Yeast Two-Hybrid Assay: Principles, Methods, and Pitfalls

By Dr. Zubair Khalid, DVM, MS, PhD ·

Yeast Two-Hybrid Assay: Principles, Methods, and Pitfalls

Introduction to Yeast Two-Hybrid Assay

What is Y2H?

The yeast two-hybrid (Y2H) assay is a molecular genetic method used to detect and characterize physical protein-protein interactions in the living nucleus of Saccharomyces cerevisiae. The assay exploits the modular architecture of eukaryotic transcription factors: a DNA-binding domain (DBD) that recognizes a specific upstream activation sequence (UAS) and a separate activation domain (AD) that recruits the RNA polymerase II holoenzyme to drive transcription. When a "bait" protein fused to a DBD interacts with a "prey" protein fused to an AD, the two domains are brought into proximity, reconstituting a functional transcription factor and activating reporter genes whose expression is quantifiable and selectable.

The technique was first described in 1989 by Fields and Song, who demonstrated that the interaction between the yeast proteins SNF1 and SNF4 could be detected by reconstituting GAL4 activity. This landmark work established that transcription factor function does not require the DBD and AD to reside on the same polypeptide chain, provided they are physically juxtaposed. Since then, the Y2H assay has become a cornerstone of interactomics, enabling genome-wide interaction screens and targeted validation of candidate interactions across diverse organisms.

Why use yeast for interaction studies?

S. cerevisiae offers several practical advantages for interaction studies. Its genetics are tractable: strains with defined auxotrophic markers and reporter gene integrations are readily available, and transformation with plasmid DNA is routine. Yeast grow rapidly on inexpensive defined media, and both haploid and diploid states permit elegant mating-based strategies. The eukaryotic protein folding and chaperone machinery in yeast is broadly conserved, allowing many heterologous proteins to fold correctly. Furthermore, the nuclear localization of the assay system means that interactions are detected in a near-physiological environment, albeit one that differs from the cytoplasm or membrane compartments of higher eukaryotes. The assay is also highly sensitive—interactions with dissociation constants (Kd) in the low micromolar range can be detected—and it is scalable from single pairwise tests to high-throughput library screens. These features make yeast an indispensable platform for interaction discovery, despite the caveats discussed later.

Underlying Principle and Mechanism

The GAL4 system

The most widely used Y2H system is based on the yeast transcription factor GAL4, which regulates genes involved in galactose metabolism. GAL4 is a modular protein of 881 amino acids: the N-terminal 147 residues constitute the DBD, which binds to the GAL4 UAS (a 17-bp palindromic sequence), and the C-terminal 114 residues (approximately residues 768–881) constitute the AD, which interacts with the Mediator complex and other components of the transcriptional machinery to stimulate transcription. The two domains are separated by a flexible linker and function independently; neither domain alone can activate transcription when tethered to DNA.

In the Y2H assay, the GAL4 DBD is fused to a protein of interest (the bait), and the GAL4 AD is fused to another protein (the prey). If the bait and prey interact, the AD is recruited to the UAS, and transcription of reporter genes downstream of the UAS is activated. The interaction is therefore reported as a gain-of-function phenotype: growth on selective media or enzymatic activity from a reporter gene.

Bait and prey fusion proteins

Bait and prey fusions are expressed from separate plasmids, each with its own selectable marker and promoter. The bait is typically expressed from a plasmid carrying a TRP1 auxotrophic marker, while the prey is expressed from a plasmid carrying a LEU2 marker. Both fusions are under the control of the constitutive ADH1 promoter, which drives moderate, stable expression in yeast. The bait fusion protein localizes to the nucleus because the GAL4 DBD contains a nuclear localization signal (NLS); the prey fusion, lacking a DBD, must also reach the nucleus to interact with the bait. This is generally achieved by passive diffusion or by the presence of an endogenous NLS in the prey protein, but it is a key limitation for proteins that are normally cytoplasmic or membrane-tethered.

