Three-Fragment Fluorescence Complementation Coupled with Photoactivated Localization Microscopy for Nanoscale Imaging of Ternary Complexes

Comprehensive Review on Bimolecular Fluorescence Complementation and Its Application in Deciphering Protein-Protein Interactions in Cell Signaling Pathways

Signaling pathways are responsible for transmitting information between cells and regulating cell growth, differentiation, and death. Proteins in cells form complexes by interacting with each other through specific structural domains, playing a crucial role in various biological functions and cell signaling pathways. Protein-protein interactions (PPIs) within cell signaling pathways are essential for signal transmission and regulation. The spatiotemporal features of PPIs in signaling pathways are crucial for comprehending the regulatory mechanisms of signal transduction. Bimolecular fluorescence complementation (BiFC) is one kind of imaging tool for the direct visualization of PPIs in living cells and has been widely utilized to uncover novel PPIs in various organisms. BiFC demonstrates significant potential for application in various areas of biological research, drug development, disease diagnosis and treatment, and other related fields. This review systematically summarizes and analyzes the technical advancement of BiFC and its utilization in elucidating PPIs within established cell signaling pathways, including TOR, PI3K/Akt, Wnt/β-catenin, NF-κB, and MAPK. Additionally, it explores the application of this technology in revealing PPIs within the plant hormone signaling pathways of ethylene, auxin, Gibberellin, and abscisic acid. Using BiFC in conjunction with CRISPR-Cas9, live-cell imaging, and ultra-high-resolution microscopy will enhance our comprehension of PPIs in cell signaling pathways.

Highly Efficient Fluorescent Probe to Visualize Protein Interactions at the Superresolution

Superresolution microscopy (SR microscopy) of protein-protein interactions (PPIs) occurring in subcellular structures is essential for understanding cellular functions. However, a powerful and useful technology for SR microscopy of PPIs remains elusive. Here, we develop a highly efficient photoconvertible fluorescent probe, named split-Dendra2, for SR microscopy of PPIs in the cell. We found that split-Dendra2 enables a highly efficient detection of PPIs, making it possible to perform SR microscopy of PPIs with high spatial resolution and high image reconstruction fidelity. We demonstrate the utility of split-Dendra2 by visualizing PPIs occurring in small subcellular structures at the superresolution, such as clathrin-coated pits and focal adhesions, which cannot be visualized by the existing tools. Split-Dendra2 offers a powerful and useful tool that greatly expands the possibility of SR microscopy and can contribute to revealing the function of PPIs at the nanoscale resolution.

Detecting and measuring of GPCR signaling - comparison of human induced pluripotent stem cells and immortal cell lines

G protein-coupled receptors (GPCRs) are a large family of transmembrane proteins that play a major role in many physiological processes, and thus GPCR-targeted drug development has been widely promoted. Although research findings generated in immortal cell lines have contributed to the advancement of the GPCR field, the homogenous genetic backgrounds, and the overexpression of GPCRs in these cell lines make it difficult to correlate the results with clinical patients. Human induced pluripotent stem cells (hiPSCs) have the potential to overcome these limitations, because they contain patient specific genetic information and can differentiate into numerous cell types. To detect GPCRs in hiPSCs, highly selective labeling and sensitive imaging techniques are required. This review summarizes existing resonance energy transfer and protein complementation assay technologies, as well as existing and new labeling methods. The difficulties of extending existing detection methods to hiPSCs are discussed, as well as the potential of hiPSCs to expand GPCR research towards personalized medicine.

