A dual-ligand-modulable fluorescent protein based on UnaG and calmodulin

A guide to genetically-encoded redox biosensors: State of the art and opportunities

Genetically-encoded redox biosensors have become invaluable tools for monitoring cellular redox processes with high spatiotemporal resolution, coupling the presence of the redox-active analyte with a change in fluorescence signal that can be easily recorded. This review summarizes the available fluorescence recording methods and presents an in-depth classification of the redox biosensors, organized by the analytes they respond to. In addition to the fluorescent protein-based architectures, this review also describes the recent advances on fluorescent, chemigenetic-based redox biosensors and other emerging chemigenetic strategies. This review examines how these biosensors are designed, the biosensors sensing mechanism, and their practical advantages and disadvantages.

Engineering of Tunable Allosteric-like Fluorogenic Protein Sensors

Optical protein sensors that enable detection of relevant biomolecules of interest play central roles in biological research. Coupling fluorescent reporters with protein sensing units has enabled the development of a wide range of biosensors that recognize analytes with high selectivity. In these sensors, analyte recognition induces a conformational change in the protein sensing unit that can modulate the optical signal of the fluorescent reporter. Here, we explore various designs for the creation of tunable allosteric-like fluorogenic protein sensors through incorporation of sensing protein units within the chemogenetic fluorescence-activating and absorption-shifting tag (FAST) that selectively binds and stabilizes the fluorescent state of 4-hydroxybenzylidene rhodanine (HBR) analogs. Conformational coupling allowed us to design analyte-responsive optical protein sensors through allosteric-like modulation of fluorogen binding.

A Genetic Engineering Toolbox for the Lignocellulolytic Anaerobic Gut Fungus Neocallimastix frontalis

Anaerobic fungi are powerful platforms for biotechnology that remain unexploited due to a lack of genetic tools. These gut fungi encode the largest number of lignocellulolytic carbohydrate active enzymes (CAZymes) in the fungal kingdom, making them attractive for applications in renewable energy and sustainability. However, efforts to genetically modify anaerobic fungi have remained limited due to inefficient methods for DNA uptake and a lack of characterized genetic parts. We demonstrate that anaerobic fungi are naturally competent for DNA and leverage this to develop a nascent genetic toolbox informed by recently acquired genomes for transient transformation of anaerobic fungi. We validate multiple selectable markers (HygR and Neo), an anaerobic reporter protein (iRFP702), enolase and TEF1A promoters, TEF1A terminator, and a nuclear localization tag for protein compartmentalization. This work establishes novel methods to reliably transform the anaerobic fungus Neocallimastix frontalis, thereby paving the way for strain development and various synthetic biology applications.

Outer Membrane Integrity-Dependent Fluorescence of the Japanese Eel UnaG Protein in Live Escherichia coli Cells

Reporter genes are important tools in many biological disciplines. The discovery of novel reporter genes is relatively rare. However, known reporter genes are constantly applied to novel applications. This study reports the performance of the bilirubin-dependent fluorescent protein UnaG from the Japanese eel Anguilla japonicas in live Escherichia coli cells in response to the disruption of outer membrane (OM) integrity at low bilirubin (BR) concentrations. Using the E. coli wild-type strain MC4100, its isogenic OM-deficient mutant strain NR698, and different OM-active compounds, we show that BR uptake and UnaG fluorescence depend on a leaky OM at concentrations of 10 µM BR and below, while fluorescence is mostly OM integrity-independent at concentrations above 50 µM BR. We suggest that these properties of the UnaG-BR couple might be applied as a biosensor as an alternative to the OM integrity assays currently in use.

Photomedicine based on heme-derived compounds

Photoimaging and phototherapy have become major platforms for the diagnosis and treatment of various health complications. These applications require a photosensitizer (PS) that is capable of absorbing light from a source and converting it into other energy forms for detection and therapy. While synthetic inorganic materials such as quantum dots and gold nanorods have been widely explored for their medical diagnosis and photodynamic (PDT) and photothermal (PTT) therapy capabilities, translation of these technologies has lagged, primarily owing to potential cytotoxicity and immunogenicity issues. Of the various photoreactive molecules, the naturally occurring endogenous compound heme, a constituent of red blood cells, and its derivatives, porphyrin, biliverdin and bilirubin, have shown immense potential as noteworthy candidates for clinically translatable photoreactive agents, as evidenced by previous reports. While porphyrin-based photomedicines have attracted significant attention and are well documented, research on photomedicines based on two other heme-derived compounds, biliverdin and bilirubin, has been relatively lacking. In this review, we summarize the unique photoproperties of heme-derived compounds and outline recent efforts to use them in biomedical imaging and phototherapy applications.

