Determination of Tripartite Interaction between Two Monomers of a MADS-box Transcription Factor and a Calcium Sensor Protein by BiFC-FRET-FLIM Assay
December 25th, 2021
3.2K views
Here we present, a method to visualize ternary complex formation between three protein partners using fluorescent-tagged proteins by BiFC based FRET-FLIM assay. This method is valuable for studying protein-protein interaction complexes in vivo.
Overview
This article demonstrates a methodology for visualizing in vivo tripartite protein-protein interactions using Bimolecular Fluorescence Complementation (BiFC) combined with Förster Resonance Energy Transfer (FRET) and Fluorescence Lifetime Imaging (FLIM). The protocol is applied in Nicotiana benthamiana leaves to study interactions among proteins, including homodimerization and interaction with a calcium sensor protein, providing quantitative evidence of complex formation in living plant cells.
Key Study Components
Area of Science
- Molecular biology
- Plant biology
- Protein-protein interaction analysis
Background
- Protein-protein interactions are fundamental to cellular processes such as signal transduction, development, and metabolic regulation.
- Transcription factors and other proteins often form complexes that influence gene expression and cellular localization.
- Traditional methods may not effectively capture dynamic or multi-protein interactions in vivo.
- BiFC and FRET-FLIM provide sensitive approaches to visualize and quantify these interactions in living cells.
Purpose of Study
- To establish a protocol for detecting and quantifying tripartite protein interactions in living plant cells.
- To combine BiFC and FRET-FLIM for robust validation of protein complex formation.
- To demonstrate the method using proteins known to homodimerize and interact with a calcium sensor protein.
Methods Used
- Cloning of genes of interest into entry and destination vectors.
- Transformation of Agrobacterium tumefaciens GV3101 with BiFC and FRET constructs.
- Agroinfiltration of Nicotiana benthamiana leaves with bacterial cultures containing the constructs and P19 strain to prevent transgene silencing.
- Confocal microscopy and FLIM to measure fluorescence lifetimes and FRET efficiency between tagged proteins.
Main Results
- Successful reconstitution of YFP fluorescence via BiFC indicated interaction between two protein partners.
- FRET-FLIM measurements showed a reduction in donor (CFP) fluorescence lifetime upon interaction with acceptor (YFP), confirming protein-protein interactions.
- Tripartite interactions were validated by observing further reduction in CFP lifetime when all three proteins were present, with FRET efficiencies around 55-56%.
- The protocol enabled visualization and quantification of complex formation and localization in living plant cells.
Conclusions
- The BiFC-based FRET-FLIM assay is a powerful tool for studying tripartite protein-protein interactions in vivo.
- This method provides quantitative and spatial information about protein complexes in plant cells.
- The approach is broadly applicable to protein interaction studies in both plant and animal systems.
Frequently Asked Questions
What is the main advantage of combining BiFC with FRET-FLIM?
Combining BiFC with FRET-FLIM allows for the visualization and quantitative measurement of tripartite protein-protein interactions in living cells, providing both spatial and interaction-specific information.
Why was Nicotiana benthamiana used as the model system?
Nicotiana benthamiana is commonly used for transient expression studies in plants due to its amenability to agroinfiltration and robust expression of introduced genes.
How does the BiFC system work in this protocol?
Two proteins are tagged with split YFP fragments; their interaction reconstitutes functional YFP, which serves as a FRET acceptor for the third protein tagged with CFP (the FRET donor).
What does a reduction in CFP fluorescence lifetime indicate?
A reduction in CFP fluorescence lifetime upon co-expression with YFP-tagged partners indicates energy transfer due to close proximity, confirming protein-protein interaction.
How is FRET efficiency calculated in this assay?
FRET efficiency is calculated by comparing the fluorescence lifetime of the donor (CFP) in the presence and absence of the acceptor (YFP), using FLIM data and standard equations.
Can this protocol be adapted for animal systems?
Yes, the BiFC-based FRET-FLIM approach is applicable to both plant and animal systems for studying complex protein interactions in vivo.
What controls are important for this experiment?
Controls include expressing the donor alone, donor with acceptor, and all three partners to distinguish specific interactions and validate FRET efficiency measurements.
Executive Industry Relevance
Quantitative mapping of tripartite protein-protein interactions is critical for de-risking target validation and clarifying mechanistic pathways in early discovery. The BiFC-FRET-FLIM assay enables direct visualization and measurement of complex formation in living cells, supporting predictive confidence in functional protein assemblies. This capability strengthens portfolio triage by providing robust evidence for multi-protein interactions relevant to transcriptional regulation and signaling.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Enables direct interrogation of multi-protein complex formation in live cellular contexts.