Near-Infrared Luciferase Complementation Assay with Enhanced Bioluminescence for Studying Protein-Protein Interactions and Drug Evaluation Under Physiological Conditions

Identification of protein-protein interactions (PPIs) that occur in various cellular processes helps to reveal their potential molecular mechanisms, and there is still an urgent need to develop the assays to explore PPIs in living subjects. Here, we reported a near-infrared split luciferase complementation assay (SLCA) with enhanced bioluminescence produced by cleaving a luciferase, Akaluc, for exploring and visualizing PPIs in living cells and live mice. Compared with the previously developed and widely used red SLCA based on split firefly luciferase (Fluc-SLCA), the signal intensities for PPI recognition in living cells and live mice of the Akaluc-SLCA increased by ∼3.79-fold and ∼18.06-fold in the measured condition, respectively. Additionally, the interactions between the nucleocapsid protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and cellular RNA processing proteins were identified, and the drug evaluation assays were also performed in living cells using Akaluc-SLCA. This study provides a new tool in the near-infrared region for the identification of PPIs in living cells and in vivo and new information for the understanding and treatment of SARS-CoV-2.

The smallest near-infrared fluorescence complementation system for imaging protein-protein and RNA-protein interactions

Bimolecular fluorescence complementation (BiFC) and its derivative molecular biosensor systems provide effective tools for visualizing biomolecular interactions. The introduction of red and near-infrared fluorescence emission proteins has expanded the spectrum of signal generating modules, enabling BiFC for in vivo imaging. However, the large size of the signal module of BiFC can hinder the interaction between proteins under investigation. In this study, we constructed the near-infrared BiFC and TriFC systems by splitting miRFP670nano, the smallest cyanobacteriochrome-evolved phytochrome available. The miRFP670nano-BiFC sensor system identified and enabled visualization of protein–protein interactions in living cells and live mice, and afforded a faster maturation rate and higher photostability and cellular stability when compared with those of reported near-infrared BiFC systems. We used the miRFP670nano-BiFC sensor system to identify interactions between the nucleocapsid (N) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and cellular stress granule proteins in living cells and found that the N protein downregulated the expression level of granule protein G3BP1. With the advantages of small size and long wavelength emission of the signal module, the proposed molecular biosensor system should be suitable for various applications in cell imaging studies.

The smallest near-infrared fluorescence complementation system for imaging protein–protein and RNA–protein interactions in living cells and live mice.

Rational engineering and synthetic applications of a high specificity BiFC probe derived from Springgreen-M

A high signal-to-noise (S/N) ratio BiFC assay was developed for efficient detection and flexible visualization of protein–protein interactions under physiological conditions in live cells.

Nanoscale Imaging of RNA-Protein Interactions with a Photoactivatable Trimolecular Fluorescence Complementation System

Imaging RNA-protein interaction in the cellular space with single molecule sensitivity is attractive for studying gene expression and regulation, but remains a challenge. In this study, we reported a photoactivatable trimolecular fluorescence complementation (TriFC) system based on fluorescent protein, mIrisFP, to identify and visualize RNA-protein interactions in living mammalian cells. We also combined this TriFC system with photoactivated localization microscopy (PALM), named the TriFC-PALM technique, which allowed us to image the RNA-protein interactions with single molecule sensitivity. Using this TriFC-PALM technique, we identified the actin-bundling protein, FSCN1, specifically interacting with the HOX Transcript Antisense RNA (HOTAIR). The TriFC-PALM imaging acquired a higher resolution compared with the traditional method of total internal reflection (TIRF) imaging. The TriFC-PALM thus provides a useful tool for imaging and identifying the RNA-protein interactions inside cells at the nanometer scale.

TagBiFC technique allows long-term single-molecule tracking of protein-protein interactions in living cells

Protein-protein interactions (PPIs) are critical for cellular activity regulation. Visualization of PPIs using bimolecular fluorescence complementation (BiFC) techniques helps to understand how PPIs implement their functions. However, current BiFC is based on fluorescent proteins and the brightness and photostability are suboptimal for single molecule tracking experiments, resulting in either low spatiotemporal resolution or incapability of tracking for extended time course. Here, we developed the TagBiFC technique based on split HaloTag, a self-labeling tag that could conjugate an organic dye molecule and thus offered better brightness and photostability than fluorescent proteins for PPI visualization inside living cells. Through screening and optimization, we demonstrated that the reconstituted HaloTag exhibited higher localization precision and longer tracking length than previous methods. Using TagBiFC, we reveal that the dynamic interactions of transcription factor dimers with chromatin DNA are distinct and closely related to their dimeric states, indicating a general regulatory mechanism for these kinds of transcription factors. In addition, we also demonstrated the advantageous applications of TagBiFC in single nucleosome imaging, light-burden imaging of single mRNA, low background imaging of cellular structures. We believe these superior properties of our TagBiFC system will have broad applications in the studies of single molecule imaging inside living cells.

