Artifactual sulfation of silver-stained proteins: implications for the assignment of phosphorylation and sulfation sites

Tyrosine O-sulfation proteoforms affect HIV-1 monoclonal antibody potency

CAP256V2LS, a broadly neutralizing monoclonal antibody (bNAb), is being pursued as a promising drug for HIV-1 prevention. The total level of tyrosine-O-sulfation, a post-translational modification, was known to play a key role for antibody biological activity. More importantly, here wedescribe for the first time the significance of the tyrosine-O-sulfation proteoforms. We developed a hydrophobic interaction chromatography (HIC) method to separate and quantify different sulfation proteoforms, which led to the direct functionality assessment of tyrosine-sulfated species. The fully sulfated (4-SO₃) proteoform demonstrated the highest in vitro relative antigen binding potency and neutralization efficiency against a panel of HIV-1 viruses. Interestingly, highly variable levels of 4-SO₃ were produced by different clonal CHO cell lines, which helped the bNAb process development towards production of a highly potent CAP256V2LS clinical product with high 4-SO₃ proteoform. This study presents powerful insight for any biotherapeutic protein development where sulfation may play an important role in product efficacy.

Evaluation of Chemically-Cleavable Linkers for Quantitative Mapping of Small Molecule-Cysteinome Reactivity

Numerous reagents have been developed to enable chemical proteomic analysis of small molecule-protein interactomes. However, the performance of these reagents has not been systematically evaluated and compared. Herein, we report our efforts to conduct a parallel assessment of two widely used chemically cleavable linkers equipped with dialkoxydiphenylsilane (DADPS linker) and azobenzene (AZO linker) moieties. Profiling a cellular cysteinome using the iodoacetamide alkyne probe demonstrated a significant discrepancy between the experimental results obtained through the application of each of the reagents. To better understand the source of observed discrepancy, we evaluated the key sample preparation steps. We also performed a mass tolerant database search strategy using MSFragger software. This resulted in identifying a previously unreported artifactual modification on the residual mass of the azobenzene linker. Furthermore, we conducted a comparative analysis of enrichment modes using both cleavable linkers. This effort determined that enrichment of proteolytic digests yielded a far greater number of identified cysteine residues than the enrichment conducted prior to protein digest. Inspired by recent studies where multiplexed quantitative labeling strategies were applied to cleavable biotin linkers, we combined this further optimized protocol using the DADPS cleavable linker with tandem mass tag (TMT) labeling to profile the FDA-approved covalent EGFR kinase inhibitor dacomitinib against the cysteinome of an epidermoid cancer cell line. Our analysis resulted in the detection and quantification of over 10,000 unique cysteine residues, a nearly 3-fold increase over previous studies that used cleavable biotin linkers for enrichment. Critically, cysteine residues corresponding to proteins directly as well as indirectly modulated by dacomitinib treatment were identified. Overall, our study suggests that the dialkoxydiphenylsilane linker could be broadly applied wherever chemically cleavable linkers are required for chemical proteomic characterization of cellular proteomes.

Recent advances in phosphoproteomics and application to neurological diseases

Phosphorylation has an incredible impact on the biological behavior of proteins, altering everything from intrinsic activity to cellular localization and complex formation. It is no surprise then that this post-translational modification has been the subject of intense study and that, with the advent of faster, more accurate instrumentation, the number of large-scale mass spectrometry-based phosphoproteomic studies has swelled over the past decade. Recent developments in sample preparation, phosphorylation enrichment, quantification, and data analysis strategies permit both targeted and ultra-deep phosphoproteome profiling, but challenges remain in pinpointing biologically relevant phosphorylation events. We describe here technological advances that have facilitated phosphoproteomic analysis of cells, tissues, and biofluids and note applications to neuropathologies in which the phosphorylation machinery may be dysregulated, much as it is in cancer.

