is deposited to ProteomeXchange (PXD001907). The developed node is available for testing at https://github.com/caetera/SuperQuantNode.

MS2PIP prediction server: compute and visualize MS2 peak intensity predictions for CID and HCD fragmentation

We present an MS² peak intensity prediction server that computes MS² charge 2+ and 3+ spectra from peptide sequences for the most common fragment ions. The server integrates the Unimod public domain post-translational modification database for modified peptides. The prediction model is an improvement of the previously published MS²PIP model for Orbitrap-LTQ CID spectra. Predicted MS² spectra can be downloaded as a spectrum file and can be visualized in the browser for comparisons with observations. In addition, we added prediction models for HCD fragmentation (Q-Exactive Orbitrap) and show that these models compute accurate intensity predictions on par with CID performance. We also show that training prediction models for CID and HCD separately improves the accuracy for each fragmentation method. The MS²PIP prediction server is accessible from http://iomics.ugent.be/ms2pip.

Proteomic challenges: sample preparation techniques for microgram-quantity protein analysis from biological samples

Proteins regulate many cellular functions and analyzing the presence and abundance of proteins in biological samples are central focuses in proteomics. The discovery and validation of biomarkers, pathways, and drug targets for various diseases can be accomplished using mass spectrometry-based proteomics. However, with mass-limited samples like tumor biopsies, it can be challenging to obtain sufficient amounts of proteins to generate high-quality mass spectrometric data. Techniques developed for macroscale quantities recover sufficient amounts of protein from milligram quantities of starting material, but sample losses become crippling with these techniques when only microgram amounts of material are available. To combat this challenge, proteomicists have developed micro-scale techniques that are compatible with decreased sample size (100 μg or lower) and still enable excellent proteome coverage. Extraction, contaminant removal, protein quantitation, and sample handling techniques for the microgram protein range are reviewed here, with an emphasis on liquid chromatography and bottom-up mass spectrometry-compatible techniques. Also, a range of biological specimens, including mammalian tissues and model cell culture systems, are discussed.

Characterization of protein N-glycosylation by tandem mass spectrometry using complementary fragmentation techniques

The analysis of post-translational modifications (PTMs) by proteomics is regarded as a technically challenging undertaking. While in recent years approaches to examine and quantify protein phosphorylation have greatly improved, the analysis of many protein modifications, such as glycosylation, are still regarded as problematic. Limitations in the standard proteomics workflow, such as use of suboptimal peptide fragmentation methods, can significantly prevent the identification of glycopeptides. The current generation of tandem mass spectrometers has made available a variety of fragmentation options, many of which are becoming standard features on these instruments. We have used three common fragmentation techniques, namely CID, HCD, and ETD, to analyze a glycopeptide and highlight how an integrated fragmentation approach can be used to identify the modified residue and characterize the N-glycan on a peptide.

Reconstitution of active human core Mediator complex reveals a critical role of the MED14 subunit

The evolutionarily conserved Mediator complex is a critical coactivator for RNA polymerase II (Pol II)-mediated transcription. Here, we report the reconstitution of a functional 15-subunit human core Mediator complex and its characterization by functional assays and chemical cross-linking coupled to mass spectrometry (CX-MS). Whereas the reconstituted head and middle modules can stably associate, only with incorporation of MED14 into the bi-modular complex does it acquire basal and coactivator functions. This results from a dramatically enhanced ability of MED14-containing complexes to associate with Pol II. Altogether, our analyses identify MED14 as both an architectural and a functional backbone of the Mediator complex. We further establish a conditional requirement for metazoan-specific MED26 that becomes evident in the presence of heterologous nuclear factors. This general approach paves the way for systematically dissecting the multiple layers of functionalities associated with the Mediator complex.

Photo-cross-linking and high-resolution mass spectrometry for assignment of RNA-binding sites in RNA-binding proteins

RNA–protein complexes play pivotal roles in many central biological processes. While methods based on next-generation sequencing have profoundly advanced our ability to identify the specific RNAs bound by a particular protein, there is a dire need for precise and systematic ways to identify RNA interaction sites on proteins. We have developed an integrated experimental and computational workflow combining photo-induced cross-linking, high-resolution mass spectrometry, and automated analysis of the resulting mass spectra for the identification of cross-linked peptides and exact amino acids with their cross-linked RNA oligonucleotide moiety of such RNA-binding proteins. The generic workflow can be applied to any RNA–protein complex of interest. Application to human and yeast mRNA–protein complexes in vitro and in vivo demonstrates the powerful utility of the approach by identification of 257 cross-linking sites on 124 distinct RNA-binding proteins. The software pipeline developed for this purpose is available as open-source software as part of the OpenMS project.