The choice of fusion orientation (N-terminal or C-terminal) is critical. The DBD and AD must remain accessible for interaction, and the fusion must not disrupt the folding or function of the protein of interest. For this reason, both N-terminal and C-terminal fusions are often constructed and tested in parallel. Additionally, the linker region between the DBD/AD and the protein of interest should be flexible; standard vectors include a short glycine-rich linker to reduce steric hindrance.

Reporter gene activation

The readout of a Y2H assay is the activation of reporter genes placed under the control of GAL4 UAS elements. In the most common strain backgrounds (AH109, Y187, and their derivatives), three reporters are used:

  • HIS3: encodes imidazoleglycerol-phosphate dehydratase, an enzyme in histidine biosynthesis. Its expression allows growth on medium lacking histidine. The HIS3 promoter in these strains contains a weak GAL4 UAS, so growth requires a relatively strong interaction. Background growth can be suppressed by adding 3-amino-1,2,4-triazole (3-AT), a competitive inhibitor of the HIS3 enzyme, at concentrations typically ranging from 1 to 100 mM.
  • ADE2: encodes phosphoribosylaminoimidazole carboxylase, an enzyme in adenine biosynthesis. The ADE2 promoter contains a strong GAL4 UAS, so its activation requires a weaker interaction than HIS3. Additionally, ade2 mutants accumulate a red pigment in their vacuoles; colonies that activate ADE2 are white, providing a visual readout.
  • lacZ: encodes β-galactosidase from E. coli. Its expression is quantifiable using the chromogenic substrate X-gal (5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside) in a colorimetric assay, or using the chemiluminescent substrate Galacton-Star for more sensitive quantification.

The use of multiple reporters with different promoter strengths provides a semi-quantitative measure of interaction strength. A strong interaction activates all three reporters; a weak interaction may activate only ADE2 and lacZ but not HIS3. This differential readout is valuable for ranking interaction strengths and for distinguishing specific interactions from background.

Key Components and Vectors

Bait and prey vectors

Standard Y2H vectors are derived from the pGBKT7 (bait) and pGADT7 (prey) plasmids, or their predecessors pGBT9 and pGAD424. These vectors share several features:

  • pGBKT7: carries the TRP1 marker for selection in yeast, the ADH1 promoter and terminator, the GAL4 DBD (residues 1–147), a multiple cloning site (MCS), and an N-terminal c-Myc epitope tag for detection of the fusion protein by immunoblotting. The vector also contains the kanMX gene for selection in E. coli using kanamycin.
  • pGADT7: carries the LEU2 marker, the ADH1 promoter and terminator, the GAL4 AD (residues 768–881), an MCS, and an N-terminal HA epitope tag. Selection in E. coli is via ampicillin resistance.

Both vectors contain the ori and ampR (or kanR) sequences for propagation in bacteria, and the 2µ origin for high-copy replication in yeast. The choice of vector determines the selectable markers, which must be compatible with the auxotrophic markers in the yeast strain.

Yeast strains (e.g., AH109, Y187)

The two most commonly used Y2H strains are AH109 and Y187, both derived from the PJ69-2A lineage. These strains carry deletions in the endogenous GAL4 and GAL80 genes to eliminate endogenous GAL4 activity, and they contain integrated reporter genes under GAL4 UAS control:

  • AH109 (MATa): carries HIS3, ADE2, and lacZ reporters integrated at the TRP1, ADE2, and URA3 loci, respectively. It is used as the bait strain.
  • Y187 (MATα): carries lacZ and HIS3 reporters (but not ADE2) and is used as the prey strain.

The mating-based approach exploits the opposite mating types: a bait strain (AH109) is mated with a prey strain (Y187) to produce diploids that contain both plasmids. Diploid selection is achieved on medium lacking tryptophan and leucine (SD/-Trp/-Leu), and interaction is scored on medium additionally lacking histidine and/or adenine (SD/-Trp/-Leu/-His or SD/-Trp/-Leu/-His/-Ade).

Other strains, such as PJ69-4A and L40, are also used. L40 carries HIS3 and lacZ reporters but lacks ADE2; PJ69-4A carries all three reporters and is considered the gold standard for stringent selection.

