Yeast two-hybrid (Y2H) assay represents a pivotal technology for detecting binary protein-protein interactions (PPIs) in living cells. This method emerged in 1989 and exploited the modular nature of transcription factors. It provided a simple yet powerful approach to uncovering direct molecular interactions. The assay relies on Saccharomyces cerevisiae as a cellular host. Yeast cells can be engineered to express hybrid proteins that reconstitute a transcription factor only when they interact. The resultant transcription drives reporter gene expression. This arrangement yields a clear phenotypic readout. Researchers can thus screen large libraries of potential interacting partners. The Y2H assay accelerated functional genomics in the era of whole-genome sequencing. It enabled the mapping of extensive protein interaction networks. The assay also supported drug discovery efforts by identifying novel targets.
The classical Y2H system leverages the GAL4 transcription factor from yeast. GAL4 comprises two independent modules: a DNA-Binding Domain (DBD) and an Activation Domain (AD). The DBD binds specifically to upstream activating sequences (UAS) in reporter gene promoters. The AD recruits the transcriptional machinery, including RNA polymerase II. In its native form, GAL4 binds to UAS and activates the transcription of target genes in the presence of galactose. When the DBD and AD are separated, each module cannot activate transcription alone. However, when brought into proximity, they reconstitute a functional transcription factor. The Y2H assay uses two hybrid proteins: a "bait" and a "prey." The bait consists of a protein of interest fused to GAL4-DBD. The prey consists of a candidate interactor fused to GAL4-AD. Yeast cells are co-transformed with vectors encoding these fusions. If the bait and prey physically interact, the GAL4-DBD and AD modules converge. This event reconstitutes the transcription factor. The reconstituted factor binds to UAS upstream of reporter genes. Recruitment of RNA polymerase II follows. The process yields reporter gene expression. Absent interaction, the DBD and AD remain separate. No transcription occurs. Reporter genes provide a visible or selectable phenotype. lacZ: Encodes β-galactosidase. HIS3: Encodes imidazole glycerol phosphate dehydratase. ADE2: Encodes phosphoribosylaminoimidazole carboxylase. URA3: Encodes orotidine 5'-phosphate decarboxylase.
Library Screening vs. Pairwise Testing: Researchers test specific bait-prey combinations. This method is suited to confirm interactions suggested by other techniques (e.g., co-immunoprecipitation or computational prediction). Library Screening: A single bait is screened against an entire prey library. Yeast cells are co-transformed with the bait vector and the pooled prey library. Transformants are plated on selective media. Positive colonies indicate potential interactors. These colonies are isolated, and prey plasmids are recovered and sequenced to identify the interacting partner. AH109: Carries HIS3, ADE2, and lacZ reporters. Y2HGold: Contains HIS3, AUR1-C, ADE2, and MEL1 reporters. These strains are engineered to minimize endogenous transcription factor noise. They also lack proteases that degrade hybrid proteins. Lithium acetate transformation suffices for most Y2H assays. Y2H assays employ defined synthetic dropout media to select for plasmid uptake and reporter activation. Qualitative detection relies on the growth of yeast colonies on selective plates. When bait and prey interact, reporter genes enable auxotrophy rescue. Quantitative assessment uses the β-galactosidase assay. Yeast colonies or liquid cultures are subjected to a colorimetric assay using ONPG (o-nitrophenyl-β-D-galactopyranoside). The reaction yields o-nitrophenol, which absorbs at 420 nm. Researchers measure absorbance and calculate Miller Units to quantify transcriptional activation.
< p>Retransformation: Reintroduce purified bait and prey plasmids into fresh yeast cells. Co-IP serves as an orthogonal validation method. Researchers express epitope-tagged bait and prey in yeast or mammalian cells. Cell lysates are incubated with an antibody against the bait tag. The immune complex is captured on Protein A/G beads. Western blot analysis with an antibody against the prey reveals co-precipitation. Positive Co-IP corroborates Y2H results in a different biological context. It confirms that interactions occur in native membranes and multi-protein complexes. BiFC exploits fragments of fluorescent proteins (e.g., split-YFP or split-GFP). Each fragment is fused to one of the interacting proteins. Upon interaction, the fragments reassemble into a functional fluorescent protein. This event emits fluorescence detectable by microscopy or flow cytometry. BiFC offers subcellular localization information. It confirms Y2H-identified interactions in mammalian cells. Affinity Purification-Mass Spectrometry (AP/MS) complements Y2H by identifying multi-protein complexes. Researchers generate a cell line stably expressing an epitope-tagged bait. The bait and its interactors are isolated via affinity purification. Mass spectrometry then identifies co-purified proteins. AP/MS captures direct and indirect interactors within a native complex. Networking Y2H and AP/MS data provides both binary interactions and multi-component assemblies. Integrating Y2H with quantitative proteomics (e.g., SILAC, TMT labeling) yields dynamic interaction maps under various conditions.- In Saccharomyces cerevisiae, systematic array-based screening of 6,000 open reading frames (ORFs) yielded over 5,600 interactions covering 70 % of the proteome. Similar efforts in Caenorhabditis elegans, Drosophila melanogaster, and human cells have revealed thousands of interactions. These interactomes serve as blueprints for cellular wiring diagrams. They facilitate identification of network hubs, modules, and pathways.
- Y2H assays have identified interactors of disease-associated proteins. For example, the interaction between p53 and MDM2 was confirmed in Y2H. This discovery highlighted MDM2 as a therapeutic target. Screening against oncogenic Ras uncovered the Ras-Raf interaction. This knowledge spurred the development of Raf inhibitors. Library-based Y2H screens of viral proteins have identified host-pathogen interactions. Integration of Y2H data with gene expression profiles, genetic screens, and phenotypic assays creates a multi-omic view of cellular processes.
- In proteomics, Y2H data complement mass spectrometry-based methods. They provide binary interaction validation for complexes identified by affinity purification. Comprehensive maps of PPIs are essential to understand how genotype gives rise to phenotype. Existing datasets were either limited to small-scale, hypothesis-driven studies or skewed by protein expression biases.
The yeast two-hybrid (Y2H) system is a genetic technique used to determine protein-protein interactions. Interaction mapping gives clear explanations about the organization of the proteome (organisms, tissues, and cells express the full complement of proteins) into functional units. The term “two-hybrid” is coined from the two classes of hybrid or chimeric proteins. The variations in the Y2H system depend on the interaction of proteins. This method has also been utilized to identify inhibitions of four independent protein-protein interactions in screens that contained a 23,000 small molecule compound library. The Polymerase III (Pol III) transcription system allows interaction of transcription factors with proteins by activating transcription that is based on RNA polymerase II. The protein X is fused to bait (Gal4-DBD) in the traditional Y2H system. τ138p exists as one of the two transcription factors that undergoes (RNA polymerase III) Pol III-mediated transcription. The interaction that occurs between the two hybrid proteins results in reconstitution of Ire1p dimerization and activation of unfolded protein response (UPR) downstream signaling. The SOS recruitment system involves the fusion of a soluble protein X into the mammalian SOS. The RAS recruitment system involves fusion of active mammalian RAS and the soluble protein X. Both RRS and SRS help in analyzing interactions that occur between membrane or soluble preys and soluble baits. Further ReadingAll Mycology ContentTwo Hybrid Screening: Advantages and DisadvantagesGenetic Studies with S. cerevisiae and Mitotic InsightsApplications of Yeast Two-Hybrid ScreeningGenetic Studies with S.