Structural characterization by cross-linking reveals the detailed architecture of a coatomer-related heptameric module from the nuclear pore complex

Most cellular processes are orchestrated by macromolecular complexes. However, structural elucidation of these endogenous complexes can be challenging because they frequently contain large numbers of proteins, are compositionally and morphologically heterogeneous, can be dynamic, and are often of low abundance in the cell. Here, we present a strategy for the structural characterization of such complexes that has at its center chemical cross-linking with mass spectrometric readout. In this strategy, we isolate the endogenous complexes using a highly optimized sample preparation protocol and generate a comprehensive, high-quality cross-linking dataset using two complementary cross-linking reagents. We then determine the structure of the complex using a refined integrative method that combines the cross-linking data with information generated from other sources, including electron microscopy, X-ray crystallography, and comparative protein structure modeling. We applied this integrative strategy to determine the structure of the native Nup84 complex, a stable hetero-heptameric assembly (∼ 600 kDa), 16 copies of which form the outer rings of the 50-MDa nuclear pore complex (NPC) in budding yeast. The unprecedented detail of the Nup84 complex structure reveals previously unseen features in its pentameric structural hub and provides information on the conformational flexibility of the assembly. These additional details further support and augment the protocoatomer hypothesis, which proposes an evolutionary relationship between vesicle coating complexes and the NPC, and indicates a conserved mechanism by which the NPC is anchored in the nuclear envelope.

Molecular architecture and function of the SEA complex, a modulator of the TORC1 pathway

The TORC1 signaling pathway plays a major role in the control of cell growth and response to stress. Here we demonstrate that the SEA complex physically interacts with TORC1 and is an important regulator of its activity. During nitrogen starvation, deletions of SEA complex components lead to Tor1 kinase delocalization, defects in autophagy, and vacuolar fragmentation. TORC1 inactivation, via nitrogen deprivation or rapamycin treatment, changes cellular levels of SEA complex members. We used affinity purification and chemical cross-linking to generate the data for an integrative structure modeling approach, which produced a well-defined molecular architecture of the SEA complex and showed that the SEA complex comprises two regions that are structurally and functionally distinct. The SEA complex emerges as a platform that can coordinate both structural and enzymatic activities necessary for the effective functioning of the TORC1 pathway.

A method to determine lysine acetylation stoichiometries

Lysine acetylation is a common protein posttranslational modification that regulates a variety of biological processes. A major bottleneck to fully understanding the functional aspects of lysine acetylation is the difficulty in measuring the proportion of lysine residues that are acetylated. Here we describe a mass spectrometry method using a combination of isotope labeling and detection of a diagnostic fragment ion to determine the stoichiometry of protein lysine acetylation. Using this technique, we determined the modification occupancy for ~750 acetylated peptides from mammalian cell lysates. Furthermore, the acetylation on N-terminal tail of histone H4 was cross-validated by treating cells with sodium butyrate, a potent deacetylase inhibitor, and comparing changes in stoichiometry levels measured by our method with immunoblotting measurements. Of note we observe that acetylation stoichiometry is high in nuclear proteins, but very low in mitochondrial and cytosolic proteins. In summary, our method opens new opportunities to study in detail the relationship of lysine acetylation levels of proteins with their biological functions.

Glycoform analysis of recombinant and human immunodeficiency virus envelope protein gp120 via higher energy collisional dissociation and spectral-aligning strategy