Mass Spectrometry of Intact Proteins Reveals +98 u Chemical Artifacts Following Precipitation in Acetone

Protein precipitation in acetone is frequently employed ahead of mass spectrometry for sample preconcentration and purification. Unfortunately, acetone is not chemically inert; mass artifacts have previously been observed on glycine-containing peptides when exposed to acetone under acidic conditions. We herein report a distinct chemical modification occurring at the level of intact proteins when incubated in acetone. This artifact manifests as one or more satellite peaks in the MS spectrum of intact protein, spaced 98 u above the mass of the unmodified protein. Other artifacts (+84, +112 u) also appear upon incubation of proteins or peptides in acetone. The reaction is pH-sensitive, being suppressed when proteins are exposed to acetone under acidic conditions. The +98 u artifact is speculated to originate through an intermediate product of aldol condensation of acetone to form diacetone alcohol and mesityl oxide. A +98 u product could originate from nucleophilic attack on mesityl oxide or through condensation with diacetone alcohol. Given the extent of modification possible upon exposure of proteins to acetone, particularly following overnight solvent exposure or incubation at room temperature, an awareness of the variables influencing this novel modification is valued by proteomics researchers who employ acetone precipitation for protein purification.

Common errors in mass spectrometry-based analysis of post-translational modifications

Mass spectrometry (MS) is a powerful tool to analyze complex mixtures of proteins in a high-throughput fashion. Proteome analysis has already become a routine task in biomedical research with the emergence of proteomics core facilities in most research institutions. Post-translational modifications (PTMs) represent a mechanism by which complex biological processes are orchestrated dynamically at the systems level. MS is rapidly becoming popular to discover new modifications and novel sites of known PTMs, revolutionizing the current understanding of diverse signaling pathways and biological processes. However, MS-based analysis of PTMs has its own caveats and pitfalls that can lead to erroneous conclusions. Here, we review the most common errors in MS-based PTM analyses with the goal of adopting strategies that maximize correct interpretation in the context of biological questions that are being addressed. Finally, we provide suggestions that should help mass spectrometrists, bioinformaticians and biologists to perform and interpret MS-based PTM analyses more accurately.

Why phosphoproteomics is still a challenge

Despite continuous improvements phosphoproteomics still faces challenges that are often neglected, e.g. partially poor recovery of phosphopeptide enrichment, assessment of phosphorylation stoichiometry, label-free quantification, poor behavior during chromatography, and general limitations of peptide-centric proteomics. Here we critically discuss current limitations that need consideration in both qualitative and quantitative studies.

The application of targeted mass spectrometry-based strategies to the detection and localization of post-translational modifications

This review describes some of the more interesting and imaginative ways in which mass spectrometry has been utilized to study a number of important post-translational modifications over the past two decades; from circa 1990 to 2013. A diverse range of modifications is covered, including citrullination, sulfation, hydroxylation and sumoylation. A summary of the biological role of each modification described, along with some brief mechanistic detail, is also included. Emphasis has been placed on strategies specifically aimed at detecting target modifications, as opposed to more serendipitous modification discovery approaches, which rely upon straightforward product ion scanning methods. The authors have intentionally excluded from this review both phosphorylation and glycosylation since these major modifications have been extensively reviewed elsewhere.

Surfactant-Induced Artifacts during Proteomic Sample Preparation

Bottom-up proteomics is a powerful tool for characterization of protein post-translational modifications (PTMs), where PTMs are identified at the peptide level by mass spectrometry (MS) following protein digestion. However, enzymatic digestion is associated with additional sample processing steps that may potentially introduce artifactual modifications. Here, during an MS study of the PTMs of the regulator of G-protein signaling 4, we discovered that the use of ProteaseMAX, which is an acid-labile surfactant commonly used to improve protein solubilization and digestion efficiency, can lead to in vitro modifications on cysteine residues. These hydrophobic modifications resemble S-palmitoylation and hydroxyfarnesylation, thus discouraging the use of ProteaseMAX in studies of lipid modifications of proteins. Furthermore, since they target the cysteine thiol group, the presence of these artifacts will inevitably lead to inaccuracies in quantitative analysis of cysteine modifications.

