Detection and quantitation of protein-ligand binding interactions is important in many areas of biological research. The Stability of Proteins from Rates of Oxidation (SPROX) technique is an energetics-based technique for identifying the proteins targets of ligands in complex biological mixtures. Knowing the false positive rate of protein target discovery in proteome-wide SPROX experiments is important for the correct interpretation of results. Reported here are the results of a control SPROX experiment in which chemical denaturation data is obtained on the proteins in two samples that originated from the same yeast lysate, as would be done in a typical SPROX experiment except that one sample would be spiked with the test ligand. False positive rates of 1.2–2.2% and <0.8% are calculated for SPROX experiments using Q-TOF and orbitrap mass spectrometer systems, respectively. Our results indicate that the false positive rate is largely determined by random errors associated with the mass spectral analysis of the isobaric mass tag (e.g., iTRAQ®) reporter ions used for peptide quantitation. Our results also suggest that technical replicates can be used to effectively eliminate such false positives that result from this random error, as is demonstrated in a SPROX experiment to identify yeast protein targets of the drug, manassantin A. The impact of ion purity in the tandem mass spectral analyses and of background oxidation on the false positive rate of protein target discovery using SPROX is also discussed.
The detection and quantitation of protein-ligand binding interactions is critical in a number of different areas of biochemical research from fundamental studies of biological processes to drug discovery efforts. Described here is a protocol that can be used to identify the protein targets of biologically relevant ligands (e.g. drugs like tamoxifen or cyclosporin A) in complex protein mixtures such as cell lysates. The protocol utilizes quantitative, bottom-up, shotgun proteomics technologies (iTRAQ) with a covalent labeling technique, termed Stability of Proteins from Rates of Oxidation (SPROX). In SPROX, the thermodynamic properties of proteins and protein-ligand complexes are assessed using the hydrogen peroxide-mediated oxidation of methionine residues as a function of the chemical denaturant (e.g. guanidine Hydrochloride or urea) concentration. The proteome-wide SPROX experiments described here enable the ligand binding properties of hundreds of proteins to be simultaneously assayed in the context of complex biological samples. The proteomic capabilities of the protocol render it amenable to detection of both the on- and off-target effects of ligand binding.
Protein-ligand; protein-drug; interaction; mass spectrometry; iTRAQ; shotgun proteomics; hydrogen peroxide; H2O2; methionine oxidation; denaturant; guanidine; urea; protein folding/unfolding
IL-23, a heterodimer of IL-12 p40 and IL-23 p19, is critical for an effective immune response to many infections and has been implicated in several autoimmune diseases, however, little is known about the regulation of IL-23 gene expression in monocytes. We found that poly I:C, LPS, flagellin, and zymogen activated significant IL-23 production in primary human monocytes. Using chromatin immunoprecipitation, we found that a distal upstream region of the IL-23 p19 promoter at −601 to −521 underwent extensive histone modifications in response to stimuli. This distal region of the promoter is not highly conserved between species and has not been previously implicated in the regulation of IL-23 expression. Knockdown of CBP markedly decreased IL-23 p19 responses to poly I:C but had a less dramatic effect on LPS responses, confirming different chromatin responses to these two stimuli. Our data suggest that one of the mechanisms regulating IL-23 expression is the regulation of histone modifications at this distal upstream region of the promoter.
human; monocyte/macrophages; cytokines; histones; chromatin; gene regulation; molecular biology; CBP
Described here is a mass spectrometry based protocol to study the thermodynamic stability of proteins and protein-ligand complexes using the slow H/D exchange reaction of the imidazole C2 proton in histidine side chains. The protocol, which involves evaluating the denaturant dependence of this slow H/D exchange reaction in proteins, allows the global and/or subglobal unfolding/refolding properties of proteins and protein-ligand complexes to be probed. The protocol is developed using several model protein systems including: ribonuclease (Rnase) A, myoglobin, bovine carbonic anhydrase (BCA) II, hemoglobin, and the hemoglobin-haptoglobin protein complex. The compatibility of the protocol with conventional mass spectrometry-based proteomic sample preparation and analysis methods is also evaluated in an experiment in which the protocol is applied to proteins in a yeast cell lysate and used to detect the binding of Zn to superoxide dismutase in the yeast cell lysate sample. The yeast cell sample analyses also helped define the scope of the technique, which requires the presence of globally protected histidine residues in a protein’s three-dimensional structure for successful application.