Envelope protein gp120 of human immunodeficiency virus (HIV) is armored with a dense glycan shield, which plays critical roles in envelope folding, immune-evasion, infectivity, and immunogenicity. Site-specific glycosylation profiling of recombinant gp120 is very challenging. Therefore, glycoproteomic analysis of native viral gp120 is still formidable to date. This challenge promoted us to employ a Q-Exactive mass spectrometer to identify low abundant glycopeptides from virion-associated gp120. To search the HCD-MS data for glycopeptides, a novel spectral-aligning strategy was developed. This strategy depends on the observation that glycopeptides and the corresponding deglycosylated peptides share very similar MS/MS pattern in terms of b- and y-ions that do not contain the site of glycosylation. Moreover, glycopeptides with an identical peptide backbone show nearly resembling spectra regardless of the attached glycan structures. For the recombinant gp120, this “copy–paste” spectral pattern of glycopeptides facilitated identification of 2224 spectra using only 18 spectral templates, and after precursor mass correction, 1268 (57%) spectra were assigned to 460 unique glycopeptides accommodating 19 N-linked and one O-linked glycosylation sites (glycosites). Strikingly, we were able to observe five N- and one O-linked glycosites in native gp120. We further revealed that except for Asn276 in the C2 region, glycans were processed to contain both high mannose and hybrid/complex glycans; an additional four N-linked glycosites were decorated with high mannose type. Core 1 O-linked glycan Gal₁GalNAc₁ was seen for the O-linked glycosite at Thr499. This direct observation of site-specific glycosylation of virion-derived gp120 has implications in HIV glycobiology and vaccine design.

Analytical utility of mass spectral binning in proteomic experiments by SPectral Immonium Ion Detection (SPIID)

Unambiguous identification of tandem mass spectra is a cornerstone in mass-spectrometry-based proteomics. As the study of post-translational modifications (PTMs) by means of shotgun proteomics progresses in depth and coverage, the ability to correctly identify PTM-bearing peptides is essential, increasing the demand for advanced data interpretation. Several PTMs are known to generate unique fragment ions during tandem mass spectrometry, the so-called diagnostic ions, which unequivocally identify a given mass spectrum as related to a specific PTM. Although such ions offer tremendous analytical advantages, algorithms to decipher MS/MS spectra for the presence of diagnostic ions in an unbiased manner are currently lacking. Here, we present a systematic spectral-pattern-based approach for the discovery of diagnostic ions and new fragmentation mechanisms in shotgun proteomics datasets. The developed software tool is designed to analyze large sets of high-resolution peptide fragmentation spectra independent of the fragmentation method, instrument type, or protease employed. To benchmark the software tool, we analyzed large higher-energy collisional activation dissociation datasets of samples containing phosphorylation, ubiquitylation, SUMOylation, formylation, and lysine acetylation. Using the developed software tool, we were able to identify known diagnostic ions by comparing histograms of modified and unmodified peptide spectra. Because the investigated tandem mass spectra data were acquired with high mass accuracy, unambiguous interpretation and determination of the chemical composition for the majority of detected fragment ions was feasible. Collectively we present a freely available software tool that allows for comprehensive and automatic analysis of analogous product ions in tandem mass spectra and systematic mapping of fragmentation mechanisms related to common amino acids.

NovoHCD: de novo peptide sequencing from HCD spectra

In recent years, de novo peptide sequencing from mass spectrometry data has developed as one of the major peptide identification methods with the emergence of new instruments and advanced computational methods. However, there are still limitations to this method; for example, the typically used spectrum graph model cannot represent all the information and relationships inherent in tandem mass spectra (MS/MS spectra). Here, we present a new method named NovoHCD which applies a spectrum graph model with multiple types of edges (called a multi-edge graph), and integrates into it amino acid combination (AAC) information and peptide tags. In addition, information on immonium ions observed particularly in higher-energy collisional dissociation (HCD) spectra is incorporated. Comparisons between NovoHCD and another successful de novo peptide sequencing method for HCD spectra, pNovo, were performed. Experiments were conducted on five HCD spectral datasets. Results show that NovoHCD outperforms pNovo in terms of full length peptide identification accuracy; specifically, the accuracy increases 13%-21% over the five datasets.

N-terminal sequence tagging using reliably determined b2 ions: a useful approach to deconvolute tandem mass spectra of co-fragmented peptides in proteomics

With the recent introduction of higher-energy collisional dissociation (HCD) in Orbitrap mass spectrometry, the popularity of that technique has grown tremendously in the proteomics society. HCD spectra, however, are characterized by a limited distribution of bn-type ions, which permit the generation of reliable sequence tags based on complementary b,y pairs both for de novo sequencing and sequence tagging strategies. Instead, most peptide HCD spectra (~95%) are dominated with b2 ions. In this work, we analyzed positive predictive values of b2 ions in HCD, and found that b2 ions can be determined with >97% certainty in the presence of a2 and its complementary yn-2 ions. Analytically, b2 ions provide information on the composition of the first two N-terminal amino acids in peptides. Their utilization in N-terminal sequence tagging leads to a significant decrease in false discovery rate by filtering out false positives while retaining true positive identifications. As a consequence, the number of peptide spectrum matches (PSMs) increased by 4.8% at fixed FDR (1%). This approach allows for deconvolution of mixture spectra and increased the number of PSM to 9.2% in a complex human sample and to 24% in a complex sample of synthetic peptides at 1% FDR.

