Comparative proteomics of glycoproteins based on lectin selection and isotope coding

Lectins have been widely used in glycan structure analysis. The studies described here exploit this fact to select glycopeptides carrying disease-associated modifications in their oligosaccharides. Coupling lectin affinity selection with recent advances in stable isotope coding for quantitative proteomics allowed a comparative proteomics method to be developed for examining aberrant glycosylation in cancer. Control and experimental samples were individually tryptic digested and differentially coded with stable isotope coding agents before they were mixed and affinity selected with a lectin affinity chromatography column. Glycopeptides carrying an alpha-L-fucose residue were selected with Lotus tetragonolobus agglutinin (LTA) immobilized on a chromatography matrix. Because the oligosaccharides of glycoproteins are generally heterogeneous and often of unknown structure, it was necessary to deglycosylate the selected peptides with PNGase F before they could be compared to sequences in DNA and protein databases. After deglycosylated peptides were transferred to a reversed phase chromatography (RPC) column and fractionated by gradient elution with increasing amounts of acetonitrile. The RPC fractions were then analyzed by both matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) and electrospray ionization mass spectrometry (ESI-MS). When this method was applied to a study of lymphosarcoma in canines, it was found that during chemotherapy, a series of fucosylated proteins in the blood of patients decreased in concentration more than 2-fold. Two of the proteins identified, CD44 and E-selectin, are known to be involved in cell adhesion and cancer cell migration. The observed aberrant fucosylation of these proteins is consistent with the hypothesis that CD44 and E-selectin play a key role in metastasis and the spread of cancer cells to remote sites.     

A method for proteomic identification of membrane-bound proteins containing Asn-linked oligosaccharides

Glycosylated proteins on the cell surface have been shown to be essential for cell-cell interactions in development and differentiation. Our ultimate goal is to identify Asn-linked oligosaccharides that are directly involved in these critical in vivo functions. Because such oligosaccharides would be expected to reside on the integral plasma membrane proteins, and conventional two-dimensional gel techniques are ineffective at separating such proteins, we have developed a new approach to their identification on a proteomics scale from Caenorhabditis elegans. Membrane proteins are solubilized in guanidine-HCl, precipitated, and digested with trypsin. The glycopeptides are then separated by lectin chromatography. Next, glycopeptidase F digestion removes the oligosaccharides from the peptides and converts to Asp each Asn to which one was attached. The peptides are then analyzed by matrix-assisted laser desorption/ionization quadrupole time-of-flight (MALDI-Q-TOF) mass spectrometry. Thus, the membrane glycoproteins are identified through the sequence tags of these peptides and the conversion of at least one deduced Asn residue to Asp at the Asn-X-Ser/Thr consensus sequence. To validate the utility of this approach, we have identified 13 membrane-bound N-glycosylated proteins from the major peaks observed on MALDI-Q-TOF analysis of our total glycopeptide fraction.     

A new strategy for identification of N-glycosylated proteins and unambiguous assignment of their glycosylation sites using HILIC enrichment and partial deglycosylation

Characterization of glycoproteins using mass spectrometry ranges from determination of carbohydrate-protein linkages to the full characterization of all glycan structures attached to each glycosylation site. In a novel approach to identify N-glycosylation sites in complex biological samples, we performed an enrichment of glycosylated peptides through hydrophilic interaction liquid chromatography (HILIC) followed by partial deglycosylation using a combination of endo-beta-N-acetylglucosaminidases (EC 3.2.1.96). After hydrolysis with these enzymes, a single N-acetylglucosamine (GlcNAc) residue remains linked to the asparagine residue. The removal of the major part of the glycan simplifies the MS/MS fragment ion spectra of glycopeptides, while the remaining GlcNAc residue enables unambiguous assignment of the glycosylation site together with the amino acid sequence. We first tested our approach on a mixture of known glycoproteins, and subsequently the method was applied to samples of human plasma obtained by lectin chromatography followed by 1D gel-electrophoresis for determination of 62 glycosylation sites in 37 glycoproteins.     