- Supports mechanistic de-risking by confirming physical interactions among candidate targets and cofactors.
- Provides quantitative FRET efficiency data to inform target engagement and pathway mapping.
- Facilitates functional validation of transcription factor assemblies and signaling nodes.
Screening & Assay Development
- Establishes validated cellular systems for downstream screening of modulators affecting protein complex formation.
- Delivers reproducible, quantitative readouts of interaction strength via fluorescence lifetime measurements.
- Enables standardization of assay conditions for high-content screening platforms.
- Supports scalability and reuse across diverse protein interaction studies.
Translational & Preclinical Research
- Aligns with disease-relevant systems by enabling in vivo analysis of protein complexes in plant or animal models.
- Provides continuity from discovery through preclinical validation by tracking complex localization and dynamics.
- Reduces translational risk by confirming functional assemblies in physiologically relevant contexts.
Pipeline & Workflow Integration
This method integrates at the interface of early discovery and lead identification, bridging hypothesis-driven target validation with quantitative assay development for complex protein interactions.
- Discovery Biology: Supports hypothesis testing and pathway clarification by quantifying tripartite interactions in live cells.
- Screening: Delivers reproducible, quantitative outputs suitable for compound screening targeting protein complexes.
- Analytics: Provides fluorescence lifetime and FRET efficiency measurements for robust comparative analysis.
- Translational Research: Enables assessment of protein complex formation in disease-relevant or model systems.
- Enterprise Reuse: Offers a reusable platform for diverse protein interaction and localization studies across R&D programs.
Operational & Enterprise Impact
- Scientific Value: Increases predictive confidence and reduces mechanistic ambiguity in target validation.
- Operational Value: Standardizes quantitative interaction assays for reproducibility and scalability.
- Strategic Value: Informs go/no-go decisions and enhances capital efficiency by clarifying functional assemblies early.
- Portfolio Impact: Supports risk-adjusted prioritization and advancement of validated targets and pathways.
Implementation Considerations
- Requires expertise in molecular cloning, fluorescence microscopy, and quantitative imaging analysis.
- Demands access to confocal microscopes with FLIM capabilities and appropriate analytical software.
- Necessitates cross-team standardization of assay conditions and controls for reproducibility.
- Adaptable to various model systems, but optimization may be needed for different cell types or organisms.
- Dependent on robust expression and detection of tagged protein partners in the chosen system.
Biopharma FAQs
Why is null hypothesis testing critical in BiFC-FRET-FLIM target validation?
Null hypothesis testing in BiFC-FRET-FLIM ensures that observed reductions in donor fluorescence lifetime are statistically significant, confirming that protein-protein interactions are not due to random proximity or background signal. This rigor supports confident target validation and reduces false positives in early discovery. Quantitative thresholds derived from lifetime measurements provide objective criteria for interaction assessment.
How does independent variable isolation enhance discovery in FRET-FLIM assays?
Isolating variables such as individual protein expression or specific construct combinations allows teams to attribute changes in fluorescence lifetime directly to defined interactions. This clarity is essential for mapping mechanistic pathways and distinguishing true tripartite interactions from binary or nonspecific associations in the discovery pipeline.
What do quantitative dependent variable measurements enable in this workflow?
Quantitative measurements of donor fluorescence lifetime and FRET efficiency enable precise comparison of interaction strengths across conditions, constructs, or compounds. These outputs support data-driven decisions in target validation, assay optimization, and lead identification by providing reproducible, interpretable metrics.
Why are replication requirements important for cross-functional collaboration in BiFC-FRET-FLIM?
Replication ensures that observed protein-protein interactions are robust and reproducible across experiments, operators, and laboratories. This reliability is essential for cross-functional teams to trust the data, align on mechanistic hypotheses, and advance validated targets through the R&D pipeline.
What statistical analysis capabilities are required before implementing FRET-FLIM assays?
Teams must be equipped to perform statistical comparisons of fluorescence lifetimes, calculate FRET efficiencies, and assess significance against appropriate controls. These analytical capabilities are necessary to validate interaction specificity and support rigorous decision-making in biopharma R&D.
Chapters in this video
0:08
Introduction
1:17
Cloning of Genes in Entry and Destination Vectors
2:01
Growth Conditions for Nicotiana benthamiana Plants
3:05
Prepare Bacterial Strains for Agroinfiltration
6:20
Prepare Slides for Fluorescence Visualization
6:45
Fret-Flim Analysis Using a Confocal Laser Scanning Microscope
10:51
Fret-Flim Analysis Using a Confocal Laser Scanning Microscope: The Tripartite Interaction
12:31
Representative Results
13:42
Conclusion
