The identification of molecular targets is a critical step in drug discovery, particularly for bioactive natural products, which often display complex structures, polypharmacology, and unconventional mechanisms of action. Unlike synthetic compounds designed for specific targets, natural products frequently interact with multiple proteins, making unbiased, proteome-wide target identification technologies essential. Over the past decade, several label-free or minimally perturbative proteomics-based approaches have emerged to address this challenge. Among the most widely adopted are DARTS (Drug Affinity Responsive Target Stability), SPROX (Stability of Proteins from Rates of Oxidation), CETSA (Cellular Thermal Shift Assay), and TPP (Thermal Proteome Profiling). Although all four techniques rely on ligand-induced changes in protein stability, they differ significantly in experimental design, throughput, biological context, and applicability to natural product research. Here, we provide a systematic comparison of these technologies, highlighting their principles, strengths, limitations, and optimal use cases.
DARTS: Drug Affinity Responsive Target Stability
Principle
DARTS is based on the concept that ligand binding protects proteins from proteolytic degradation. When a small molecule binds to its target protein, it stabilizes the protein's conformation, rendering it less susceptible to limited protease digestion. By comparing proteolysis patterns in the presence and absence of a compound, potential targets can be identified.
Workflow
Cells or tissues are lysed and incubated with or without the compound of interest. Samples are then subjected to limited proteolysis (commonly using pronase). Stabilized proteins are enriched and identified via SDS-PAGE followed by mass spectrometry.
Strengths
- Completely label-free, ideal for natural products that are difficult to derivatize
- Simple experimental setup
- Suitable for low-abundance natural compounds
- Preserves near-native protein conformations
Limitations
- Lower proteome coverage compared to thermal profiling methods
- Sensitive to protease conditions and experimental variability
- Primarily performed in cell lysates, not intact cells
Applications
DARTS is particularly effective for initial target discovery of bioactive natural products and for validating direct binding to candidate proteins.
SPROX: Stability of Proteins from Rates of Oxidation
Principle
SPROX measures ligand-induced changes in protein folding stability by monitoring the oxidation rates of methionine residues under increasing concentrations of chemical denaturants. Ligand binding shifts the protein's folding equilibrium, which can be detected as changes in oxidation behavior.
Workflow
Proteins are exposed to a gradient of denaturant concentrations in the presence or absence of a compound, followed by hydrogen peroxide-mediated oxidation. Oxidized peptides are quantified using mass spectrometry to determine stability shifts.
Strengths
- Quantitative assessment of protein thermodynamic stability
- Capable of detecting both stabilizing and destabilizing interactions
- Applicable to complex proteomes
Limitations
- Limited to proteins containing oxidizable methionine residues
- Technically complex and mass spectrometry-intensive
- Typically performed in lysates rather than live cells
Applications
SPROX is valuable for mechanistic studies, especially when understanding how natural products modulate protein folding and conformational stability.
CETSA: Cellular Thermal Shift Assay
Principle
CETSA exploits the fact that ligand binding increases a protein's resistance to thermal denaturation. By heating cells or lysates across a temperature range, bound proteins remain soluble at higher temperatures compared to unbound proteins.
Workflow
Cells or lysates are treated with a compound, heated to defined temperatures, and centrifuged to separate soluble and aggregated proteins. Target engagement is assessed by immunoblotting or mass spectrometry.
Strengths
- Can be performed in intact cells, preserving physiological context
- Direct measurement of target engagement in vivo
- Widely accepted for validation in drug discovery pipelines
Limitations
- Traditionally low throughput when antibody-based
- Requires high-quality antibodies unless coupled to MS
- Limited proteome coverage in classical CETSA formats
Applications
CETSA is ideal for confirming target engagement of natural products in living cells and bridging the gap between biochemical binding and cellular activity.
TPP: Thermal Proteome Profiling
Principle
TPP is an extension of CETSA combined with quantitative mass spectrometry, enabling proteome-wide analysis of thermal stability shifts. It provides an unbiased view of how a compound affects protein stability across the entire proteome.
Workflow
Cells or lysates are treated with a compound, subjected to a temperature gradient, digested, labeled (often with TMT), and analyzed by high-resolution LC-MS/MS to generate protein melting curves.
Strengths
- Proteome-wide, unbiased target identification
- High quantitative accuracy
- Detects direct and indirect interactions
- Compatible with intact cells
Limitations
- High cost and technical complexity
- Large data sets requiring advanced bioinformatics
- Lower sensitivity for very low-abundance proteins
Applications
TPP is considered the gold standard for global target discovery and pathway analysis of bioactive natural products.
Comparative Overview
| Feature | DARTS | SPROX | CETSA | TPP |
| Label-free | Yes | Yes | Yes | Yes |
| Proteome coverage | Low–Medium | Medium | Low–Medium | High |
| Cellular context | Lysate | Lysate | Cells/Lysate | Cells/Lysate |
| Quantitative | Semi | Yes | Semi | Yes |
| Throughput | Low | Medium | Medium | High |
| Readout | SDS-PAGE / MS | MS (Methionine focus) | Western Blot / ELISA | Quantitative MS |
Strategic Selection for Natural Product R&D
For a natural product with an unknown mechanism, the ideal strategy usually involves a discovery-to-validation pipeline. We often recommend starting with TPP to scan the entire proteome for potential binders without any prior bias. Once candidates are identified, CETSA is utilized to confirm these interactions in live cells, while DARTS or SPROX can be employed to narrow down the specific structural domains of the binding interaction.
Partner With Us
As a company dedicated to bioactive natural products target identification, we provide integrated DARTS, SPROX, and TPP services, supported by advanced mass spectrometry, robust bioinformatics pipelines, and extensive experience in natural product research. Our platform enables seamless transition from unbiased target discovery to cellular validation, accelerating the translation of natural products into drug candidates.
