CHalf: Folding Stability Made Simple.

The structure of a protein defines its function and integrity and correlates with the protein folding stability (PFS). Quantifying PFS allows researchers to assess differential stability of proteins in different disease or ligand binding states, providing insight into protein efficacy and potentially serving as a metric of protein quality. There are a number of mass spectrometry (MS)-based methods to assess PFS, such as Thermal Protein Profiling (TPP), Stability of Proteins from Rates of Oxidation (SPROX), and Iodination Protein Stability Assay (IPSA). Despite the critical value that PFS studies add to the understanding of mechanisms of disease and treatment development, proteomics research is still primarily dominated by concentration-based studies. We found that a major reason for the lack of PFS studies is the lack of a user-friendly data processing tool. Here we present the first user-friendly software, CHalf, with a graphical user interface for calculating PFS. Besides calculating site-specific PFS of a given protein from chemical denature folding stability assays, CHalf is also compatible with thermal denature folding stability assays. CHalf also includes a set of data visualization tools to help identify changes in PFS across protein sequences and in between different treatment conditions. We expect the introduction of CHalf to lower the barrier of entry for researchers to investigate PFS, promoting the usage of PFS in studies. In the long run, we expect this increase in PFS research to accelerate our understanding of the pathogenesis and pathophysiology of disease.

Analysis of Brain Protein Stability Changes in Mouse Models of Normal Aging and α-Synucleinopathy Reveals Age- and Disease-Related Differences.

Here, we utilize the stability of proteins from rates of oxidation (SPROX) technique, to profile the thermodynamic stabilities of proteins in brain tissue cell lysates from Huα-Syn(A53T) transgenic mice at three time points including at 1 month (n = 9), at 6 months (n = 7), and at the time (between 9 and 16 months) a mouse became symptomatic (n = 8). The thermodynamic stability profiles generated here on 332 proteins were compared to thermodynamic stability profiles generated on the same proteins from similarly aged wild-type mice using a two-way unbalanced analysis of variance (ANOVA) analysis. This analysis identified a group of 22 proteins with age-related protein stability changes and a group of 11 proteins that were differentially stabilized in the Huα-Syn(A53T) transgenic mouse model. A total of 9 of the 11 proteins identified here with disease-related stability changes have been previously detected in human cerebral spinal fluid and thus have potential utility as biomarkers of Parkinson's disease (PD). The differential stability observed for one protein, glutamate decarboxylase 2 (Gad2), with an age-related change in stability, was consistent with the differential presence of a known, age-related truncation product of this protein, which is shown here to have a higher folding stability than full-length Gad2. Mass spectrometry data were deposited at ProteomeXchange (PXD016985).

[Label-free small molecule probe and target discovery of traditional Chinese medicine].

Target discovery is the core of elucidating the mechanism of traditional Chinese medicine( TCM),and it is also the key to correlate the chemical composition and pharmacological action of TCM. The traditional target screening methods such as the activitybased probe profiling,affinity chromatography,and protein microarray are commonly used in the past,however,which are limited in TCM due to the complexity of small molecules existed in the herbal medicine. The label-free small molecule probe is a recently well-applied technology in the target discovery of natural products,which is characterized by discovering the small molecule-protein ligation without any structural modification at the ligands,and is therefore suitable to the complex chemical constituents in TCM. Furthermore,this method is conducted on the basis of proteome,which is advanced in the discovery of new or multiple target proteins of TCM. Owing to the potential of label-free probe in the target discovery of TCM,its analytical principle,application status,and general protocol were reviewed in this paper. The label-free probe technology is anticipated to accelerate the mechanism-uncovering of TCM.

Proteome-Wide Characterization of Phosphorylation-Induced Conformational Changes in Breast Cancer.

Because of the close link between protein function and protein folding stability, knowledge about phosphorylation-induced protein folding stability changes can lead to a better understanding of the functional effects of protein phosphorylation. Here, the stability of proteins from rates of oxidation (SPROX) and limited proteolysis (LiP) techniques are used to compare the conformational properties of proteins in two MCF-7 cell lysates including one that was and one that was not dephosphorylated with alkaline phosphatase. A total of 168 and 251 protein hits were identified with dephosphorylation-induced stability changes using the SPROX and LiP techniques, respectively. Many protein hits are previously known to be differentially phosphorylated or differentially stabilized in different human breast cancer subtypes, suggesting that the phosphorylation-induced stability changes detected in this work are disease related. The SPROX hits were enriched in proteins with aminoacyl-tRNA ligase activity. These enriched protein hits included many aminoacyl-tRNA synthetases (aaRSs), which are known from previous studies to have their catalytic activity modulated by phosphorylation. The SPROX results revealed that the magnitudes of the destabilizing effects of dephoshporylation on the different aaRSs were directly correlated with their previously reported aminoacylation activity change upon dephosphorylation. This substantiates the close link between protein folding and function.

Discovery of Tamoxifen and N-Desmethyl Tamoxifen Protein Targets in MCF-7 Cells Using Large-Scale Protein Folding and Stability Measurements.