A tandem near-infrared fluorescence complementation system with enhanced fluorescence for imaging protein-protein interactions in vivo

Bimolecular fluorescence complementation (BiFC) is an effective tool for visualizing protein-protein interactions (PPIs). However, a BiFC system with long wavelength and high fluorescence intensity is yet to be developed for in vivo imaging. In this study, we constructed a tandem near-infrared BiFC (tBiFC) system by splitting a near-infrared phytochrome, IFP2.0. This system allows the identification and visualization of PPIs in live cells and living mice. The photophysical properties of the complementary fluorescence of the tBiFC system were similar to those of its parent protein IFP2.0, but the intensity was twice that of a single-copy IFP2.0-based BiFC system. Compared with previously reported near infrared BiFC systems-iRFP-BiFC and IFP1.4-BiFC-the signal intensity of the tBiFC system increased by ~1.48- and ~400-fold for weak PPIs in living cells, and ~1.51- and ~8-fold for strong PPIs, respectively. When applied to imaging PPIs in live mice, the complementary fluorescence intensity of the tBiFC system was also significantly higher than that of the other near-infrared BiFC systems. The use of this bright phytochrome in a tandem arrangement constitutes a powerful tool for imaging PPIs in the near infrared region.

Genetically Encodable Fluorescent and Bioluminescent Biosensors Light Up Signaling Networks

Cell signaling networks are intricately regulated in time and space to determine the responses and fates of cells to different cues. Genetically encodable fluorescent and bioluminescent biosensors enable the direct visualization of these spatiotemporal signaling dynamics within the native biological context, and have therefore become powerful molecular tools whose unique benefits are being used to address challenging biological questions. We first review the basis of biosensor design and remark on recent technologies that are accelerating biosensor development. We then discuss a few of the latest advances in the development and application of genetically encodable fluorescent and bioluminescent biosensors that have led to scientific or technological breakthroughs.

Fluorescent Protein-Based Indicators for Functional Super-Resolution Imaging of Biomolecular Activities in Living Cells

Super-resolution light microscopy (SRM) offers a unique opportunity for diffraction-unlimited imaging of biomolecular activities in living cells. To realize such potential, genetically encoded indicators were developed recently from fluorescent proteins (FPs) that exhibit phototransformation behaviors including photoactivation, photoconversion, and photoswitching, etc. Super-resolution observations of biomolecule interactions and biochemical activities have been demonstrated by exploiting the principles of bimolecular fluorescence complementation (BiFC), points accumulation for imaging nanoscale topography (PAINT), and fluorescence fluctuation increase by contact (FLINC), etc. To improve functional nanoscopy with the technology of genetically encoded indicators, it is essential to fully decipher the photo-induced chemistry of FPs and opt for innovative indicator designs that utilize not only fluorescence intensity but also multi-parametric readouts such as phototransformation kinetics. In parallel, technical improvements to both the microscopy optics and image analysis pipeline are promising avenues to increase the sensitivity and versatility of functional SRM.

Visualizing the inner life of microbes: practices of multi-color single-molecule localization microscopy in microbiology

In this review, we discuss multi-color single-molecule imaging and tracking strategies for studying microbial cell biology. We first summarize and compare the methods in a detailed literature review of published studies conducted in bacteria and fungi. We then introduce a guideline on which factors and parameters should be evaluated when designing a new experiment, from fluorophore and labeling choices to imaging routines and data analysis. Finally, we give some insight into some of the recent and promising applications and developments of these techniques and discuss the outlook for this field.