Exploring structural dynamics and optical properties of UnaG fluorescent protein upon N57 mutations

UnaG is a new class of fluorescence protein in which an endogenous ligand, namely bilirubin (BLR), plays the role of chromophore. Upon photoexcitation, holoUnaG emits green light. A single mutation at residue 57 induces a decrease in the fluorescence quantum yield. To our knowledge, no atomic simulation at the atomic level has been carried out to date to explain this fluorescence decay in N57A and N57Q mutants. Herein molecular dynamics simulations were carried out on wild-type (WT) UnaG and both mutants to investigate the structural impact of the mutation on its global structure, on BLR and the absorption spectra. Our study reveals significant global changes upon mutation at the protein entrance (L3, H2, and, H3) governing a BLR modification. BLR in WT UnaG is rather rigid while when embedded into N57A or N57Q, dihedral angles between endo and exo vinyl moieties and between A and B rings at the entrance of UnaG are strongly modified along with the number of inter-/intramolecular interactions. The water molecules play an important role in the modification of the shape of the binding cavity. For the first time, we show that the structural modifications upon ligand mutations are tightly related to the key structural changes in the protein such as Loop3 (L3), β sheet 2 (B2), and β sheet 3 (B3) dynamics. The present work suggests that the quenching of the fluorescence properties of UnaG mutants is mainly a non-radiative process closely related to the BLR flexibility induced by global structural changes.

Structure-Based Rational Design of Two Enhanced Bacterial Lipocalin Blc Tags for Protein-PAINT Super-resolution Microscopy

Super-resolution fluorescent imaging in living cells remains technically challenging, largely due to the photodecomposition of fluorescent tags. The recently suggested protein-PAINT is the only super-resolution technique available for prolonged imaging of proteins in living cells. It is realized with complexes of fluorogen-activating proteins, expressed as fusions, and solvatochromic synthetic dyes. Once photobleached, the dye in the complex is replaced with a fresh fluorogen available in the sample. With suitable kinetics, this replacement creates fluorescence blinking required for attaining super-resolution and overcomes photobleaching associated with the loss of an irreplaceable fluorophore. Here we report on the rational design of two protein-PAINT tags based on the 1.58 Å crystal structure of the DiB1:M739 complex, an improved green-emitting DiB3/F74V:M739 and a new orange-emitting DiB3/F53L:M739. They outperform previously reported DiB-based tags to become best in class biomarkers for protein-PAINT. The new tags advance protein-PAINT from the proof-of-concept to a reliable tool suitable for prolonged super-resolution imaging of intracellular proteins in fixed and living cells and two-color PAINT-like nanoscopy with a single fluorogen.

Illuminating Cellular Biochemistry: Fluorogenic Chemogenetic Biosensors for Biological Imaging

Cellular activity is defined by the precise spatiotemporal regulation of various components, such as ions, small molecules, or proteins. Studying cell physiology consequently requires the optical recording of these processes, notably by using fluorescent biosensors. The recent development of various fluorogenic systems greatly expanded the palette of reporters to be included in these sensors design. Fluorogenic reporters consist of a protein or RNA tag that can complex either an endogenous or a synthetic fluorogenic dye (so-called fluorogen). The intrinsic nature of these tags, along with the high tunability of their cognate chromophore provide interesting features such as far-red to near-infrared emission, oxygen independence, or unprecedented color versatility. These engineered photoreceptors, self-labelling proteins, or noncovalent aptamers and protein tags were rapidly identified as promising reporters to observe biological events. This Minireview focuses on the new perspectives they offer to design unique and innovative biosensors, thus pushing the boundaries of cellular imaging.