The proteins in an MCF-7 cell line were probed for tamoxifen (TAM) and n-desmethyl tamoxifen (NDT) induced stability changes using the Stability of Proteins from Rates of Oxidation (SPROX) technique in combination with two different quantitative proteomics strategies, including one based on SILAC and one based on isobaric mass tags. Over 1000 proteins were assayed for TAM- and NDT-induced protein stability changes, and a total of 163 and 200 protein hits were identified in the TAM and NDT studies, respectively. A subset of 27 high-confidence protein hits were reproducibly identified with both proteomics strategies and/or with multiple peptide probes. One-third of the high-confidence hits have previously established experimental links to the estrogen receptor, and nearly all of the high-confidence hits have established links to breast cancer. One high-confidence protein hit that has known estrogen receptor binding properties, Y-box binding protein 1 (YBX1), was further validated as a direct binding target of TAM using both the SPROX and pulse proteolysis techniques. Proteins with TAM- and/or NDT-induced expression level changes were also identified in the SILAC-SPROX experiments. These proteins with expression level changes included only a small fraction of those with TAM- and/or NDT-induced stability changes.

Discovery of Age-Related Protein Folding Stability Differences in the Mouse Brain Proteome.

Described here is the application of thermodynamic stability measurements to study age-related differences in the folding and stability of proteins in a rodent model of aging. Thermodynamic stability profiles were generated for 809 proteins in brain cell lysates from mice, aged 6 (n = 7) and 18 months (n = 9) using the Stability of Proteins from Rates of Oxidation (SPROX) technique. The biological variability of the protein stability measurements was low and within the experimental error of SPROX. A total of 83 protein hits were detected with age-related stability differences in the brain samples. Remarkably, the large majority of the brain protein hits were destabilized in the old mice, and the hits were enriched in proteins that have slow turnover rates (p < 0.07). Furthermore, 70% of the hits have been previously linked to aging or age-related diseases. These results help validate the use of thermodynamic stability measurements to capture relevant age-related proteomic changes and establish a new biophysical link between these proteins and aging.

Discovery of Manassantin A Protein Targets Using Large-Scale Protein Folding and Stability Measurements.

Manassantin A is a natural product that has been shown to have anticancer activity in cell-based assays, but has a largely unknown mode-of-action. Described here is the use of two different energetics-based approaches to identify protein targets of manassantin A. Using the stability of proteins from rates of oxidation technique with an isobaric mass tagging strategy (iTRAQ-SPROX) and the pulse proteolysis technique with a stable isotope labeling with amino acids in cell culture strategy (SILAC-PP), over 1000 proteins in a MDA-MB-231 cell lysate grown under hypoxic conditions were assayed for manassantin A interactions (both direct and indirect). A total of 28 protein hits were identified with manassantin A-induced thermodynamic stability changes. Two of the protein hits (filamin A and elongation factor 1α) were identified using both experimental approaches. The remaining 26 hit proteins were only assayed in either the iTRAQ-SPROX or the SILAC-PP experiment. The 28 potential protein targets of manassantin A identified here provide new experimental avenues along which to explore the molecular basis of manassantin A's mode of action. The current work also represents the first application iTRAQ-SPROX and SILAC-PP to the large-scale analysis of protein-ligand binding interactions involving a potential anticancer drug with an unknown mode-of-action.

Label-free target protein characterization for small molecule drugs: recent advances in methods and applications.

Target protein identification is the key to identification of the mechanisms, side effects, and evaluating druglikeness of small-molecule drugs. The commonly used "labeled" target-characterization methods, including activity-based proteome profiling (ABPP), require the synthesis of a derivatized probe, which are time-consuming and may affect the active drug conformation. Label-free target identification methods do not involve any chemical modification of small-molecules drugs and have received increasing attention in recent years. We reviewed the basic principles, workflow, applications, advantages, and disadvantages of the promising label-free target identification methods, including cellular thermal shift assay (CETSA), thermal proteome profiling (TPP), pulse proteolysis (PP), stability of proteins from rates of oxidation (SPROX), drug affinity responsive target stability (DARTS), limited proteolysis-coupled mass spectrometry (LiP-MS) and solvent-induced protein precipitation (SIP). We also reviewed the prospective applications of these label-free methods for efficient target identification. The approaches based on peptide mapping using high-resolution mass spectrometry (MS) may provide more information regarding comprehensive target proteins and binding sites, which may be useful for target identification in multi-target or complex drug systems.

Emerging Methods in Chemoproteomics with Relevance to Drug Discovery.

A powerful interplay exists between the recognition of gene families, sensitive techniques in proteomics, and the interrogation of protein function using chemical probes. The most prominent methods, such as affinity capture, activity-based protein profiling and photoaffinity labeling, are extensively reviewed in the literature. Here we briefly review additional methods developed in the past 15 years. These include "stability proteomics" methods such as proteomically analyzed cellular thermal shift assays and the use of chemical oxidation as a probe of structure, the use of multiple bead-linked kinase inhibitors to analyze inhibitor specificities, and advances in the use of proteolysis-targeting chimeras for selective protein elimination.

Identification of protein binding partners of small molecules using label-free methods.

INTRODUCTION: Drug discovery efforts across the globe are chasing new drug targets and novel mechanisms of action. To support the identification of novel mechanisms of action, phenotype-based drug screening has significantly increased over the last decade. Along with the rise in phenotypic screening, methods and technologies that can help to identify drug targets of phenotypically screened 'hits' have also evolved significantly. AREAS COVERED: This article provides an overview of successful examples, limitations and advances in small-molecule target identification methodologies. Primarily, the methods are described, where small-molecules without derivatization are used as test-molecules for identifying their direct binding protein partners, the targets, in detail. A brief discussion of other affinity chromatography coupled mass-spectrometry based target identification methods are also presented for comparative appreciation of label-free methods. EXPERT OPINION: Label-free methods do not require (a) extensive structure activity analysis of phenotypically screened 'hits' and (b) preparation of tool compounds or target capturing probes for target identification. These methods are significantly shortening the time required for the identification and the downstream validation of targets and hence are gaining popularity as the method of choice for target identification.