Reporter genes (lacZ, HIS3, ADE2)

The reporter genes in Y2H strains are not the native yeast genes; they are chimeric constructs in which the coding sequence of the reporter is placed under the control of a minimal promoter containing GAL4 UAS elements. The specific promoter architecture determines the sensitivity of each reporter:

  • ***HIS3* reporter**: driven by the GAL1 UAS with a truncated HIS3 promoter. This reporter has relatively low basal activity, but it can be further suppressed by 3-AT. Growth on SD/-His medium typically requires an interaction with a Kd in the low micromolar range.
  • ***ADE2* reporter**: driven by the GAL2 UAS. This reporter has the highest sensitivity and can detect weak interactions. The red/white colony phenotype provides an immediate visual readout.
  • ***lacZ* reporter**: driven by the GAL1 UAS. β-galactosidase activity is quantified by a colorimetric assay using ONPG (o-nitrophenyl-β-D-galactoside) as substrate, or by a filter lift assay using X-gal. The assay is performed on permeabilized yeast cells and provides a quantitative measure of transcriptional activation.

The use of three reporters with different sensitivities is essential for distinguishing true interactions from background and for estimating relative interaction strength.

Step-by-Step Protocol Overview

Constructing bait and prey

  1. Clone the gene of interest into the MCS of pGBKT7 (bait) and pGADT7 (prey) in the correct reading frame. Verify the fusion by Sanger sequencing across the junction.
  2. Test for autoactivation: Transform the bait plasmid alone into AH109 and plate on SD/-Trp/-His and SD/-Trp/-Ade. If colonies grow, the bait alone activates the reporters (autoactivation), and the assay cannot proceed without modification (see Common Pitfalls).
  3. Confirm protein expression: Prepare whole-cell lysates from yeast expressing the bait and prey fusions and detect the fusion proteins by immunoblotting using anti-c-Myc (bait) and anti-HA (prey) antibodies. Lack of expression is a common cause of false negatives.

Transformation and mating

  1. Transform bait into AH109 (MATa) and prey into Y187 (MATα) using the lithium acetate/single-stranded carrier DNA/PEG method. Plate transformants on SD/-Trp (bait) and SD/-Leu (prey).
  2. Mating: Pick a single colony of each transformant and co-inoculate into 1 mL of YPD (rich medium) in a microcentrifuge tube. Incubate at 30°C with shaking for 4–6 hours, then plate 100 µL onto SD/-Trp/-Leu to select for diploids. Alternatively, perform a "patch mating" by replica-plating bait and prey patches onto a YPD plate, incubating overnight, and then replica-plating onto SD/-Trp/-Leu.
  3. Diploid selection: Incubate SD/-Trp/-Leu plates at 30°C for 2–3 days. Diploid colonies should appear; these contain both bait and prey plasmids.

Selection and screening

  1. Interaction selection: Replica-plate or streak diploid colonies onto SD/-Trp/-Leu/-His and SD/-Trp/-Leu/-His/-Ade plates. For the HIS3 reporter, include 3-AT at a concentration determined empirically (typically 1–10 mM) to suppress background growth.
  2. Incubate at 30°C for 3–7 days. Growth on SD/-Trp/-Leu/-His/-Ade indicates a strong interaction; growth only on SD/-Trp/-Leu/-His indicates a weaker interaction.
  3. β-galactosidase assay: Perform a filter lift assay or liquid culture assay to quantify lacZ activation. For the filter lift assay, transfer colonies to a nitrocellulose membrane, permeabilize by freezing in liquid nitrogen, and incubate in Z-buffer (60 mM Na₂HPO₄, 40 mM NaH₂PO₄, 10 mM KCl, 1 mM MgSO₄, pH 7.0) containing 1 mg/mL X-gal and 0.27% β-mercaptoethanol at 30°C. Blue color develops within 30 minutes to 8 hours depending on interaction strength.