Mass spectrometry-based identification and characterisation of lysine and arginine methylation in the human proteome

Protein methylation is a post-translational modification (PTM) by which a variable number of methyl groups are transferred to lysine and arginine residues within proteins. Despite increased interest in this modification due to its reversible nature and its emerging role in a diverse set of biological pathways beyond chromatin, global identification of protein methylation has remained an unachieved goal. To characterise sites of lysine and arginine methylation beyond histones, we employed an approach that combines heavy methyl stable isotope labelling by amino acids in cell culture (hmSILAC) with high-resolution mass spectrometry-based proteomics. Through a broad evaluation of immuno-affinity enrichment and the application of two classical protein separation techniques prior to mass spectrometry, to nucleosolic and cytosolic fractions separately, we identified a total of 501 different methylation types, on 397 distinct lysine and arginine sites, present on 139 unique proteins. Our results considerably extend the number of known in vivo methylation sites and indicate their significant presence on several protein complexes involved at all stages of gene expression, from chromatin remodelling and transcription to splicing and translation. In addition, we describe the potential of the hmSILAC approach for accurate relative quantification of methylation levels between distinct functional states.

Expanding proteome coverage with orthogonal-specificity α-lytic proteases

Bottom-up proteomics studies traditionally involve proteome digestion with a single protease, trypsin. However, trypsin alone does not generate peptides that encompass the entire proteome. Alternative proteases have been explored, but most have specificity for charged amino acid side chains. Therefore, additional proteases that improve proteome coverage through cleavage at sequences complementary to trypsin's may increase proteome coverage. We demonstrate the novel application of two proteases for bottom-up proteomics: wild type α-lytic protease (WaLP) and an active site mutant of WaLP, M190A α-lytic protease (MaLP). We assess several relevant factors, including MS/MS fragmentation, peptide length, peptide yield, and protease specificity. When data from separate digestions with trypsin, LysC, WaLP, and MaLP were combined, proteome coverage was increased by 101% relative to that achieved with trypsin digestion alone. To demonstrate how the gained sequence coverage can yield additional post-translational modification information, we show the identification of a number of novel phosphorylation sites in the Schizosaccharomyces pombe proteome and include an illustrative example from the protein MPD2 wherein two novel sites are identified, one in a tryptic peptide too short to identify and the other in a sequence devoid of tryptic sites. The specificity of WaLP and MaLP for aliphatic amino acid side chains was particularly valuable for coverage of membrane protein sequences, which increased 350% when the data from trypsin, LysC, WaLP, and MaLP were combined.

Top-down proteomics by means of Orbitrap mass spectrometry

Top-down proteomics has become a popular approach for the analysis of intact proteins. The term "top down" has been coined for the analysis of proteins not involving any enzymatic or chemical cleavage but rather the ionization of the protein as a sound molecule and mass analysis of intact species and fragment ions thereof produced upon dissociation inside a mass spectrometer. One or several charge states of the protein are mass-isolated and subjected to dissociation (MS/MS) in the gas phase. The obtained fragment masses, predominantly from cleavages of the protein along its amino acid backbone, are directly related to the intact protein. Using bioinformatics tools the fragment masses are matched against a known protein sequence or can alternatively be used for partial or full de novo sequencing, depending on the size of the protein and the number of fragment ions obtained. Moreover, this approach provides global information about modification states of proteins including the number and types of isoforms and their stoichiometry and allows for the precise localization of modifications within the amino acid sequence. Top-down analysis of a single, purified protein can be performed by matrix-assisted laser desorption ionization or electrospray ionization upon direct infusion without online chromatographic separation, whereas top-down analysis of complex protein mixtures makes pre-fractionation combined with an efficient front-end chromatographic separation coupled online to the mass spectrometer inevitable.