Exploring proteomes and analyzing protein processing by mass spectrometric identification of sorted N-terminal peptides

Current non-gel techniques for analyzing proteomes rely heavily on mass spectrometric analysis of enzymatically digested protein mixtures. Prior to analysis, a highly complex peptide mixture is either separated on a multidimensional chromatographic system or it is first reduced in complexity by isolating sets of representative peptides. Recently, we developed a peptide isolation procedure based on diagonal electrophoresis and diagonal chromatography. We call it combined fractional diagonal chromatography (COFRADIC). In previous experiments, we used COFRADIC to identify more than 800 Escherichia coli proteins by tandem mass spectrometric (MS/MS) analysis of isolated methionine-containing peptides. Here, we describe a diagonal method to isolate N-terminal peptides. This reduces the complexity of the peptide sample, because each protein has one N terminus and is thus represented by only one peptide. In this new procedure, free amino groups in proteins are first blocked by acetylation and then digested with trypsin. After reverse-phase (RP) chromatographic fractionation of the generated peptide mixture, internal peptides are blocked using 2,4,6-trinitrobenzenesulfonic acid (TNBS); they display a strong hydrophobic shift and therefore segregate from the unaltered N-terminal peptides during a second identical separation step. N-terminal peptides can thereby be specifically collected for further liquid chromatography (LC)-MS/MS analysis. Omitting the acetylation step results in the isolation of non-lysine-containing N-terminal peptides from in vivo blocked proteins.     

Reversible labeling of cysteine-containing peptides allows their specific chromatographic isolation for non-gel proteome studies

We report upon a novel procedure to specifically isolate cysteine-containing peptides from a complex peptide mixture. Cysteines are converted to hydrophobic residues by mixed disulfide formation with Ellman's reagent. Proteins are subsequently digested with trypsin and the generated peptide mixture is a first time fractionated by reverse-phase high-performance liquid chromatography. Cysteinyl-peptides are isolated out of each primary fraction by a reduction step followed by a secondary peptide separation on the same column, performed under identical conditions as for the primary separation. The reducing agent removes the covalently attached group from the cysteine side chain, making cysteine-peptides more hydrophilic and, thereby, such peptides can be specifically collected during the secondary separation and are finally used to identify their precursor proteins using automated liquid chromatography tandem mass spectrometry. We show that this procedure efficiently isolates cysteine-peptides, making the sample mixture less complex for further analysis. This method was applied for the analysis of the proteomes of human platelets and enriched human plasma. In both proteomes, a significant number of low abundance proteins were identified next to extremely abundant ones. A dynamic range for protein identification spanning 4-5 orders of magnitude is demonstrated.     

Lectin affinity capture,isotope-coded tagging and mass spectrometry to identify N-linked glycoproteins

We describe here a strategy for the large-scale identification of N-glycosylated proteins from a complex biological sample. The approach, termed isotope-coded glycosylation-site-specific tagging (IGOT), is based on the lectin column-mediated affinity capture of a set of glycopeptides generated by tryptic digestion of protein mixtures, followed by peptide-N-glycosidase-mediated incorporation of a stable isotope tag, 18O, specifically into the N-glycosylation site. The 18O-tagged peptides are then identified by multi-dimensional liquid chromatography-mass spectrometry (LC-MS)-based technology. The application of this method to the characterization of N-linked high-mannose and/or hybrid-type glycoproteins from an extract of Caenorhabditis elegans proteins allowed the identification of 250 glycoproteins, including 83 putative transmembrane proteins, with the simultaneous determination of 400 unique N-glycosylation sites. Because the method is applicable to the systematic identification of a wide range of glycoproteins, it should facilitate basic glycobiology research and may be useful for diagnostic applications, such as genome-wide screening for disease-related glycoproteins.     