Tyrosine sulfation as a protein post-translational modification

Integration of inorganic sulfate into biological molecules plays an important role in biological systems and is directly involved in the instigation of diseases. Protein tyrosine sulfation (PTS) is a common post-translational modification that was first reported in the literature fifty years ago. However, the significance of PTS under physiological conditions and its link to diseases have just begun to be appreciated in recent years. PTS is catalyzed by tyrosylprotein sulfotransferase (TPST) through transfer of an activated sulfate from 3'-phosphoadenosine-5'-phosphosulfate to tyrosine in a variety of proteins and peptides. Currently, only a small fraction of sulfated proteins is known and the understanding of the biological sulfation mechanisms is still in progress. In this review, we give an introductory and selective brief review of PTS and then summarize the basic biochemical information including the activity and the preparation of TPST, methods for the determination of PTS, and kinetics and reaction mechanism of TPST. This information is fundamental for the further exploration of the function of PTS that induces protein-protein interactions and the subsequent biochemical and physiological reactions.

Systematic analysis of the in situ crosstalk of tyrosine modifications reveals no additional natural selection on multiply modified residues

Recent studies have indicated that different post-translational modifications (PTMs) synergistically orchestrate specific biological processes by crosstalks. However, the preference of the crosstalk among different PTMs and the evolutionary constraint on the PTM crosstalk need further dissections. In this study, the in situ crosstalk at the same positions among three tyrosine PTMs including sulfation, nitration and phosphorylation were systematically analyzed. The experimentally identified sulfation, nitration and phosphorylation sites were collected and integrated with reliable predictions to perform large-scale analyses of in situ crosstalks. From the results, we observed that the in situ crosstalk between sulfation and nitration is significantly under-represented, whereas both sulfation and nitration prefer to co-occupy with phosphorylation at same tyrosines. Further analyses suggested that sulfation and nitration preferentially co-occur with phosphorylation at specific positions in proteins, and participate in distinct biological processes and functions. More interestingly, the long-term evolutionary analysis indicated that multi-PTM targeting tyrosines didn't show any higher conservation than singly modified ones. Also, the analysis of human genetic variations demonstrated that there is no additional functional constraint on inherited disease, cancer or rare mutations of multiply modified tyrosines. Taken together, our systematic analyses provided a better understanding of the in situ crosstalk among PTMs.

Regulation of the histone deacetylase Hst3 by cyclin-dependent kinases and the ubiquitin ligase SCFCdc4

In Saccharomyces cerevisiae, histone H3 lysine 56 acetylation (H3K56ac) is a modification of new H3 molecules deposited throughout the genome during S-phase. H3K56ac is removed by the sirtuins Hst3 and Hst4 at later stages of the cell cycle. Previous studies indicated that regulated degradation of Hst3 plays an important role in the genome-wide waves of H3K56 acetylation and deacetylation that occur during each cell cycle. However, little is known regarding the mechanism of cell cycle-regulated Hst3 degradation. Here, we demonstrate that Hst3 instability in vivo is dependent upon the ubiquitin ligase SCF(Cdc4) and that Hst3 is phosphorylated at two Cdk1 sites, threonine 380 and threonine 384. This creates a diphosphorylated degron that is necessary for Hst3 polyubiquitylation by SCF(Cdc4). Mutation of the Hst3 diphospho-degron does not completely stabilize Hst3 in vivo, but it nonetheless results in a significant fitness defect that is particularly severe in mutant cells treated with the alkylating agent methyl methanesulfonate. Unexpectedly, we show that Hst3 can be degraded between G2 and anaphase, a window of the cell cycle where Hst3 normally mediates genome-wide deacetylation of H3K56. Our results suggest an intricate coordination between Hst3 synthesis, genome-wide H3K56 deacetylation by Hst3, and cell cycle-regulated degradation of Hst3 by cyclin-dependent kinases and SCF(Cdc4).

