Towards proteomic approaches for the identification of structural disorder

Intrinsically unstructured/disordered proteins (IUPs) and protein domains lack a well-defined three-dimensional structure under physiological conditions. Structural disorder imparts advantages in many non-conventional functions, which poses a significant challenge to our understanding of the structure-function relationship of proteins. The general appreciation of this fact, however, is hampered by the large gap in our knowledge on IUPs, as we have biophysical data on less than 500 of them, whereas bioinformatic predictions suggest at least several thousand such proteins in the human proteome alone. Thus, proteomic-scale identification and characterization of IUPs will need to be implemented to fill this gap and advance our knowledge in this important field. In this review we give an insight into the various rationales of proteomic efforts of identifying IUPs, and survey the handful of attempts that combined enrichment of extracts for IUPs by heat- or acid treatment with a subsequent two-dimensional electrophoresis/mass spectrometry identification. Advantages and drawbacks of the various approaches are outlined in anticipation of future inventions in the field that will hopefully elevate IUP research to the truly proteomic level.     

Genetic and genomic approaches to identify and study the targets of bioactive small molecules

Natural and synthetic bioactive small molecules form the backbone of modern therapeutics. These drugs primarily exert their effect by targeting cellular host or foreign proteins that are critical for the progression of disease. Therefore, a crucial step in the process of recognizing valuable new drug leads is identification of their protein targets; this is often a time consuming and difficult task. This report is intended to provide a comprehensive review of recent developments in genetic and genomic approaches to overcome the hurdle of discovering the protein targets of bioactive small molecules.     

Computational identification of 48 potato microRNAs and their targets

MicroRNAs (miRNAs) are a new family of small RNA molecules known in animals and plants, whose conservation among species suggests that they bear conserved biological functions. So far, little is known about miRNA in Solanum tuberosum species. Using previously known miRNAs from Arabidopsis, rice and other plant species against expressed sequence tags (ESTs), genomic survey sequence (GSS) and nucleotide databases, we identified 48 potential miRNAs in S. tuberosum. These potato miRNAs may regulate 186 potential targets, which are involved in floral, leaf, root, and stem development, signal transduction, metabolism pathways, and stress responses. To validate the prediction of miRNAs in potato, we performed a RT-PCR analysis and found that potato miRNAs have diverse expression patterns during development.     

Identification of human hepatocellular carcinoma-related proteins by proteomic approaches

Hepatocellular carcinoma (HCC) is the most common malignant liver tumor. Analysis of human serum from HCC patients using two-dimensional gel electrophoresis (2DE) combined with nano-high-performance liquid chromatography electrospray ionization tandem mass spectrometry (nano-HPLC–ESI-MS/MS) identified fourteen different proteins differentially expressed between HCC patients and the control group. Twelve proteins were up-regulated and two down-regulated. By using nano-HPLC–MS/MS system to analyze proteome in human serum, 317 proteins were identified, twenty-nine of which to high confidence levels (protein matched at last two unique peptide sequences). Of these twenty-nine proteins, six were present only in HCC patients and may serve as biomarkers for HCC. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Design and synthesis of an affinity probe that targets caspases in proteomic experiments

The field of proteomics aims to study all proteins in the human proteome. This huge task may be accelerated by using active-site directed probes which profile proteins in an activity-dependent manner. Herein, we have developed a fluorescently-labeled affinity probe containing chemical reactivity specific towards caspases. Preliminary assays and proof-of-concept experiments demonstrated that this probe exhibits strong chemical reactivity towards caspase-1 over other enzymes, capable of covalently labeling caspapse-1 over other non-caspase enzymes. This thus demonstrates its selectivity and potential in high-throughput screenings of other unknown caspases in a large-scale proteomics experiment.     

