Periplasmic-binding protein-based biosensors and bioanalytical assay platforms: Advances, considerations, and strategies for optimal utility

Discovery of periplasmic solute binding proteins with specificity for ketone bodies: β-hydroxybutyrate binding proteins

Polyhydroxyalkanoates are a class of biodegradable, thermoplastic polymers which represent a major carbon source for various bacteria. Proteins which mediate the translocation of polyhydroxyalkanoate breakdown products, such as β-hydroxybutyrate (BHB)-a ketone body which in humans serves as an important biomarker, have not been well characterized. In our investigation to screen a solute-binding protein (SBP) which can act as a suitable recognition element for BHB, we uncovered insights at the intersection of bacterial metabolism and diagnostics. Herein, we identify SBPs associated with putative ATP-binding cassette transporters that specifically recognize BHB, with the potential to serve as recognition elements for continuous quantification of this analyte. Through bioinformatic analysis, we identified candidate SBPs from known metabolizers of polyhydroxybutyrate-including proteins from Cupriavidus necator, Ensifer meliloti, Paucimonas lemoignei, and Thermus thermophilus. After recombinant expression in Escherichia coli, we demonstrated with intrinsic tryptophan fluorescence spectroscopy that four candidate proteins interacted with BHB, ranging from nanomolar to micromolar affinity. Tt.2, an intrinsically thermostable protein from Thermus thermophilus, was observed to have the tightest binding and specificity for BHB, which was confirmed by isothermal calorimetry. Structural analyses facilitated by AlphaFold2, along with molecular docking and dynamics simulations, were used to hypothesize key residues in the binding pocket and to model the conformational dynamics of substrate unbinding. Overall, this study provides strong evidence identifying the cognate ligands of SBPs which we hypothesize to be involved in prokaryotic cellular translocation of polyhydroxyalkanoate breakdown products, while highlighting these proteins' promising biotechnological application.

An automatic glucose monitoring system based on periplasmic binding proteins for online bioprocess monitoring

Glucose is one of the most vital nutrients in all living organisms, so its monitoring is critical in healthcare and bioprocessing. Enzymatic sensors are more popular as a technology solution to meet the requirement. However, periplasmic binding proteins have been investigated extensively for their high sensitivity, enabling microdialysis sampling to replace existing complex and expensive glucose monitoring solutions based on enzymatic sensors. The binding proteins are used as optical biosensors by introducing an environment-sensitive fluorophore to the protein. The biosensor's construction, characterization, and potential application are well studied, but a complete glucose monitoring system based on it is yet to be reported. This work documents the development of the first glucose sensor prototype based on glucose binding protein (GBP) for automatic and continuous glucose measurements. The development includes immobilizing the protein into reusable chips and a low-cost solution for non-invasive glucose sampling in bioprocesses using microdialysis sampling technique. A program was written in LabVIEW to accompany the prototype for the complete automation of measurement. The sampling technique allowed glucose measurements of a few micromolar to 260 mM glucose levels. A thorough analysis of the sampling mode and the device's performance was conducted. The reported measurement accuracy was 81.78%, with an RSD of 1.83%. The prototype was also used in online glucose monitoring of E. coli cell culture. The mode of glucose sensing can be expanded to the measurement of other analytes by switching the binding proteins.

Computational design of Periplasmic binding protein biosensors guided by molecular dynamics

Periplasmic binding proteins (PBPs) are bacterial proteins commonly used as scaffolds for substrate-detecting biosensors. In these biosensors, effector proteins (for example fluorescent proteins) are inserted into a PBP such that the effector protein's output changes upon PBP-substate binding. The insertion site is often determined by comparison of PBP apo/holo crystal structures, but random insertion libraries have shown that this can miss the best sites. Here, we present a PBP biosensor design method based on residue contact analysis from molecular dynamics. This computational method identifies the best previously known insertion sites in the maltose binding PBP, and suggests further previously unknown sites. We experimentally characterise fluorescent protein insertions at these new sites, finding they too give functional biosensors. Furthermore, our method is sufficiently flexible to both suggest insertion sites compatible with a variety of effector proteins, and be applied to binding proteins beyond PBPs.