Confirmation of positive clones

  1. Retest in fresh diploids: Isolate plasmid DNA from positive colonies, transform into E. coli, and retransform the purified bait and prey plasmids into fresh AH109 and Y187 strains. Repeat the mating and selection to confirm that the interaction is reproducible and not due to a mutation or contamination.
  2. Test specificity: Perform a "bait swap" or use a non-interacting protein (e.g., lamin C or an irrelevant protein) as a negative control. A true interaction should not be detected with the negative control bait.
  3. Quantify interaction strength: Use a liquid β-galactosidase assay to measure activity in Miller units. This provides a numerical value that can be compared across experiments.

Applications of Yeast Two-Hybrid Assay

Interaction mapping

Y2H is widely used to map interaction domains within a protein. By generating a series of N-terminal or C-terminal truncation mutants of the bait and testing them against the full-length prey (or vice versa), the minimal region required for interaction can be defined. This approach is particularly useful for identifying the binding sites of modular domains such as SH3, PDZ, or WW domains. For example, a bait spanning the full-length protein can be truncated into overlapping fragments of 50–100 residues, and each fragment is tested for interaction with the prey. The results define the interaction interface at the resolution of the fragment size. This information is valuable for designing mutagenesis experiments to disrupt the interaction specifically.

Library screening

The most powerful application of Y2H is the screening of cDNA or genomic libraries to identify novel interaction partners. In a typical library screen, a bait is expressed in AH109, and a prey library (e.g., a cDNA library from a tissue of interest, cloned into pGADT7) is transformed into Y187. The two strains are mated, and diploids are plated on selective medium. Colonies that grow on SD/-Trp/-Leu/-His/-Ade are candidate interactors. The prey plasmid is then recovered from each colony, sequenced, and the identity of the prey protein is determined. Library screens can identify dozens to hundreds of candidate interactors, which must then be validated by independent methods.

Key considerations for library screening include the complexity of the library (typically 10⁶–10⁷ independent clones), the representation of full-length versus partial clones, and the need to suppress autoactivation with 3-AT. False positives are a significant issue in library screens, and rigorous filtering (e.g., requiring activation of all three reporters and retesting in fresh cells) is essential.

Validation of predicted interactions

Y2H is also used to validate interactions predicted by computational methods, co-expression analyses, or other high-throughput screens. For example, if a protein interaction is predicted by AlphaFold or by co-immunoprecipitation-mass spectrometry, Y2H can provide independent confirmation that the two proteins interact directly (i.e., without an intermediary bridging protein). This is a key advantage of Y2H over co-immunoprecipitation, which detects complexes that may contain multiple proteins. However, because Y2H detects interactions in the yeast nucleus, a positive result does not prove that the interaction occurs in the native cellular context; it only demonstrates that the two proteins have the intrinsic capacity to bind each other.

Advantages and Limitations

Advantages

  • In vivo context: Interactions are detected in a living eukaryotic cell, allowing proper protein folding and, in some cases, post-translational modifications that may be required for interaction.
  • High sensitivity: Y2H can detect weak or transient interactions (Kd in the low micromolar range) that may be missed by biochemical methods such as co-immunoprecipitation.
  • Direct interaction: Y2H detects binary interactions, providing evidence that two proteins bind directly rather than as part of a larger complex.
  • Scalability: The assay is amenable to high-throughput screening, enabling genome-wide interaction mapping.
  • Cost-effectiveness: Y2H requires only standard microbiology equipment and reagents, making it accessible to most laboratories.
  • Quantitative readout: Reporter gene activity provides a semi-quantitative measure of interaction strength.

Limitations and challenges

  • False positives: Y2H is prone to false positives arising from autoactivation, "sticky" proteins (e.g., proteins with exposed hydrophobic patches), or interactions that occur only in the artificial context of the yeast nucleus.
  • False negatives: Many true interactions are missed because the proteins do not fold correctly in yeast, require post-translational modifications absent in yeast, or are toxic when overexpressed. Membrane proteins are particularly problematic because they cannot fold properly in the nucleus.
  • Nuclear localization requirement: Both bait and prey must localize to the nucleus. Proteins with strong cytoplasmic retention signals or transmembrane domains are refractory to standard Y2H.
  • Post-translational modifications: Yeast lacks many mammalian-specific modifications (e.g., phosphorylation by tyrosine kinases, glycosylation), so interactions dependent on these modifications will be missed.
  • Toxicity: Overexpression of some proteins is toxic to yeast, preventing growth and yielding false negatives.