Novel staining-free proteomic method for simultaneous identification of proteins and determination of their pI values by using low-molecular-mass pI markers

A new proteomic staining-free method for simultaneous identification of proteins and determination of their pI values by using low-molecular-mass pI markers is described. It is based on separation of proteins in gels by IEF in combination with mass spectrometric analysis of both peptides derived by in-gel digestion and low-molecular-mass pI markers extracted form the same piece excised from the gel. In this method, the pI markers are mixed with a protein mixture (a commercial malted barley protein extract) deposited on a gel and separated in a pH gradient. Color pI markers enable supervision of progress of focusing process. Several separated bands of the pI markers (including separated proteins) were excised and the pI markers were eluted from each gel piece by water/ethanol and identified by MALDI-TOF/TOF MS. The remaining carrier ampholytes were then washed out from gel pieces and proteins were in-gel digested with trypsin or chymotrypsin. Obtained peptides were measured by MALDI-TOF/TOF MS and proteins were identified via protein database search. This procedure allows omitting time-consuming protein staining and destaining procedures, which shortens the analysis time. For comparison, other IEF gels were stained with CBB R 250 and proteins in the gel bands were identified. Similarity of the results confirmed that our approach can give information about the correct pI values of particular proteins in complex samples at significantly shorter analysis times. This method can be very useful for identification of proteins and their post-translational modifications in prefractioned samples, where post-translational modifications (e.g., glycation) are frequent.     

DeepSIRT: A deep neural network for identification of sirtuin targets and their subcellular localizations

Sirtuins are a family of proteins that play a key role in regulating a wide range of cellular processes including DNA regulation, metabolism, aging/longevity, cell survival, apoptosis, and stress resistance. Sirtuins are protein deacetylases and include in the class III family of histone deacetylase enzymes (HDACs). The class III HDACs contains seven members of the sirtuin family from SIRT1 to SIRT7. The seven members of the sirtuin family have various substrates and are present in nearly all subcellular localizations including the nucleus, cytoplasm, and mitochondria. In this study, a deep neural network approach using one-dimensional Convolutional Neural Networks (CNN) was proposed to build a prediction model that can accurately identify the outcome of the sirtuin protein by targeting their subcellular localizations. Therefore, the function and localization of sirtuin targets were analyzed and annotated to compartmentalize into distinct subcellular localizations. We further reduced the sequence similarity between protein sequences and three feature extraction methods were applied in datasets. Finally, the proposed method has been tested and compared with various machine-learning algorithms. The proposed method is validated on two independent datasets and showed an average of up to 85.77 % sensitivity, 97.32 % specificity, and 0.82 MCC for seven members of the sirtuin family of proteins.     

Intracellular protein scaffold-mediated display of random peptide libraries for phenotypic screens in mammalian cells

BACKGROUND: Mammalian cell screens of peptide libraries for changes in cellular phenotype may identify novel functional peptides and their cognate binding partners, and allow identification of signal transduction network members or proteins important in disease processes. RESULTS: Green fluorescent protein (GFP) peptide libraries with different structural biases were tested by retroviral expression in A549 carcinoma cells, HUVEC and other cell types. Three different loop replacement libraries, containing 12 or 18 random residues, were compatible with enhanced GFP (EGFP) folding, as was a C-terminally fused random 20-mer library. Library concentrations in A549 cells ranged from ca. 1 to 54 microM. Replacement of loop 3 with known nuclear localization sequence (NLS) peptides, but not with inactive mutants, directed EGFP to the nucleus. Microscopy-based screens of three different libraries for non-uniform localization revealed novel NLS peptides, novel variants of a peroxisomal localization motif, a variety of partial NLS peptides, peptides localized to the nucleolus, and nuclear-excluded peptides. CONCLUSIONS: Peptides can be presented by EGFP in conformations that can functionally interact with cellular constituents in mammalian cells. A phenotypic screen resulting in the discovery of novel localization peptides that were not cell type-specific suggests that this methodology may be applied to other screens in cells derived from diseased organisms, and illustrates the use of intracellular combinatorial peptide chemistry in mammalian cells.     