Split Green Fluorescent Proteins: Scope, Limitations, and Outlook

Many proteins can be split into fragments that spontaneously reassemble, without covalent linkage, into a functional protein. For split green fluorescent proteins (GFPs), fragment reassembly leads to a fluorescent readout, which has been widely used to investigate protein-protein interactions. We review the scope and limitations of this approach as well as other diverse applications of split GFPs as versatile sensors, molecular glues, optogenetic tools, and platforms for photophysical studies.

Spying on protein interactions in living cells with reconstituted scarlet light

The BiFC (bimolecular fluorescence complementation) assay and BiFC combined with FRET (fluorescence resonance energy transfer) technique have become important tools for molecular interaction studies in live cells. However, the real detection and cellular imaging performances of most existing red fluorescent protein-derived BiFC assays still suffer from relatively low ensemble brightness, high cytotoxicity, the red fluorescent proteins being prone-to-aggregation or severe residual dimerization, inefficient complementation and slow maturation at 37 °C physiological temperature in live mammalian cells. We developed a BiFC assay based on a recently evolved truly monomeric red fluorescent protein (FP) mScarlet-I with excellent cellular performances such as low cytotoxicity, fast and efficient chromophore maturation and the highest in-cell brightness among all previously reported monomeric red fluorescent proteins. In this work, a classic β-Fos/β-Jun constitutive heterodimerization model and a rapamycin-inducible FRB/FKBP interaction system were used to establish and test the performance of the mScarlet-I-based BiFC assay in live mammalian cells. Furthermore, simply by adopting the large-Stokes-shift fluorescent protein mAmetrine as the donor, β-Jun-β-Fos-NFAT1 ternary protein complex formation could be readily and efficiently detected and visualized with minimal spectral cross-talk in live HeLa cells by combining live-cell sensitized-emission FRET measurement with the mScarlet-I-based BiFC assay. The currently established BiFC assay in this work was also shown to be able to detect and visualize various protein-protein interactions (PPIs) at different subcellular compartments with high specificity and sensitivity at 37 °C physiological temperature in live mammalian cells.

Optimal fluorescent protein tags for quantifying protein oligomerization in living cells

Fluorescence fluctuation spectroscopy has become a popular toolbox for non-disruptive analysis of molecular interactions in living cells. The quantification of protein oligomerization in the native cellular environment is highly relevant for a detailed understanding of complex biological processes. An important parameter in this context is the molecular brightness, which serves as a direct measure of oligomerization and can be easily extracted from temporal or spatial fluorescence fluctuations. However, fluorescent proteins (FPs) typically used in such studies suffer from complex photophysical transitions and limited maturation, inducing non-fluorescent states. Here, we show how these processes strongly affect molecular brightness measurements. We perform a systematic characterization of non-fluorescent states for commonly used FPs and provide a simple guideline for accurate, unbiased oligomerization measurements in living cells. Further, we focus on novel red FPs and demonstrate that mCherry2, an mCherry variant, possesses superior properties with regards to precise quantification of oligomerization.

Live Cell Visualization of Multiple Protein-Protein Interactions with BiFC Rainbow

As one of the most powerful tools to visualize PPIs in living cells, bimolecular fluorescence complementation (BiFC) has gained great advancement during recent years, including deep tissue imaging with far-red or near-infrared fluorescent proteins or super-resolution imaging with photochromic fluorescent proteins. However, little progress has been made toward simultaneous detection and visualization of multiple PPIs in the same cell, mainly due to the spectral crosstalk. In this report, we developed novel BiFC assays based on large-Stokes-shift fluorescent proteins (LSS-FPs) to detect and visualize multiple PPIs in living cells. With the large excitation/emission spectral separation, LSS-FPs can be imaged together with normal Stokes shift fluorescent proteins to realize multicolor BiFC imaging using a simple illumination scheme. We also further demonstrated BiFC rainbow combining newly developed BiFC assays with previously established mCerulean/mVenus-based BiFC assays to achieve detection and visualization of four PPI pairs in the same cell. Additionally, we prove that with the complete spectral separation of mT-Sapphire and CyOFP1, LSS-FP-based BiFC assays can be readily combined with intensity-based FRET measurement to detect ternary protein complex formation with minimal spectral crosstalk. Thus, our newly developed LSS-FP-based BiFC assays not only expand the fluorescent protein toolbox available for BiFC but also facilitate the detection and visualization of multiple protein complex interactions in living cells.