Shotgun proteomics protocols are widely used for the identification and/or quantitation of proteins in complex biological samples. Described here is a shotgun proteomics protocol that can be used to identify the protein targets of biologically relevant ligands in complex protein mixtures. The protocol combines a quantitative proteomics platform with a covalent modification strategy, termed Stability of Proteins from Rates of Oxidation (SPROX), which utilizes the denaturant dependence of hydrogen peroxide-mediated oxidation of methionine side chains in proteins to assess the thermodynamic properties of proteins and protein-ligand complexes. The quantitative proteomics platform involves the use of isobaric mass tags and a methionine-containing peptide enhancement strategy. The protocol is evaluated in a ligand binding experiment designed to identify the proteins in a yeast cell lysate that bind the well-known enzyme co-factor, β-nicotinamide adenine dinucleotide (NAD+). The protocol is also used to investigate the protein targets of resveratrol, a biologically active ligand with less well-understood protein targets. A known protein target of resveratrol, cytosolic aldehyde dehydrogenase, was identified in addition to six other potential new proteins targets including four that are associated with the protein translation machinery, which has previously been implicated as a target of resveratrol.
Protein-ligand binding; resveratrol; NAD+; methionine oxidation; iTRAQ; SPROX
Described here is a stable isotope labeling protocol that can be used with a chemical modification- and mass spectrometry-based protein–ligand binding assay for detecting and quantifying both the direct and indirect binding events that result from protein–ligand binding interactions. The protocol utilizes an H216O2 and H218O2 labeling strategy to evaluate the chemical denaturant dependence of methionine oxidation in proteins both in the presence and absence of a target ligand. The differential denaturant dependence to the oxidation reactions performed in the presence and absence of ligand provides a measure of the protein stability changes that occur as a result of direct interactions of proteins with the target ligand and/or as a result of indirect interactions involving other protein–ligand interactions that are either induced or disrupted by the ligand. The described protocol utilizes the 18O/16O ratio in the oxidized protein samples to quantify the ligand-induced protein stability changes. The ratio is determined using the isotopic distributions observed for the methionine-containing peptides used for protein identification in the LC-MS-based proteomics readout. The strategy is applied to a multi-component protein mixture in this proof-of-principle experiment, which was designed to evaluate the technique’s ability to detect and quantify the direct binding interaction between cyclosporin A and cyclophilin A and to detect the indirect binding interaction between cyclosporin A and calcineurin (i.e., the protein–protein interaction between cyclophilin A and calcineurin that is induced by cyclosporin A binding to cyclophilin A).
Protein folding; Ligand binding; Thermodynamics; Covalent labeling; Quantitative proteomics; Cyclophilin A; Cyclosporin A; Calcineurin
Cyclophilin A (CypA) is an overexpressed protein in lung cancer tumors and as a result is a potential therapeutic and diagnostic target. Here we utilize an H/D exchange- and MALDI mass spectrometry-based assay, termed single-point SUPREX (Stability of Unpurified Proteins from Rates of H/D Exchange), to screen two chemical libraries, including the 1280-compound LOPAC library and the 9600 compound DIVERSet library, for binding to CypA. This work represents the first application of single-point SUPREX using a pooled ligand approach, which we demonstrate is capable of screening rates as fast as six seconds/ligand. The false positive and false negative rates determined in the current work using a set of control samples were 0% and 9%, respectively. A false positive rate of 20% was found in screening the actual libraries. Eight novel ligands to CypA were discovered including: 2-(α-naphthoyl)ethyltrimethyl-ammonium iodide, (E)-3-(4-t-Butylphenylsulfonyl)-2-propenenitrile, 3-(N-benzyl-N-isopropyl)amino-1-(naphthalen-2-yl)propan-1-one, cis-diammineplatinum (II) chloride, 1-(3,5-dichlorophenyl)-1H-pyrrole-2,5-dione, N-(3-chloro-1,4-dioxo-1,4-dihydro-2-naphthalenyl)-N-cyclohexylacetamide, 1-[2-(3,4-dimethoxyphenyl)ethyl]-1H-pyrrole-2,5-dione, and 4-(2-methoxy-4-nitrophenyl)-1-methyl-10-oxa-4-azatricyclo[22.214.171.124~2,6~]dec-8-ene-3,5-dione. These compounds, which had moderate binding affinities to CypA (i.e., Kd values in the low micromolar range), provide new molecular scaffolds that might be useful in the development of CypA targeted diagnostic imaging or therapeutic agents for lung cancer.