Disrupting Fluorescence by Mutagenesis in a Green Fluorescent Fatty Acid Binding Protein from a Marine Eel

Biofluorescence has been found to be an increasingly widespread phenomenon in the ocean. The reclusive Caribbean chlopsid eel, Kaupichthys hyoproroides displays bright green fluorescence in its native marine environment. We have previously shown the fluorescence to be attributed to a fluorescent fatty acid-binding protein, Chlopsid FP, part of a larger family of fluorescent fatty acid-binding proteins, including the homologous UnaG. All require the addition of exogenous bilirubin for fluorescence. Here, we report the generation of a series of point mutants, and deletions that result in the quenching of fluorescence in Chlopsid FP. In addition, we report the binding constants of bilirubin to Chlopsid FP and mutants, measured by fluorescence titration. This study provides key insights into the potential mechanism of fluorescence in this class of fluorescent fatty acid-binding proteins.

Beyond the Green Fluorescent Protein: Biomolecular Reporters for Anaerobic and Deep-Tissue Imaging

Fluorescence imaging represents cornerstone technology for studying biological function at the cellular and molecular levels. The technology's centerpiece is a prolific collection of genetic reporters based on the green fluorescent protein (GFP) and related analogs. More than two decades of protein engineering have endowed the GFP repertoire with an incredible assortment of fluorescent proteins, allowing scientists immense latitude in choosing reporters tailored to various cellular and environmental contexts. Nevertheless, GFP and derivative reporters have specific limitations that hinder their unrestricted use for molecular imaging. These challenges have inspired the development of new reporter proteins and imaging mechanisms. Here, we review how these developments are expanding the frontiers of reporter gene techniques to enable nondestructive studies of cell function in anaerobic environments and deep inside intact animals-two important biological contexts that are fundamentally incompatible with the use of GFP-based reporters.

Bright ligand-activatable fluorescent protein for high-quality multicolor live-cell super-resolution microscopy

We introduce UnaG as a green-to-dark photoswitching fluorescent protein capable of high-quality super-resolution imaging with photon numbers equivalent to the brightest photoswitchable red protein. UnaG only fluoresces upon binding of a fluorogenic metabolite, bilirubin, enabling UV-free reversible photoswitching with easily controllable kinetics and low background under Epi illumination. The on- and off-switching rates are controlled by the concentration of the ligand and the excitation light intensity, respectively, where the dissolved oxygen also promotes the off-switching. The photo-oxidation reaction mechanism of bilirubin in UnaG suggests that the lack of ligand-protein covalent bond allows the oxidized ligand to detach from the protein, emptying the binding cavity for rebinding to a fresh ligand molecule. We demonstrate super-resolution single-molecule localization imaging of various subcellular structures genetically encoded with UnaG, which enables facile labeling and simultaneous multicolor imaging of live cells. UnaG has the promise of becoming a default protein for high-performance super-resolution imaging.

Photoconvertible proteins occupy two color channels thereby limiting multicolour localisation microscopy applications. Here the authors present UnaG, a new green-to-dark photoswitching fluorescent protein for super-resolution imaging, whose activation is based on a noncovalent binding with bilirubin.

Next-Generation Fluorogen-Based Reporters and Biosensors for Advanced Bioimaging

Our ability to observe biochemical events with high spatial and temporal resolution is essential for understanding the functioning of living systems. Intrinsically fluorescent proteins such as the green fluorescent protein (GFP) have revolutionized the way biologists study cells and organisms. The fluorescence toolbox has been recently extended with new fluorescent reporters composed of a genetically encoded tag that binds endogenously present or exogenously applied fluorogenic chromophores (so-called fluorogens) and activates their fluorescence. This review presents the toolbox of fluorogen-based reporters and biosensors available to biologists. Various applications are detailed to illustrate the possible uses and opportunities offered by this new generation of fluorescent probes and sensors for advanced bioimaging.

Circularly Permuted Fluorogenic Proteins for the Design of Modular Biosensors

Fluorescent reporters are essential components for the design of optical biosensors that are able to image intracellular analytes in living cells. Herein, we describe the development of circularly permuted variants of Fluorescence-Activating and absorption-Shifting Tag (FAST) and demonstrate their potential as reporting module in biosensors. Circularly permutated FAST (cpFAST) variants allow one to condition the binding and activation of a fluorogenic ligand (and thus fluorescence) to analyte recognition by coupling them with analyte-binding domains. We demonstrated their use for biosensor design by generating multicolor plug-and-play fluorogenic biosensors for imaging the intracellular levels of Ca²⁺ in living mammalian cells in real time.