Influence of various endogenous and artefact modifications on large-scale proteomics analysis

RATIONALE

Some large-scale proteomics studies in which strong cation exchange chromatography has been applied are used to determine proteomes and post-translational modification dynamics. Although such datasets favour the characterisation of thousands of modified peptides, e.g., phosphorylated and N-α-acetylated, a large fraction of the acquired spectra remain unexplained by standard proteomics approaches. Thus, advanced data processing allows characterisation of a significant part of these unassigned spectra.

METHODS

Our recent investigation of the N-α-acetylation status of plant proteins gave a dataset of choice to investigate further the in-depth characterisation of peptide modifications using Mascot tools associated with relevant validation processes. Such an approach allows to target frequently occurring modifications such as methionine oxidation, phosphorylation or N-α-acetylation, but also the less usual peptide cationisation. Finally, this dataset offers the unique opportunity to determine the overall influence of some of these modifications on the identification score.

RESULTS

Although methionine oxidation has no influence and tends to favour the characterisation of protein N-terminal peptides, peptide alkalinisation shows an adverse effect on peptide average score. Nevertheless, peptide cationisation appears to favour the characterisation of protein C-terminal peptides with a limited to no direct influence on the identification score. Unexpectedly, our investigation reveals the unfortunate combination of the molecular weight of N-α-acetylation and potassium cation that mimics the mass increment of a phosphorylation group.

CONCLUSIONS

Since these characterisations rely upon computational treatment associated with statistical validation approaches such as 'False discovery rates' calculation or post-translational modification position validation, our investigation highlights the limitation of such treatment which is biased by the initial searched hypotheses.

Simultaneous identification of tyrosine phosphorylation and sulfation sites utilizing tyrosine-specific bromination

Tyrosine phosphorylation and sulfation play many key roles in the cell. Isobaric phosphotyrosine and sulfotyrosine residues in peptides were determined by mass spectrometry using phosphatase or sulfatase to remove the phosphate or the sulfate group. Unique Br signature was introduced to the resulting tyrosine residues by incubation with 32% HBr at -20 °C for 20 min. MS/MS analysis of the brominated peptide enabled unambiguous determination of the phosphotyrosine and the sulfotyrosine sites. When phosphotyrosine and sulfotyrosine as well as free tyrosine were present in the same peptide, they could be determined simultaneously using either phosphatase or sulfatase following acetylation of the free tyrosine.

Tyrosine 656 in topoisomerase IIβ is important for the catalytic activity of the enzyme: Identification based on artifactual +80-Da modification at this site

Topoisomerase (topo) II catalyzes topological changes in DNA. Although both human isozymes, topo IIα and β are phosphorylated, site-specific phosphorylation of topo IIβ is poorly characterized. Using LC-MS/MS analysis of topo IIβ, cleaved with trypsin, Arg C or cyanogen bromide (CNBr) plus trypsin, we detected four +80-Da modified sites: tyr656, ser1395, thr1426 and ser1545. Phosphorylation at ser1395, thr1426 and ser1545 was established based on neutral loss of H(3) PO(4) (-98 Da) in the CID spectra and on differences in 2-D-phosphopeptide maps of (32) P-labeled wild-type (WT) and S1395A or T1426A/S1545A mutant topo IIβ. However, phosphorylation at tyr656 could not be verified by 2-D-phosphopeptide mapping of (32) P-labeled WT and Y656F mutant protein or by Western blotting with phosphotyrosine-specific antibodies. Since the +80-Da modification on tyr656 was observed exclusively during cleavage with CNBr and trypsin, this modification likely represented bromination, which occurred during CNBr cleavage. Re-evaluation of the CID spectra identified +78/+80-Da fragment ions in CID spectra of two peptides containing tyr656 and tyr711, confirming bromination. Interestingly, mutation of only tyr656, but not ser1395, thr1326 or ser1545, decreased topo IIβ activity, suggesting a functional role for tyr656. These results, while identifying an important tyrosine in topo IIβ, underscore the importance of careful interpretation of modifications having the same nominal mass.