Cyclophilin A; Matrix-Assisted Laser Desorption/Ionization; amide H/D exchange; high-throughput screening
Molecular chaperones are a highly diverse group of proteins that recognize and bind unfolded proteins in order to facilitate protein folding and prevent non-specific protein aggregation. The mechanisms by which chaperones bind their protein substrates have been studied for decades. However, there are few reports on the affinity of molecular chaperones for their unfolded protein substrates. Thus, little is known about the relative binding affinities of different chaperones and about the relative binding affinities of chaperones for different unfolded protein substrates. Here we describe the application of SUPREX (stability of unpurified proteins from rates of H/D exchange), an H/D exchange and MALDI-based technique, to study the binding interaction between the molecular chaperone Hsp33 and four different unfolded protein substrates including citrate synthase, lactate dehydrogenase, malate dehydrogenase, and aldolase. The results of our studies suggest that the cooperativity of the Hsp33 folding/unfolding reaction increases upon binding with denatured protein substrates. This is consistent with the burial of significant hydrophobic surface area in Hsp33 when it interacts with its substrate proteins. The SUPREX derived Kd-values for Hsp33 complexes with four different substrates were found to be all within a range of 3-300 nM.
A protease digestion strategy was incorporated into single-point SUPREX (stability of unpurified proteins from rates of H/D exchange), which is an H/D exchange- and mass spectrometry-based assay for the detection of protein-ligand binding. Single-point SUPREX is an abbreviated form of SUPREX in which protein-ligand binding interactions are detected by measuring the increase in a protein’s thermodynamic stability upon ligand binding. The new protease digestion protocol provides a noteworthy increase in the efficiency of single-point SUPREX because peptide masses can be determined with greater precision than intact protein masses in the MALDI readout of single-point SUPREX. The protocol was evaluated in test screens on two model protein systems, including cyclophilin A (CypA) and the minor allele variant of human alanine:glyoxylate aminotransferase (AGTmi). The test screening results obtained on both proteins revealed that the peptide readout of the single-point SUPREX-protease digestion protocol was more efficient than the intact protein readout of the original single-point SUPREX protocol at discriminating hits and non-hits. In addition to this improvement in screening efficiency, the protease digestion strategy described here is expected to significantly increase the generality of the single-point SUPREX assay.
An H/D exchange- and MALDI mass spectrometry-based screening assay was applied to search for novel ligands that bind to cyclophilin A, a potential therapeutic and diagnostic target in lung cancer. The assay is based on SUPREX (stability of unpurified proteins from rates of H/D exchange), which exploits the H/D exchange properties of amide protons to measure the increase in a protein's thermodynamic stability upon ligand binding in solution. The current study evaluates the throughput and efficiency with which 880 potential ligands from the Prestwick Chemical Library could be screened for binding to cyclophilin A. Screening was performed at a rate of 3 min/ligand using a conventional MALDI mass spectrometer. False positive and false negative rates, based on a set of control data, were as low as 0% and 9%, respectively. Based on the 880-member library screening, a false positive rate of 0% was observed when a 2-tier selection strategy was implemented. Although novel ligands for cyclophilin A were not discovered, cyclosporin A, a known ligand to CypA and a blind control in the library, was identified as a hit. We also describe a new strategy to eliminate some of the complications related to back exchange that can arise in screening applications of SUPREX.
Neisseria gonorrhoeae is an obligate pathogen that hijacks iron from the human iron transport protein, holo-transferrin (Fe2-Tf), by expressing TonB-dependent outer membrane receptor proteins, TbpA and TbpB. Homologous to other TonB-dependent outer membrane transporters, TbpA is thought to consist of a β-barrel with an N-terminal plug domain. Previous reports by our laboratories show that the sequence EIEYE in the plug domain is highly conserved among various bacterial species that express TbpA and plays a crucial role in iron utilization for gonococci. We hypothesize that this highly conserved EIEYE sequence in the TbpA plug, rich in hard oxygen donor groups, binds with Fe3+ through the transport process across the outer membrane through the β-barrel. Sequestration of Fe3+ by the TbpA-plug supports the paradigm that the ferric iron must always remain chelated and controlled throughout the transport process. In order to test this hypothesis here we describe the ability of both the recombinant wild-type plug, and three small peptides that encompass the sequence EIEYE of the plug, to bind Fe3+. This is the first report of the expression/isolation of the recombinant wild-type TbpA plug. Although CD and SUPREX spectroscopies suggest that a non-native structure is observed for the recombinant plug, fluorescence quenching titrations indicate that the wild-type recombinant TbpA plug binds Fe3+ with a conditional log Kd = 7 at pH 7.5, with no evidence of binding at pH 6.3. A recombinant TbpA plug with mutated sequence (NEIEYEN → NEIAAAN) shows no evidence of Fe3+ binding under our experimental set up. Interestingly, in silico modeling with the wild-type plug also predicts a flexible loop structure for the EIEYE sequence under native conditions which once again supports the Fe3+ binding hypothesis. These in vitro observations are consistent with the hypothesis that the EIEYE sequence in the wild-type TbpA plug binds Fe3+ during the outer membrane transport process in vivo.