Common Pitfalls and Troubleshooting

Autoactivation

Autoactivation occurs when the bait fusion protein alone activates transcription of the reporter genes, independent of prey interaction. This is caused by an intrinsic transcriptional activation domain in the bait protein or by nonspecific DNA binding. Autoactivation is detected by transforming the bait alone into AH109 and plating on SD/-Trp/-His and SD/-Trp/-Ade. If colonies grow, the bait is autoactivating.

Solutions:

  • Use a higher concentration of 3-AT (up to 100 mM) to suppress HIS3 background. This is often sufficient for weak autoactivation.
  • Truncate the bait to remove the activating region. Identify the minimal non-activating fragment by testing deletion mutants.
  • Use a different DBD, such as the LexA DBD, which has different DNA-binding specificity and may reduce autoactivation.
  • Switch to a strain with a weaker HIS3 reporter or use only the ADE2 reporter for selection.

False positives

False positives are interactions detected by Y2H that do not occur in the native cellular context. Common causes include:

  • Sticky proteins: Proteins with exposed hydrophobic surfaces or low complexity regions can interact nonspecifically with many prey proteins. These are often identified by their appearance in many unrelated screens.
  • Overexpression artifacts: The high copy number of Y2H plasmids leads to overexpression, which can drive weak, non-physiological interactions.
  • Bridging by yeast proteins: A yeast protein may bridge the bait and prey, creating an apparent direct interaction.

Solutions:

  • Require activation of all three reporters (HIS3, ADE2, lacZ).
  • Retest positive clones in fresh cells with purified plasmids.
  • Use a "bait swap" or test against a panel of unrelated baits to assess specificity.
  • Perform a co-immunoprecipitation assay to validate the interaction in a native context.

False negatives

False negatives are true interactions that are missed by Y2H. Common causes include:

  • Poor protein expression: The bait or prey may be expressed at very low levels or may be degraded. Verify expression by immunoblotting.
  • Improper folding: The fusion protein may not fold correctly, especially if the protein of interest is large or contains multiple domains.
  • Steric hindrance: The DBD or AD may block the interaction interface. Test both N-terminal and C-terminal fusions.
  • Toxicity: The fusion protein may be toxic to yeast, leading to slow growth and poor transformation efficiency.
  • Requirement for post-translational modifications: If the interaction requires phosphorylation or other modifications not present in yeast, Y2H will fail.

Solutions:

  • Test both N-terminal and C-terminal fusions for both bait and prey.
  • Use a lower copy number vector (e.g., centromeric plasmids) to reduce toxicity.
  • Use a different yeast strain or growth temperature (e.g., 25°C instead of 30°C) to improve protein folding.
  • Consider using a Yeast 3 Hybrid System if a bridging molecule (e.g., a small molecule or a third protein) is required for the interaction.

Optimization tips

  • Titrate 3-AT: Determine the minimal concentration of 3-AT that suppresses background growth of the bait alone. This is done by plating the bait strain on SD/-Trp/-His plates containing increasing concentrations of 3-AT (0, 1, 5, 10, 25, 50, 100 mM) and identifying the lowest concentration that prevents growth.
  • Optimize mating efficiency: Use fresh cultures (OD₆₀₀ = 0.8–1.0) and a 4:1 ratio of prey to bait cells. Mating efficiency should be >5% for library screens.
  • Include proper controls: Always include a known interacting pair (e.g., p53 with SV40 large T antigen) as a positive control, and a non-interacting pair (e.g., p53 with lamin C) as a negative control.
  • Verify plasmid integrity: After screening, recover plasmids and confirm that the bait and prey inserts are intact and in the correct reading frame.