Methods for analyzing peptides and proteins on a chromatographic timescale by electron-transfer dissociation mass spectrometry

Advancement in proteomics research relies on the development of new, innovative tools for identifying and characterizing proteins. Here, we describe a protocol for analyzing peptides and proteins on a chromatographic timescale by coupling nanoflow reverse-phase (RP) liquid chromatography (LC) to electron-transfer dissociation (ETD) mass spectrometry. For this protocol, proteins can be proteolytically digested before ETD analysis, although digestion is not necessary for all applications. Proteins 相似文献   

Mass spectrometric identification of N-linked glycopeptides using lectin-mediated affinity capture and glycosylation site-specific stable isotope tagging

Protein post-translational modifications (PTMs), such as glycosylation and phosphorylation, are crucial for various signaling and regulatory events, and are therefore an important objective of proteomics research. We describe here a protocol for isotope-coded glycosylation site-specific tagging (IGOT), a method for the large-scale identification of N-linked glycoproteins from complex biological samples. The steps of this approach are: (1) lectin column-mediated affinity capture of glycopeptides generated by protease digestion of protein mixtures; (2) purification of the enriched glycopeptides by hydrophilic interaction chromatography (HIC); (3) peptide-N-glycanase-mediated incorporation of a stable isotope tag, 18O18O, specifically at the N-glycosylation site; and (4) identification of 18O-tagged peptides by liquid chromatography-coupled mass spectrometry (LC/MS)-based proteomics technology. The application of this protocol to the characterization of N-linked glycoproteins from crude extracts of the nematode Caenorhabditis elegans or mouse liver provides a list of hundreds to a thousand glycoproteins and their sites of glycosylation within a week.     

A method for determination of N-glycosylation sites in glycoproteins by collision-induced dissociation analysis in fast atom bombardment mass spectrometry: identification of the positions of carbohydrate-linked asparagine in recombinant alpha-amylase by treatment with peptide-N-glycosidase F in 18O-labeled water.

Previously, a combined use of fast atom bombardment (FAB) mass spectrometry and peptide N-glycosidase F, an enzyme that cleaves the beta-aspartylglycosylamine linkage of Asn-linked carbohydrates, was successfully applied to identification of N-glycosylation sites in a glycoprotein with the known or DNA-derived sequence (S. A. Carr and G. D. Roberts, 1986, Anal. Biochem. 157, 396-406). Here, we extended the method for easier identification of N-glycosylation sites in a glycoprotein even with unknown sequence. The glycoprotein is digested with peptide-N-glycosidase F in buffer containing 40 at% H2 18O, to yield a deglycosylated protein whose carbohydrate-linked Asn residues are converted to Asp partly labeled with 18O at their beta-carboxyl group during this digestion. The deglycosylated protein is further digested with proteolytic enzymes in an appropriate buffer prepared with normal water, and then peptides are separated on a reversed-phase column by HPLC. Peptides in which carbohydrate-linked Asn has been converted to Asp show a pair of signals ([M + 1]+ and [M + 3]+) in FAB mass spectra due to the partial incorporation of 18O into the beta-carboxyl groups of Asp residues, while the other peptides show normal isotopic ion distributions. Thus, both formally N-glycosylated peptides and, using collision-induced dissociation analysis, N-glycosylation sites can be identified. The application of the present method to the determination of N-glycosylation sites in a recombinant glycoprotein, Bacillus licheniformis alpha-amylase, is described.     

Integrating 'top-down" and "bottom-up" mass spectrometric approaches for proteomic analysis of Shewanella oneidensis

Here we present a comprehensive method for proteome analysis that integrates both intact protein measurement ("top-down") and proteolytic fragment characterization ("bottom-up") mass spectrometric approaches, capitalizing on the unique capabilities of each method. This integrated approach was applied in a preliminary proteomic analysis of Shewanella oneidensis, a metal-reducing microbe of potential importance to the field of bioremediation. Cellular lysates were examined directly by the "bottom-up" approach as well as fractionated via anion-exchange liquid chromatography for integrated studies. A portion of each fraction was proteolytically digested, with the resulting peptides characterized by on-line liquid chromatography/tandem mass spectrometry. The remaining portion of each fraction containing the intact proteins was examined by high-resolution Fourier transform mass spectrometry. This "top-down" technique provided direct measurement of the molecular masses for the intact proteins and thereby enabled confirmation of post-translational modifications, signal peptides, and gene start sites of proteins detected in the "bottom-up" experiments. A total of 868 proteins from virtually every functional class, including hypotheticals, were identified from this organism.     