Novel oxidative modifications in redox-active cysteine residues

Redox-active cysteine, a highly reactive sulfhydryl, is one of the major targets of ROS. Formation of disulfide bonds and other oxidative derivatives of cysteine including sulfenic, sulfinic, and sulfonic acids, regulates the biological function of various proteins. We identified novel low-abundant cysteine modifications in cellular GAPDH purified on 2-dimensional gel electrophoresis (2D-PAGE) by employing selectively excluded mass screening analysis for nano ultraperformance liquid chromatography-electrospray-quadrupole-time of flight tandem mass spectrometry, in conjunction with MODi and MODmap algorithm. We observed unexpected mass shifts (Δm=-16, -34, +64, +87, and +103 Da) at redox-active cysteine residue in cellular GAPDH purified on 2D-PAGE, in oxidized NDP kinase A, peroxiredoxin 6, and in various mitochondrial proteins. Mass differences of -16, -34, and +64 Da are presumed to reflect the conversion of cysteine to serine, dehydroalanine (DHA), and Cys-SO2-SH respectively. To determine the plausible pathways to the formation of these products, we prepared model compounds and examined the hydrolysis and hydration of thiosulfonate (Cys-S-SO2-Cys) either to DHA (Δm=-34 Da) or serine along with Cys-SO2-SH (Δm=+64 Da). We also detected acrylamide adducts of sulfenic and sulfinic acids (+87 and +103 Da). These findings suggest that oxidations take place at redox-active cysteine residues in cellular proteins, with the formation of thiosulfonate, Cys-SO2-SH, and DHA, and conversion of cysteine to serine, in addition to sulfenic, sulfinic and sulfonic acids of reactive cysteine.

Subcellular phosphoproteomics

Protein phosphorylation represents one of the most extensively studied post-translational modifications, primarily due to the emergence of sensitive methods enabling the detection of this modification both in vitro and in vivo. The availability of enrichment methods combined with sensitive mass spectrometry instrumentation has played a crucial role in uncovering the dynamic changes and the large expanding repertoire of this reversible modification. The structural changes imparted by the phosphorylation of specific residues afford exquisite mechanisms for the regulation of protein functions by modulating new binding sites on scaffold proteins or by abrogating protein-protein interactions. However, the dynamic interplay of protein phosphorylation is not occurring randomly within the cell but is rather finely orchestrated by specific kinases and phosphatases that are unevenly distributed across subcellular compartments. This spatial separation not only regulates protein phosphorylation but can also control the activity of other enzymes and the transfer of other post-translational modifications. While numerous large-scale phosphoproteomics studies highlighted the extent and diversity of phosphoproteins present in total cell lysates, the further understanding of their regulation and biological activities require a spatio-temporal resolution only achievable through subcellular fractionation. This review presents a first account of the emerging field of subcellular phosphoproteomics where cell fractionation approaches are combined with sensitive mass spectrometry methods to facilitate the identification of low abundance proteins and to unravel the intricate regulation of protein phosphorylation.

Characterization of Nitrotyrosine-Modified Proteins in Cerebrospinal Fluid

Background

HIV-associated neurocognitive disorders (HAND) has been associated with the up-regulation of various oxidative stress pathways. Previous studies have linked the neuronal damage observed in individuals diagnosed with HAND to increased nitrotyrosine modification of neuronal proteins.

Materials and methods

Tyrosine nitration alters protein structure and function, affects biological half-life, and potentially prevents the phosphorylation of key tyrosine residues involved in signal transduction pathways. Therefore, in this study we employed proteomics-based experimental approaches to investigate nitrotyrosine-modified proteins in pooled cerebrospinal fluid (CSF) of individuals diagnosed with HAND. To identify specific nitrotyrosine-modified proteins in the CSF of individuals diagnosed with HAND, affinity purification and high-performance tandem mass spectrometry are utilized in a “bottom-up” proteomics approach.