Manufacturing Multienzymatic Complex Reactors In Vivo by Self-Assembly To Improve the Biosynthesis of Itaconic Acid in Escherichia coli

The self-assembly of multienzyme into bioreactors is of extensive interest to spatially regulate valuable reactions. Despite the important progresses achieved, methods to precisely manufacture multienzymatic complex reactors (MECRs) are still poorly proposed both in vivo and in vitro, particularly for more than three biocatalytically relevant enzymes. Here, we developed a sequential self-assembly system to form multitude MECRs involving three enzymes in the itaconic acid (IA) pathway with two pairs of protein-peptide interactions. The MECRs were identified as nanoscale particle-like structures when self-assembled in vitro and produced higher IA production than the unassembled and linearly assembled systems when applied in vivo coupling with CRISPR-Cas9 based metabolic engineering. This work provides novel insights into the construction of multifarious multienzyme complex into bioreactors by the self-assembly strategy for multistep cascades to sequentially control metabolic fluxes inside cells.

Modular fluorescence complementation sensors for live cell detection of epigenetic signals at endogenous genomic sites

Investigation of the fundamental role of epigenetic processes requires methods for the locus-specific detection of epigenetic modifications in living cells. Here, we address this urgent demand by developing four modular fluorescence complementation-based epigenetic biosensors for live-cell microscopy applications. These tools combine engineered DNA-binding proteins with domains recognizing defined epigenetic marks, both fused to non-fluorescent fragments of a fluorescent protein. The presence of the epigenetic mark at the target DNA sequence leads to the reconstitution of a functional fluorophore. With this approach, we could for the first time directly detect DNA methylation and histone 3 lysine 9 trimethylation at endogenous genomic sites in live cells and follow dynamic changes in these marks upon drug treatment, induction of epigenetic enzymes and during the cell cycle. We anticipate that this versatile technology will improve our understanding of how specific epigenetic signatures are set, erased and maintained during embryonic development or disease onset.Tools for imaging epigenetic modifications can shed light on the regulation of epigenetic processes. Here, the authors present a fluorescence complementation approach for detection of DNA and histone methylation at endogenous genomic sites allowing following of dynamic changes of these marks by live-cell microscopy.

Development of bimolecular fluorescence complementation using rsEGFP2 for detection and super-resolution imaging of protein-protein interactions in live cells

Direct visualization of protein-protein interactions (PPIs) at high spatial and temporal resolution in live cells is crucial for understanding the intricate and dynamic behaviors of signaling protein complexes. Recently, bimolecular fluorescence complementation (BiFC) assays have been combined with super-resolution imaging techniques including PALM and SOFI to visualize PPIs at the nanometer spatial resolution. RESOLFT nanoscopy has been proven as a powerful live-cell super-resolution imaging technique. With regard to the detection and visualization of PPIs in live cells with high temporal and spatial resolution, here we developed a BiFC assay using split rsEGFP2, a highly photostable and reversibly photoswitchable fluorescent protein previously developed for RESOLFT nanoscopy. Combined with parallelized RESOLFT microscopy, we demonstrated the high spatiotemporal resolving capability of a rsEGFP2-based BiFC assay by detecting and visualizing specifically the heterodimerization interactions between Bcl-x_L and Bak as well as the dynamics of the complex on mitochondria membrane in live cells.