Mass spectrometric methods for the analysis of nucleoside-protein cross-links: application to oxopropenyl-deoxyadenosine

Electrophilic DNA adducts produced following oxidative stress can form DNA-protein cross-links (DPCs), dramatically altering genomic maintenance pathways. Complete characterization of DPCs has been hindered, in part, because of a lack of comprehensive techniques for their analysis. We have, therefore, established a proteomics approach to investigate sites of cross-link formation using N(6)-(3-oxo-1-propenyl)-2'-deoxyadenosine (OPdA), an electrophilic DNA adduct produced from oxidative stress. OPdA was reacted with albumin and reduced with NaBH4 to stabilize DPCs. Using LC-MS/MS proteomics techniques, high-resolution peptide sequence data were obtained; however, using a database searching strategy, adducted peptides were only identified in samples subjected to chemical depurination. This strategy revealed multiple oxopropenyl adenine-lysine adducts and oxopropenyl-lysine adducts with the most reactive lysines identified to be Lys256 and Lys548. Manual interrogation of the mass spectral data provided evidence of OPdA deoxynucleoside conjugates to lysines and cross-links that underwent facile collision-induced dissociation to release an unmodified peptide without subsequent fragmentation. These fragmentations precluded adduct detection and peptide sequencing using database searching methods. Thus, comprehensive analysis of DPCs requires chemical depurination of DNA-protein reaction mixtures followed by a combination of database-dependent and manual interrogation of LC-MS/MS data using higher-energy collision-induced dissociation. In the present case, this approach revealed that OPdA selectively modifies surface lysine residues and produces nucleoside-protein cross-links and oxopropenyl lysine.

Post-translational modifications and mass spectrometry detection

In this review, we provide a comprehensive bibliographic overview of the role of mass spectrometry and the recent technical developments in the detection of post-translational modifications (PTMs). We briefly describe the principles of mass spectrometry for detecting PTMs and the protein and peptide enrichment strategies for PTM analysis, including phosphorylation, acetylation and oxidation. This review presents a bibliographic overview of the scientific achievements and the recent technical development in the detection of PTMs is provided. In order to ascertain the state of the art in mass spectrometry and proteomics methodologies for the study of PTMs, we analyzed all the PTM data introduced in the Universal Protein Resource (UniProt) and the literature published in the last three years. The evolution of curated data in UniProt for proteins annotated as being post-translationally modified is also analyzed. Additionally, we have undertaken a careful analysis of the research articles published in the years 2010 to 2012 reporting the detection of PTMs in biological samples by mass spectrometry.

Energy dependence of HCD on peptide fragmentation: stepped collisional energy finds the sweet spot

An understanding of the process of peptide fragmentation and what parameters are best to obtain the most useful information is important. This is especially true for large-scale proteomics where data collection and data analysis are most often automated, and manual interpretation of spectra is rare because of the vast amounts of data generated. We show herein that collisional cell peptide fragmentation, in this case higher collisional dissociation (HCD) in the Q Exactive, is significantly affected by the normalized energy applied. Both peptide sequence and energy applied determine what ion fragments are observed. However, by applying a stepped normalized collisional energy scheme and combining ions from low, medium, and high collision energies, we are able to increase the diversity of fragmentation ions generated. Application of stepped collision energy to HEK293T lysate demonstrated a minimal effect on peptide and protein identification in a large-scale proteomics dataset, but improved phospho site localization through increased sequence coverage. Stepped HCD is also beneficial for tandem mass tagged (TMT) experiments, increasing intensity of TMT reporters used for quantitation without adversely effecting peptide identification.

Fragmentation reactions of methionine-containing protonated octapeptides and fragment ions therefrom: an energy-resolved study

The fragmentation reactions of the MH(+) ions as well as the b7, a7, and a7* ions derived therefrom have been studied in detail for the octapeptides MAAAAAAA, AAMAAAAA, AAAAMAAA, and AAAAAAMA. Ionization was by electrospray using a QqToF mass spectrometer, which allowed a study of the evolution of the fragmentation channels as a function of the collision energy. Not surprisingly, the product ion mass spectra for the b7 ions are independent of the original precursor sequence, indicating macrocyclization and reopening to the same mixture of protonated oxazolones prior to fragmentation. The results show that this sequence scrambling results in a distinct preference to place the Met residue in the C-terminal position of the protonated oxazolones. The a7 and a7* ions also produce product ion mass spectra independent of the original peptide sequence. The results for the a7 ions indicate that fragmentation occurs primarily from an amide structure analogous to that observed for a4 ions (Bythell et al. in J Am Chem Soc 132:14766-14779, 2010). Clearly, the rearrangement reaction they have proposed applies equally well to an ions as large as a7. The major fragmentation modes of the MH(+) ions at low collision energies produce b7, b6, and b5 ions. As the collision energy is increased further fragmentation of these primary products produces, in part, non-direct sequence ions, which become prominent at lower m/z values, particularly for the peptides with the Met residue near the N-terminus.