Characterization of a digested protein complex with quantitative aspects: an approach based on accurate mass chromatographic analysis with Fourier transform-ion cyclotron resonance mass spectrometry

We describe a new approach for the characterization of a digested protein complex with quantitative aspects. Accurate masses of tryptic peptides in the digested complex were acquired by nano-liquid chromatography Fourier transform-ion cyclotron resonance mass spectrometry (MS). The conditions of the electrospray ion source were alternated to acquire normal and fragment-ion-rich mass spectra concurrently. This, alternating-scan method, which includes no tandem mass spectrometry (MS/MS), allowed us to retain the integrity of the mass chromatograms and averted missed peptides due to MS and MS/MS switching. Tentative assignments of accurate peptide masses were verified with the concurrently acquired fragment-ion-rich spectra, and the identities of the protein components were established. For each identified protein component, mass chromatograms attributable to the validated accurate peptide masses were extracted, and the peak areas of multiple mass chromatograms were standardized. The standardized peak areas appeared to reasonably reflect the molar ratio of the protein components in standard mixtures. This new approach was successfully applied to the characterization of a cyanobacterial photosystem II complex preparation. A clear difference in the standardized peak areas was observed between the two groups of identified components, namely eight stoichiometric photosystem II proteins and two minor copurified phycobiliproteins.     

Chemical derivatization of histones for facilitated analysis by mass spectrometry

Histone post-translational modifications have been recently intensely studied owing to their role in regulating gene expression. Here, we describe protocols for the characterization of histone modifications in both qualitative and semiquantitative manners using chemical derivatization and tandem mass spectrometry. In these procedures, extracted histones are first derivatized using propionic anhydride to neutralize charge and block lysine residues, and are subsequently digested using trypsin, which, under these conditions, cleaves only the arginine residues. The generated peptides can be easily analyzed using online LC-electrospray ionization-tandem mass spectrometry to identify the modification site. In addition, a stable isotope-labeling step can be included to modify carboxylic acid groups allowing for relative quantification of histone modifications. This methodology has the advantage of producing a small number of predicted peptides from highly modified proteins. The protocol should take approximately 15-19 h to complete, including all chemical reactions, enzymatic digestion and mass spectrometry experiments.     

A proteomic analysis of human bile

We have carried out a comprehensive characterization of human bile to define the bile proteome. Our approach involved fractionation of bile by one-dimensional gel electrophoresis and lectin affinity chromatography followed by liquid chromatography tandem mass spectrometry. Overall, we identified 87 unique proteins, including several novel proteins as well as known proteins whose functions are unknown. A large majority of the identified proteins have not been previously described in bile. Using lectin affinity chromatography and enzymatically labeling of asparagine residues carrying glycan moieties by (18)O, we have identified a total of 33 glycosylation sites. The strategy described in this study should be generally applicable for a detailed proteomic analysis of most body fluids. In combination with "tagging" approaches for differential proteomics, our method could be used for identification of cancer biomarkers from any body fluid.     

Capture of peptides with N-terminal serine and threonine: a sequence-specific chemical method for Peptide mixture simplification

The objective of this study was to evaluate a sequence-specific chemistry for the ability to specifically capture peptides that contain N-terminal serine or threonine residues from mixtures. The first step is the oxidation of the 1,2-amino alcohol structure -CH(NH(2))CH(OH)- of peptides containing N-terminal serine or threonine with periodate. The newly formed aldehyde reacts with a labeling reagent containing a hydrazide, RCONHNH(2), to form a hydrazone-peptide conjugate, RCONHN=CH-peptide. Biotin-labeled conjugates can then be isolated by affinity purification with streptavidin. The method described in this report can be useful in simplifying the complex mixtures of peptides that are generated in typical proteomic analysis, where proteins are digested with trypsin and analyzed using liquid chromatography mass spectrometry data. The sequence-specific peptide selection not only reduces the complexity of digest mixtures, but also provides additional information for peptide identification. The targeted peptides are those that have either serine or threonine adjacent to a protease cleavage site. The sequence information should greatly aid in both database matching for protein identification and for de novo sequence determination.     