Variations and Alternatives

Yeast one-hybrid

The yeast one-hybrid (Y1H) assay is a related technique used to detect protein-DNA interactions. In Y1H, a transcription factor (the "bait") is fused to the GAL4 AD, and a DNA sequence of interest (the "prey") is placed upstream of a reporter gene. If the transcription factor binds the DNA sequence, the AD is recruited to the promoter and activates transcription. Y1H is used to identify transcription factors that bind specific promoter elements or to map the DNA-binding specificity of a known transcription factor.

Yeast three-hybrid

The Yeast 3 Hybrid System extends Y2H to detect interactions that require a third molecule, such as a small-molecule ligand, an RNA, or a bridging protein. In the most common version, a bait is fused to the DBD, a prey is fused to the AD, and a third component is expressed in the same cell. If the third component bridges the bait and prey, transcription is activated. This system is useful for studying ligand-dependent interactions, RNA-protein interactions, and interactions that require a post-translational modification enzyme.

Split-ubiquitin assay

The split-ubiquitin assay (also called the mating-based split-ubiquitin system, or mbSUS) detects interactions between membrane proteins. It is based on the observation that ubiquitin can be split into two fragments (N-terminal half, Nub, and C-terminal half, Cub) that spontaneously reassemble when brought into proximity. In this assay, one protein is fused to Cub followed by a transcription factor, and the other protein is fused to Nub. If the two proteins interact, ubiquitin reassembles, and ubiquitin-specific proteases cleave the fusion, releasing the transcription factor, which then enters the nucleus and activates a reporter gene. This system allows detection of interactions at the plasma membrane, endoplasmic reticulum, or other membranes, overcoming the nuclear localization limitation of standard Y2H.

Summary and Best Practices

Key takeaways

The yeast two-hybrid assay is a powerful and versatile method for detecting protein-protein interactions. Its success depends on rigorous experimental design, careful attention to controls, and an understanding of the system's limitations. The following best practices will maximize the reliability of Y2H results:

  1. Always test for autoactivation before proceeding with a screen or pairwise test.
  2. Verify protein expression by immunoblotting for both bait and prey fusions.
  3. Use multiple reporters with different sensitivities to distinguish strong from weak interactions.
  4. Include positive and negative controls in every experiment.
  5. Retest positive clones in fresh cells with purified plasmids to eliminate false positives.
  6. Validate interactions by an independent method, such as co-immunoprecipitation or an in vitro pull-down assay.

Validation strategies

A positive Y2H result should be validated by at least one independent method before being reported as a genuine interaction. The most common validation approaches include:

  • Co-immunoprecipitation (co-IP): Express both proteins (e.g., with FLAG and HA tags) in mammalian cells, immunoprecipitate one, and detect the other by immunoblotting. This confirms the interaction in a native cellular context. See Co-immunoprecipitation Assay for a detailed protocol.
  • In vitro pull-down: Express one protein as a GST fusion in E. coli, immobilize it on glutathione-Sepharose beads, and test for binding of the other protein (expressed in vitro or in E. coli).
  • Fluorescence resonance energy transfer (FRET) or bioluminescence resonance energy transfer (BRET): These live-cell assays detect interactions in real time and provide spatial information.
  • Genetic complementation: In some cases, the interaction can be confirmed by a functional assay, such as suppression of a mutant phenotype.

Frequently Asked Questions

What is a yeast two-hybrid assay?

The yeast two-hybrid (Y2H) assay is a molecular genetic method for detecting physical interactions between two proteins. It is performed in the yeast Saccharomyces cerevisiae and relies on the reconstitution of a functional transcription factor when two fusion proteins interact. The assay is used to identify novel interaction partners, map interaction domains, and validate predicted interactions.

How does the yeast two-hybrid assay work?

The assay uses two fusion proteins: a bait protein fused to a DNA-binding domain (DBD) and a prey protein fused to an activation domain (AD). If the bait and prey interact, the DBD and AD are brought into proximity, reconstituting a functional transcription factor that activates reporter genes. The activation of these reporters (e.g., HIS3, ADE2, lacZ) provides a selectable and quantifiable readout of the interaction.