Capture-and-release of a sulfoquinovose-binding protein on sulfoquinovose-modified agarose

The solute-binding protein (SBP) components of periplasmic binding protein-dependent ATP-binding cassette (ABC)-type transporters often possess exquisite selectivity for their cognate ligands. Maltose binding protein (MBP), the best studied of these SBPs, has been extensively used as a fusion partner to enable the affinity purification of recombinant proteins. However, other SBPs and SBP-ligand based affinity systems remain underexplored. The sulfoquinovose-binding protein SmoF, is a substrate-binding protein component of the ABC transporter cassette in Agrobacterium tumefaciens involved in importing sulfoquinovose (SQ) and its derivatives for SQ catabolism. Here, we show that SmoF binds with high affinity to the octyl glycoside of SQ (octyl-SQ), demonstrating remarkable tolerance to extension of the anomeric substituent. The 3D X-ray structure of the SmoF·octyl-SQ complex reveals accommodation of the octyl chain, which projects to the protein surface, providing impetus for the synthesis of a linker-equipped SQ-amine using a thiol-ene reaction as a key step, and its conjugation to cyanogen bromide modified agarose. We demonstrate the successful capture and release of SmoF from SQ-agarose resin using SQ as competitive eluant, and selectivity for release versus other organosulfonates. We show that SmoF can be captured and purified from a cell lysate, demonstrating the utility of SQ-agarose in capturing SQ binding proteins from complex mixtures. The present work provides a pathway for development of 'capture-and-release' affinity resins for the discovery and study of SBPs.

Illuminating the brain-genetically encoded single wavelength fluorescent biosensors to unravel neurotransmitter dynamics

Understanding how neuronal networks generate complex behavior is one of the major goals of Neuroscience. Neurotransmitter and Neuromodulators are crucial for information flow between neurons and understanding their dynamics is the key to unravel their role in behavior. To understand how the brain transmits information and how brain states arise, it is essential to visualize the dynamics of neurotransmitters, neuromodulators and neurochemicals. In the last five years, an increasing number of single-wavelength biosensors either based on periplasmic binding proteins (PBPs) or on G-protein-coupled receptors (GPCR) have been published that are able to detect neurotransmitter release in vitro and in vivo with high spatial and temporal resolution. Here we review and discuss recent progress in the development of these sensors, their limitations and future directions.

In vivo protein-based biosensors: seeing metabolism in real time

Biological homeostasis is a dynamic and elastic equilibrium of countless interlinked biochemical reactions. A key goal of life sciences is to understand these dynamics; bioengineers seek to reconfigure such networks. Both goals require the ability to monitor the concentration of individual intracellular metabolites with sufficient spatiotemporal resolution. To achieve this, a range of protein or protein/DNA signalling circuits with optical readouts have been constructed. Protein biosensors can provide quantitative information at subsecond temporal and suborganelle spatial resolution. However, their construction is fraught with difficulties related to integrating the affinity- and selectivity-endowing components with the signal reporters. We argue that development of efficient approaches for construction of chemically induced dimerisation systems and reporter domains with large dynamic ranges will solve these problems.

Discovery of Thermostable, Fluorescently Responsive Glucose Biosensors by Structure-Assisted Function Extrapolation

Accurate assignment of protein function from sequence remains a fascinating and difficult challenge. The periplasmic-binding protein (PBP) superfamily present an interesting case of function prediction because they are both ubiquitous in prokaryotes and tend to diversify through gene duplication "explosions" that can lead to large numbers of paralogs in a genome. An engineered version of the moderately thermostable glucose-binding PBP from Escherichia coli has been used successfully as a reagentless fluorescent biosensor both in vitro and in vivo. To develop more robust sensors that meet the challenges of real-world applications, we report the discovery of thermostable homologues that retain a glucose-mediated conformationally coupled fluorescence response. Accurately identifying a glucose-binding PBP homologue among closely related paralogs is challenging. We demonstrate that a structure-based method that filters sequences by residues that bind glucose in an archetype structure is highly effective. Using fully sequenced bacterial genomes, we found that this filter reduced high paralog numbers to single hits in a genome, consistent with the accurate separation of glucose binding from other functions. We expressed engineered proteins for eight homologues, chosen to represent different degrees of sequence identity, and tested their glucose-mediated fluorescence responses. We accurately predicted the presence of glucose binding down to 31% sequence identity. We have also successfully identified suitable candidates for next-generation robust, fluorescent glucose sensors.