Global phosphoproteome of HT-29 human colon adenocarcinoma cells

Phosphorylation events in cellular signaling cascades triggered by a variety of cellular stimuli modulate protein function, leading to diverse cellular outcomes including cell division, growth, death, and differentiation. Abnormal regulation of protein phosphorylation due to mutation or overexpression of signaling proteins often results in various disease states. We provide here a list of protein phosphorylation sites identified from HT-29 human colon adenocarcinoma cell line by immobilized metal affinity chromatography (IMAC) combined with liquid chromatography (LC)-tandem mass spectrometry (MS/MS) analysis. In this study, proteins extracted from HT-29 whole cell lysates were digested with trypsin and carboxylate groups on the resulting peptides were converted to methyl esters. Derivatized phosphorylated peptides were enriched using Fe(3+)-chelated metal affinity resin. Phosphopeptides retained by IMAC were separated by high performance liquid chromatography (HPLC) and analyzed by electrospray ionization-quadrupole-time-of-flight (ESI-Q-TOF) mass spectrometry. We identified 238 phosphorylation sites, 213 of which could be conclusively localized to a single residue, from 116 proteins by searching MS/MS spectra against the human protein database using MASCOT. Peptide identification and phosphorylation site assignment were confirmed by manual inspection of the MS/MS spectra. Many of the phosphorylation sites identified in our results have not been described previously in the scientific literature. We attempted to ascribe functionality to the sites identified in this work by searching for potential kinase motifs with Scansite (http://scansite.mit.edu) and obtaining information on kinase substrate selectivity from Pattern Explorer (http://scansite.mit.edu/pe). The list of protein phosphorylation sites identified in the present experiment provides broad information on phosphorylated proteins under normal (asynchronous) cell culture conditions. Sites identified in this study may be utilized as surrogate bio-markers to assess the activity of selected kinases and signaling pathways from different cell states and exogenous stimuli.     

Simultaneous analysis of proteome, phospho- and glycoproteome of rat kidney tissue with electrostatic repulsion hydrophilic interaction chromatography

Protein post-translational modifications (PTMs) are regulated separately from protein expression levels. Thus, simultaneous characterization of the proteome and its PTMs is pivotal to an understanding of protein regulation, function and activity. However, concurrent analysis of the proteome and its PTMs by mass spectrometry is a challenging task because the peptides bearing PTMs are present in sub-stoichiometric amounts and their ionization is often suppressed by unmodified peptides of high abundance. We describe here a method for concurrent analysis of phosphopeptides, glycopeptides and unmodified peptides in a tryptic digest of rat kidney tissue with a sequence of ERLIC and RP-LC-MS/MS in a single experimental run, thereby avoiding inter-experimental variation. Optimization of loading solvents and elution gradients permitted ERLIC to be performed with totally volatile solvents. Two SCX and four ERLIC gradients were compared in details, and one ERLIC gradient was found to perform the best, which identified 2929 proteins, 583 phosphorylation sites in 338 phosphoproteins and 722 N-glycosylation sites in 387 glycoproteins from rat kidney tissue. Two hundred low-abundance proteins with important functions were identified only from the glyco- or phospho-subproteomes, reflecting the importance of the enrichment and separation of modified peptides by ERLIC. In addition, this strategy enables identification of unmodified and corresponding modified peptides (partial phosphorylation and N-glycosylation) from the same protein. Interestingly, partially modified proteins tend to occur on proteins involved in transport. Moreover, some membrane or extracellular proteins, such as versican core protein and fibronectin, were found to have both phosphorylation and N-glycosylation, which may permit an assessment of the potential for cross talk between these two vital PTMs and their roles in regulation.     