The redox-regulated chaperone Hsp33 protects organisms against oxidative stress that leads to protein unfolding. Activation of Hsp33 is triggered by the oxidative unfolding of its own redox-sensor domain, making Hsp33 a member of a recently discovered class of chaperones that require partial unfolding for full chaperone activity. Here we address the long-standing question of how chaperones recognize client proteins. We show that Hsp33 uses its own intrinsically disordered regions to discriminate between unfolded and partially structured folding intermediates. Binding to secondary structure elements in client proteins stabilizes Hsp33’s intrinsically disordered regions, and this stabilization appears to mediate Hsp33’s high affinity for structured folding intermediates. Return to nonstress conditions reduces Hsp33’s disulfide bonds, which then significantly destabilizes the bound client proteins and in doing so converts them into less-structured, folding-competent client proteins of ATP-dependent foldases. We propose a model in which energy-independent chaperones use internal order-to-disorder transitions to control substrate binding and release.
Neisseria gonorrhoeae has the capacity to acquire iron from its human host by removing this essential nutrient from serum transferrin. The transferrin binding proteins, TbpA and TbpB constitute the outer membrane receptor complex responsible for binding transferrin, extracting the tightly bound iron from the host-derived molecule, and transporting iron into the periplasmic space of this Gram-negative bacterium. Once iron is transported across the outer membrane, ferric binding protein A (FbpA) moves the iron across the periplasmic space and initiates the process of transport into the bacterial cytosol. The results of the studies reported here define the multiple steps in the iron transport process in which TbpA and TbpB participate. Using the SUPREX technique for assessing the thermodynamic stability of protein-ligand complexes, we report herein the first direct measurement of periplasmic FbpA binding to the outer membrane protein TbpA. We also show that TbpA discriminates between apo- and holo-FbpA; i.e. the TbpA interaction with apo-FbpA is higher affinity than the TbpA interaction with holo-FbpA. Further, we demonstrate that both TbpA and TbpB individually can deferrate transferrin and ferrate FbpA without energy supplied from TonB.
The cell autonomous response to viral infection is carefully regulated to induce type I interferons (IFNs), which in turn induce the establishment of an antiviral state. Leucine-rich repeat (in Flightless I) interacting protein-1 (LRRFIP1) and LRRFIP2 are 2 related proteins that have been identified as interacting with MyD88 and Flightless I homolog, a leucine-rich repeat protein. LRRFIP2 positively regulates NFκB and macrophage cytokine production after lipopolysaccharide, but less is known about LRRFIP1. We hypothesized that LRRFIP1 could be more important in antiviral responses, as overexpression led to type I IFN production in a pilot study. The induction of type I IFNs occurred even in the absence of virus, but was enhanced by the presence of virus. Conversely, knockdown of LRRFIP1 compromised IFN expression. We found that LRRFIP1 was rapidly recruited to influenza-containing early endosomes in a p38-dependent fashion. This was specific for virus-containing endosomes as there was almost no colocalization of LRRFIP1 with early endosomes in the absence of virus. Further, LRRFIP1 was recruited to RNA-containing vesicles. Taken together, these data suggest that LRRFIP1 participates in cell responses to virus at early time points and is important for type I IFN induction.
The use of MALDI-TOF mass spectrometry as a means of analyzing the proteome has been evaluated extensively in recent years. One of the limitations of this technique that has impeded the development of robust data analysis algorithms is the variability in the location of protein ion signals along the x-axis. We studied technical variations of MALDI-TOF measurements in the context of proteomics profiling. By acquiring a benchmark data set with five replicates, we estimated 76% to 85% of the total variance is due to phase variation. We devised a lobster plot, so named because of the resemblance to a lobster claw, to help detect the phase variation in replicates. We also investigated a peak alignment algorithm to remove the phase variation. This operation is analogous to the normalization step in microarray data analysis. Only after this critical step can features of biological interest be clearly revealed. With the help of principal component analysis, we demonstrated that after peak alignment, the differences among replicates are reduced. We compared this approach to peak alignment with a model-based calibration approach in which there was known information about peaks in common among all spectra. Finally, we examined the potential value at each point in an analysis pipeline of having a set of methods available that includes parametric, semiparametric and nonparametric methods; among such methods are those that benefit from the use of prior information.
variation; amplitude; phase; MALDI-TOF; peak alignment