The proteome of the calcified layer organic matrix of turkey (Meleagris gallopavo) eggshell

Background

Chicken eggshell mineralization is a prominent model for biomineralization not only because of its importance for avian reproduction but also because of the commercial interest associated with eggshell quality. An analysis and comparison of the protein constituents of eggshells of several species would contribute to a better understanding of the shell mineralization process. The recent publication of the turkey genome sequence now provides a basis for the in-depth analysis of the turkey eggshell proteome.

Results

Proteomic analysis of turkey acid-soluble and acid-insoluble organic eggshell matrix yielded 697 identified proteins/protein groups. However, intensity-based absolute quantification (iBAQ) results indicated that the 47 most abundant identified proteins already constituted 95% of the total turkey eggshell matrix proteome. Forty-four of these proteins were also identified in chicken eggshell matrix previously. Despite these similarities there were important and unexpected differences. While ovocleidin-116 and ovocalyxin-36 were major proteins constituting approximately 37% of the identified proteome, other members of the group of so-called eggshell-specific proteins were not identified. Thus ovocalyxin-21 and ovocalyxin-32 were missing among matrix proteins. Conversely, major turkey eggshell proteins were not detected in chicken, such as the bone protein periostin, the mammalian counterpart of which is involved in many aspects of bone metabolism and which represented 10-11% of the total identified proteome.

Conclusions

Even members of the same avian family show important differences in eggshell matrix composition and more studies on the proteome and the transcriptome level will be necessary to identify a common toolkit of eggshell mineralization and to work out species differences among functional eggshell protein sets and their role in eggshell production.

Moving away from the reference genome: evaluating a peptide sequencing tagging approach for single amino acid polymorphism identifications in the genus Populus

The genetic diversity across natural populations of the model organism, Populus, is extensive, containing a single nucleotide polymorphism roughly every 200 base pairs. When deviations from the reference genome occur in coding regions, they can impact protein sequences. Rather than relying on a static reference database to profile protein expression, we employed a peptide sequence tagging (PST) approach capable of decoding the plasticity of the Populus proteome. Using shotgun proteomics data from two genotypes of P. trichocarpa, a tag-based approach enabled the detection of 6653 unexpected sequence variants. Through manual validation, our study investigated how the most abundant chemical modification (methionine oxidation) could masquerade as a sequence variant (Ala→Ser) when few site-determining ions existed. In fact, precise localization of an oxidation site for peptides with more than one potential placement was indeterminate for 70% of the MS/MS spectra. We demonstrate that additional fragment ions made available by high energy collisional dissociation enhances the robustness of the peptide sequence tagging approach (81% of oxidation events could be exclusively localized to a methionine). We are confident that augmenting fragmentation processes for a PST approach will further improve the identification of single amino acid polymorphism in Populus and potentially other species as well.

Metaproteomics: harnessing the power of high performance mass spectrometry to identify the suite of proteins that control metabolic activities in microbial communities

The availability of extensive genome information for many different microbes, including unculturable species in mixed communities from environmental samples, has enabled systems-biology interrogation by providing a means to access genomic, transcriptomic, and proteomic information. To this end, metaproteomics exploits the power of high-performance mass spectrometry for extensive characterization of the complete suite of proteins expressed by a microbial community in an environmental sample.

HI-bone: a scoring system for identifying phenylisothiocyanate-derivatized peptides based on precursor mass and high intensity fragment ions

Peptide sequence matching algorithms used for peptide identification by tandem mass spectrometry (MS/MS) enumerate theoretical peptides from the database, predict their fragment ions, and match them to the experimental MS/MS spectra. Here, we present an approach for scoring MS/MS identifications based on the high mass accuracy matching of precursor ions, the identification of a high intensity b1 fragment ion, and partial sequence tags from phenylthiocarbamoyl-derivatized peptides. This derivatization process boosts the b1 fragment ion signal, which turns it into a powerful feature for peptide identification. We demonstrate the effectiveness of our scoring system by implementing it on a computational tool called "HI-bone" and by identifying mass spectra of an Escherichia coli sample acquired on an Orbitrap Velos instrument using Higher-energy C-trap dissociation. Following this strategy, we identified 1614 peptide spectrum matches with a peptide false discovery rate (FDR) below 1%. These results were significantly higher than those from Mascot and SEQUEST using a similar FDR.