A proteomic strategy for the identification of caspase-associating proteins

We report the efficient in vivo labelling of caspases expressed inside apoptotic HeLa cells using fluoromethyl ketone (fmk)-containing probes; preliminary results indicated that these probes may be used to identify caspase-associating proteins.     

Peptide prefractionation is essential for proteomic approaches employing multiple‐reaction monitoring of fruit proteomic research

Off‐gel? IEF has become a popular tool in proteomics research to fractionate peptides or proteins. We conducted a detailed investigation on the fruit proteomics of apple, banana, and strawberry fruit employing Off‐gel? electrophoresis (OGE) as a crucial step to improve the proteome coverage and quantitative proteomic workflows including multiple‐reaction monitoring (MRM). We provide technical details concerning the application of Off‐gel?IEF, nano‐LC–MS detection, and MRM optimization and analysis. Our results demonstrated that the application of OGE is an effective method for peptide fractionation and increased significantly the number of proteins identified by at least ten times, with more total peptides detected and collected. Furthermore, we developed a protocol combining OGE and MRM studies to identify and quantitatively investigate monodehydroascorbate reductase, a key enzyme in the redox and antioxidant system of apple fruit during fruit ripening. Using this method, the quantitative changes in this protein during ripening and in response to ethylene treatment was investigated. Our results provide direct and comprehensive evidence demonstrating the benefits of OGE and its application for both shotgun and quantitative proteomics research.     

DNA aptamers for selective identification and separation of flame retardant chemicals

Polybrominated diphenyl ethers (PBDEs) are group of chemicals which are representative persistent organic pollutants (POPs) and used as brominated flame retardants for many consumer products. PBDEs were phased out since 2009 but are still frequently observed in various environmental matrices and human body. Here, we report ssDNA aptamers which bind to BDE47, one of the PBDE congeners commonly found in various environmental matrices, and show affinity to other major tri-to hepta- BDE congeners. The PBDE specific aptamers were isolated from random library of ssDNA using Mag-SELEX. Two out of 15 sequences, based on their alignment and hairpin loop structures, were chosen to determine dissociation constant with BDE47 and showed from picomolar to nanomolar affinities (200 pM and 1.53 nM). The aptamers displayed high selectivity to the original target, BDE47, and implying general specificity to PBDE backbone with varying affinities to other congeners. Further, we showed that the use of two aptamers together could enhance the separation efficiency of BDE47 and other BDE congeners when dissolved in a solvent compared to use of single aptamer. These aptamers are expected to provide a tool for preliminary screening or quick separation of PBDEs in environmental samples prior to trace quantitative analysis.     

Mass spectrometric approaches for the identification of anthracycline analogs produced by actinobacteria

Anthracyclines are a well‐known chemical class produced by actinobacteria used effectively in cancer treatment; however, these compounds are usually produced in few amounts because of being toxic against their producers. In this work, we successfully explored the mass spectrometry versatility to detect 18 anthracyclines in microbial crude extract. From collision‐induced dissociation and nuclear magnetic resonance spectra, we proposed structures for five new and identified three more anthracyclines already described in the literature, nocardicyclins A and B and nothramicin. One new compound 8 (4‐[4‐(dimethylamino)‐5‐hydroxy‐4,6‐dimethyloxan‐2‐yl]oxy‐2,5,7,12‐tetrahydroxy‐3,10‐dimethoxy‐2‐methyl‐3,4‐dihydrotetracene‐1,6,11‐trione) was isolated and had its structure confirmed by ¹H nuclear magnetic resonance. The anthracyclines identified in this work show an interesting aminoglycoside, poorly found in natural products, 3‐methyl‐rhodosamine and derivatives. This fact encouraged to develop a focused method to identify compounds with aminoglycosides (rhodosamine, m/z 158; 3‐methyl‐rhodosamine, m/z 172; 4′‐O‐acethyl‐3‐C‐methyl‐rhodosamine, m/z 214). This method allowed the detection of four more anthracyclines. This focused method can also be applied in the search of these aminoglycosides in other microbial crude extracts. Additionally, it was observed that nocardicyclin A, nothramicin and compound 8 were able to interact to DNA through a DNA‐binding study by mass spectrometry, showing its potential as anticancer drugs. Copyright © 2016 John Wiley & Sons, Ltd.     