Results

From tandem mass spectrometric analysis, we identified major proteins that underwent nitration as a result of nitro-oxidative stress in the CSF of individuals diagnosed with HAND. We also utilized analytical and biochemical techniques to characterize the expression and modification site of in vivo nitrated lipocalin-type prostaglandin-D synthase in HAND CSF.

Proteomics analysis of the cardiac myofilament subproteome reveals dynamic alterations in phosphatase subunit distribution

Myofilament proteins are responsible for cardiac contraction. The myofilament subproteome, however, has not been comprehensively analyzed thus far. In the present study, cardiomyocytes were isolated from rodent hearts and stimulated with endothelin-1 and isoproterenol, potent inducers of myofilament protein phosphorylation. Subsequently, cardiomyocytes were “skinned,” and the myofilament subproteome was analyzed using a high mass accuracy ion trap tandem mass spectrometer (LTQ Orbitrap XL) equipped with electron transfer dissociation. As expected, a small number of myofilament proteins constituted the majority of the total protein mass with several known phosphorylation sites confirmed by electron transfer dissociation. More than 600 additional proteins were identified in the cardiac myofilament subproteome, including kinases and phosphatase subunits. The proteomic comparison of myofilaments from control and treated cardiomyocytes suggested that isoproterenol treatment altered the subcellular localization of protein phosphatase 2A regulatory subunit B56α. Immunoblot analysis of myocyte fractions confirmed that β-adrenergic stimulation by isoproterenol decreased the B56α content of the myofilament fraction in the absence of significant changes for the myosin phosphatase target subunit isoforms 1 and 2 (MYPT1 and MYPT2). Furthermore, immunolabeling and confocal microscopy revealed the spatial redistribution of these proteins with a loss of B56α from Z-disc and M-band regions but increased association of MYPT1/2 with A-band regions of the sarcomere following β-adrenergic stimulation. In summary, we present the first comprehensive proteomics data set of skinned cardiomyocytes and demonstrate the potential of proteomics to unravel dynamic changes in protein composition that may contribute to the neurohormonal regulation of myofilament contraction.

Mechanistic insights into nitrite-induced cardioprotection using an integrated metabolomic/proteomic approach

Nitrite has recently emerged as an important bioactive molecule, capable of conferring cardioprotection and a variety of other benefits in the cardiovascular system and elsewhere. The mechanisms by which it accomplishes these functions remain largely unclear. To characterize the dose response and corresponding cardiac sequelae of transient systemic elevations of nitrite, we assessed the time course of oxidation/nitros(yl)ation, as well as the metabolomic, proteomic, and associated functional changes in rat hearts following acute exposure to nitrite in vivo. Transient systemic nitrite elevations resulted in: (1) rapid formation of nitroso and nitrosyl species; (2) moderate short-term changes in cardiac redox status; (3) a pronounced increase in selective manifestations of long-term oxidative stress as evidenced by cardiac ascorbate oxidation, persisting long after changes in nitrite-related metabolites had normalized; (4) lasting reductions in glutathione oxidation (GSSG/GSH) and remarkably concordant nitrite-induced cardioprotection, which both followed a complex dose-response profile; and (5) significant nitrite-induced protein modifications (including phosphorylation) revealed by mass spectrometry-based proteomic studies. Altered proteins included those involved in metabolism (eg, aldehyde dehydrogenase 2, ubiquinone biosynthesis protein CoQ9, lactate dehydrogenase B), redox regulation (eg, protein disulfide isomerase A3), contractile function (eg, filamin-C), and serine/threonine kinase signaling (eg, protein kinase A R1alpha, protein phosphatase 2A A R1-alpha). Thus, brief elevations in plasma nitrite trigger a concerted cardioprotective response characterized by persistent changes in cardiac metabolism, redox stress, and alterations in myocardial signaling. These findings help elucidate possible mechanisms of nitrite-induced cardioprotection and have implications for nitrite dosing in therapeutic regimens.