What are the main steps in a yeast two-hybrid assay protocol?

The main steps are: (1) cloning the genes of interest into bait and prey vectors; (2) testing for autoactivation of the bait; (3) transforming the bait into a MATa strain and the prey into a MATα strain; (4) mating the two strains to produce diploids; (5) selecting diploids on medium lacking tryptophan and leucine; (6) screening for interaction on medium lacking histidine and/or adenine; (7) confirming positive clones by β-galactosidase assay and retesting in fresh cells.

What causes false positives in yeast two-hybrid assays?

False positives can arise from autoactivation (the bait alone activates transcription), "sticky" proteins that interact nonspecifically, overexpression artifacts, or bridging by endogenous yeast proteins. They are minimized by using multiple reporters, including negative controls, and retesting positive clones in fresh cells.

Can yeast two-hybrid detect interactions with membrane proteins?

Standard Y2H is generally not suitable for membrane proteins because they cannot fold correctly in the nucleus and often fail to localize there. However, the split-ubiquitin assay (mbSUS) is a variation that detects interactions between membrane proteins at their native location. Alternatively, truncated soluble domains of membrane proteins can sometimes be used in standard Y2H.

What is the difference between yeast two-hybrid and co-immunoprecipitation?

Y2H detects binary interactions in the yeast nucleus and provides evidence that two proteins can bind directly. Co-immunoprecipitation (co-IP) detects interactions in the native cellular context and can identify complexes containing multiple proteins, but it cannot distinguish direct from indirect interactions. Y2H is more sensitive for weak interactions, while co-IP is more physiologically relevant. The two methods are complementary and are often used together for validation.

How do you confirm a positive result from a yeast two-hybrid screen?

A positive result is confirmed by: (1) isolating the prey plasmid and retransforming it into fresh yeast along with the bait; (2) retesting the interaction in fresh diploids; (3) testing the prey against an unrelated bait to assess specificity; (4) validating the interaction by an independent method such as co-immunoprecipitation or in vitro pull-down.

Key Takeaways

  • The yeast two-hybrid assay detects direct protein-protein interactions by reconstituting a split transcription factor in yeast.
  • The GAL4 system uses a DBD-bait fusion and an AD-prey fusion; interaction activates HIS3, ADE2, and lacZ reporters.
  • Key components include the bait vector (pGBKT7), prey vector (pGADT7), and yeast strains AH109 and Y187.
  • The workflow involves cloning, transformation, mating, selection on dropout media, and confirmation by β-galactosidase assay.
  • Y2H is powerful for interaction mapping, library screening, and validation, but it is prone to false positives and false negatives.
  • Autoactivation, toxicity, and poor protein expression are common pitfalls that require troubleshooting.
  • Always validate Y2H results with an independent method such as co-immunoprecipitation.
  • Variations like the yeast three-hybrid and split-ubiquitin assays extend the utility of the system to RNA-mediated and membrane protein interactions.

Further Reading

  • Paiano A et al. Yeast Two-Hybrid Assay to Identify Interacting Proteins. Current protocols in protein science. 2019. PubMed 30133175
  • Coates PJ, Hall PA. The yeast two-hybrid system for identifying protein-protein interactions. The Journal of pathology. 2003. PubMed 12474220
  • Duarte CEM, Euclydes NC. Protein-Protein Interaction via Two-Hybrid Assay in Yeast. Methods in molecular biology (Clifton, N.J.). 2024. PubMed 37987907
  • Benincore-Flórez E et al. Iduronate-2-sulfatase interactome: validation by yeast two-hybrid assay. Heliyon. 2022. PubMed 35284671
  • Gnanasekaran P, Pappu HR. Yeast Two-Hybrid Technique to Identify Protein-Protein Interactions. Methods in molecular biology (Clifton, N.J.). 2023. PubMed 37450132
  • Xin X et al. A yeast two-hybrid assay reveals CMYA1 interacting proteins. Comptes rendus biologies. 2017. PubMed 28728781

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