Fully functionalized small-molecule probes for integrated phenotypic screening and target identification

Phenotypic screening offers a powerful approach to identify small molecules that perturb complex biological processes in cells and organisms. The tendency of small molecules, however, to interact with multiple protein targets, often with moderate to weak affinities, along with the lack of straightforward technologies to characterize these interactions in living systems, has hindered efforts to understand the mechanistic basis for pharmacological activity. Here we address this challenge by creating a fully functionalized small-molecule library whose membership is endowed with: (1) one or more diversity elements to promote interactions with different protein targets in cells, (2) a photoreactive group for UV light-induced covalent cross-linking to interacting proteins, and (3) an alkyne handle for reporter tag conjugation to visualize and identify cross-linked proteins. A library member was found to inhibit cancer cell proliferation selectively under nutrient-limiting (low glucose) conditions. Quantitative chemoproteomics identified MT-ND1, an integral membrane subunit of the ～1 MDa NADH:ubiquinone oxidoreductase (complex 1) involved in oxidative phosphorylation, as a specific target of the active probe. We further demonstrated that the active probe inhibits complex 1 activity in vitro (IC(50) = 720 nM), an effect that is known to induce cell death in low-glucose conditions. Based on this proof of principle study, we anticipate that the generation and integration of fully functionalized compound libraries into phenotypic screening programs should facilitate the discovery of bioactive probes that are amenable to accelerated target identification and mechanistic characterization using advanced chemoproteomic technologies.     

Application of shape-based and pharmacophore-based in silico screens for identification of Type II protein kinase inhibitors

The identification of new, potent and selective inhibitors of important protein kinase targets is a major goal of drug discovery. Here we analyze the crystal structures of 55 protein kinase complexes with Type II inhibitors and find they adopt a conserved twisted V-shape, with an angle of 121?±?8° and twist of 78?±?8°. The tightly conserved twist appears important in ensuring ligands curve around the protein backbone and towards the deep pocket. From this, we develop predictive pharmacophore- and shape-based screens to identify Type II inhibitors from a database which also contains Type I inhibitors as decoys. Both approaches exhibit a good level of discrimination for Type II molecules. The most effective pharmacophore model requires six features and three excluded volume regions. Shape-based screening using ROCS generally performs at least as well as pharmacophore approaches. There is only a moderate dependence of shape-based or pharmacophore-based screens on the underlying conformer generator (MOE, Macromodel, Omega and SPE), as well as on ligand linkage chemistry (amide and urea). Finally, we apply our approach to retrieval of Type II inhibitors from a modified version of the DUD database, containing over 104,000 compounds. We observe good enrichment, providing further evidence that the in silico screens developed here will constitute useful guides for identification of small molecule inhibitors targetting protein kinases in their inactive conformational state.     

Design and peptide-based validation of phage display antibodies for proteomic biochips

To validate potential application of phage display-antibody arrays for high-throughput screening on a novel proteomics biochip, we examined the epitopes versus the full protein of glucose-6-phosphate-dehydrogenase (G6PD) from yeast. In a predictive approach, we used the Hopp-Woods method and compared the results with antibodies directed against the entire enzyme. In total, 16 peptides of a length of 11 amino acids each fulfilling the desired criteria were identified and synthesized. Subsequently, antibodies against G6PD were raised using a phage display library. Selective interaction of the antibodies with certain peptides facilitated the identification of epitopes predicted by the hydropathic profile. The setup was adapted to a novel biochip system based on surface-enhanced absorption for direct CCD-camera based screening.