A method for the identification of glycoproteins from human serum by a combination of lectin affinity chromatography along with anion exchange and Cu-IMAC selection of tryptic peptides

This paper reports a method for identifying glycoproteins from human serum. Glycoproteins were selected with a concanavalin A (Con A) lectin column and then tryptically digested prior to sequential chromatographic selection of acidic and histidine containing peptides. Acidic peptides were selected with a strong anion exchange (SAX) column. Peptides captured by the SAX columns were then released and histidine-containing peptides in the mixture selected with a copper loaded immobilized metal affinity chromatography (Cu-IMAC) column. This serial chromatographic selection process reduced the complexity of proteolytic digests by more than an order of magnitude. Peptides selected by this serial process were then fractionated by reversed-phase chromatography (RPC) and identified by tandem mass spectrometry. The method was initially validated using human transferrin before application to human serum. The results show that all the peptides identified except one contained histidine and acidic amino acids.     

Cell surface glycoproteins from Thermoplasma acidophilum are modified with an N-linked glycan containing 6-C-sulfofucose

Thermoplasma acidophilum is a thermoacidophilic archaeon that grows optimally at pH 2 and 59°C. This extremophile is remarkable by the absence of a cell wall or an S-layer. Treating the cells with Triton X-100 at pH 3 allowed the extraction of all of the cell surface glycoproteins while keeping cells intact. The extracted glycoproteins were partially purified by cation-exchange chromatography, and we identified five glycoproteins by N-terminal sequencing and mass spectrometry of in-gel tryptic digests. These glycoproteins are positive for periodic acid-Schiff staining, have a high content of Asn including a large number in the Asn-X-Ser/Thr sequon and have apparent masses that are 34-48% larger than the masses deduced from their amino acid sequences. The pooled glycoproteins were digested with proteinase K and the purified glycopeptides were analyzed by NMR. Structural determination showed that the carbohydrate part was represented by two structures in nearly equal amounts, differing by the presence of one terminal mannose residue. The larger glycan chain consists of eight residues: six hexoses, one heptose and one sugar with an unusual residue mass of 226 Da which was identified as 6-deoxy-6-C-sulfo-D-galactose (6-C-sulfo-D-fucose). Mass spectrometry analyses of the peptides obtained by trypsin and chymotrypsin digestion confirmed the principal structures to be those determined by NMR and identified 14 glycopeptides derived from the main glycoprotein, Ta0280, all containing the Asn-X-Ser/Thr sequons. Thermoplasma acidophilum appears to have a "general" protein N-glycosylation system that targets a number of cell surface proteins.     

Strong cation exchange (SCX) based analytical methods for the targeted analysis of protein post-translational modifications

The multidimensional combination of strong cation exchange (SCX) chromatography and reversed phase chromatography has emerged as a powerful approach to separate peptides originating from complex samples such as digested cellular lysates or tissues before analysis by mass spectrometry, enabling the identification of over 10,000s of peptides and thousands of proteins in a single sample. Although, such multidimensional chromatography approaches are powerful, the in-depth analysis of protein post-translational modifications still requires additional sample preparation steps, involving the specific enrichment of peptides displaying the targeted modification. Here, we describe how in particular SCX chromatography can be used for the targeted analysis of important post-translational modifications, such as phosphorylation and N-terminal acetylation. Compared to other methods, SCX is less labor-intensive and more robust, and therefore likely more easily adaptable to main-stream research laboratories.     

Structural proteomics of macromolecular assemblies using oxidative footprinting and mass spectrometry

Understanding the composition, structure and dynamics of macromolecules and their assemblies is at the forefront of biological science today. Hydroxyl-radical-mediated protein footprinting using mass spectrometry can define macromolecular structure, macromolecular assembly and conformational changes of macromolecules in solution based on measurements of reactivity of amino acid side-chain groups with covalent-modification reagents. Subsequent to oxidation by reactive oxygen species, proteins are digested by specific proteases to generate peptides for analysis by mass spectrometry. Accurate measurements of side-chain reactivity are achieved using quantitative liquid-chromatography-coupled mass spectrometry, whereas the side-chain sites within the macromolecular probes are identified using tandem mass spectrometry. In addition, the use of footprinting data in conjunction with computational modeling approaches is a powerful new method for testing and refining structural models of macromolecules and their complexes.