Development of a Real-Time Pectic Oligosaccharide-Detecting Biosensor Using the Rapid and Flexible Computational Identification of Non-Disruptive Conjugation Sites (CINC) Biosensor Design Platform

Fluorescently labeled, solute-binding proteins that change their fluorescent output in response to ligand binding are frequently used as biosensors for a wide range of applications. We have previously developed a "Computational Identification of Non-disruptive Conjugation sites" (CINC) approach, an in silico pipeline utilizing molecular dynamics simulations for the rapid design and construction of novel protein-fluorophore conjugate-type biosensors. Here, we report an improved in silico scoring algorithm for use in CINC and its use in the construction of an oligogalacturonide-detecting biosensor set. Using both 4,5-unsaturated and saturated oligogalacturonides, we demonstrate that signal transmission from the ligand-binding pocket of the starting protein scaffold to the CINC-selected reporter positions is effective for multiple different ligands. The utility of an oligogalacturonide-detecting biosensor is shown in Carbohydrate Active Enzyme (CAZyme) activity assays, where the biosensor is used to follow product release upon polygalacturonic acid (PGA) depolymerization in real time. The oligogalacturonide-detecting biosensor set represents a novel enabling tool integral to our rapidly expanding platform for biosensor-based carbohydrate detection, and moving forward, the CINC pipeline will continue to enable the rational design of biomolecular tools to detect additional chemically distinct oligosaccharides and other solutes.

Subcellular Localization Defects Characterize Ribose-Binding Mutant Proteins with New Ligand Properties in Escherichia coli

Periplasmic binding proteins have been previously proclaimed as a general scaffold to design sensor proteins with new recognition specificities for nonnatural compounds. Such proteins can be integrated in bacterial bioreporter chassis with hybrid chemoreceptors to produce a concentration-dependent signal after ligand binding to the sensor cell. However, computationally designed new ligand-binding properties ignore the more general properties of periplasmic binding proteins, such as their periplasmic translocation, dynamic transition of open and closed forms, and interactions with membrane receptors. In order to better understand the roles of such general properties in periplasmic signaling behavior, we studied the subcellular localization of ribose-binding protein (RbsB) in Escherichia coli in comparison to a recently evolved set of mutants designed to bind 1,3-cyclohexanediol. As proxies for localization, we calibrated and deployed C-terminal end mCherry fluorescent protein fusions. Whereas RbsB-mCherry coherently localized to the periplasmic space and accumulated in (periplasmic) polar regions depending on chemoreceptor availability, mutant RbsB-mCherry expression resulted in high fluorescence cell-to-cell variability. This resulted in higher proportions of cells devoid of clear polar foci and of cells with multiple fluorescent foci elsewhere, suggesting poorer translocation, periplasmic autoaggregation, and mislocalization. Analysis of RbsB mutants and mutant libraries at different stages of directed evolution suggested overall improvement to more RbsB-wild-type-like characteristics, which was corroborated by structure predictions. Our results show that defects in periplasmic localization of mutant RbsB proteins partly explain their poor sensing performance. Future efforts should be directed to predicting or selecting secondary mutations outside computationally designed binding pockets, taking folding, translocation, and receptor interactions into account. IMPORTANCE Biosensor engineering relies on transcription factors or signaling proteins to provide the actual sensory functions for the target chemicals. Since for many compounds there are no natural sensory proteins, there is a general interest in methods that could unlock routes to obtaining new ligand-binding properties. Bacterial periplasmic binding proteins (PBPs) form an interesting family of proteins to explore for this purpose, because there is a large natural variety suggesting evolutionary trajectories to bind new ligands. PBPs are conserved and amenable to accurate computational binding pocket predictions. However, studying ribose-binding protein in Escherichia coli, we discovered that designed variants have defects in their proper localization in the cell, which can impair appropriate sensor signaling. This indicates that functional sensing capacity of PBPs cannot be obtained solely through computational design of the ligand-binding pocket but must take other properties of the protein into account, which are currently very difficult to predict.

Fluorescence-based sensing of the bioenergetic and physicochemical status of the cell

Fluorescence-based sensors play a fundamental role in biological research. These sensors can be based on fluorescent proteins, fluorescent probes or they can be hybrid systems. The availability of a very large dataset of fluorescent molecules, both genetically encoded and synthetically produced, together with the structural insights on many sensing domains, allowed to rationally design a high variety of sensors, capable of monitoring both molecular and global changes in living cells or in in vitro systems. The advancements in the fluorescence-imaging field helped researchers to obtain a deeper understanding of how and where specific changes occur in a cell or in vitro by combining the readout of the fluorescent sensors with the spatial information provided by fluorescent microscopy techniques. In this review we give an overview of the state of the art in the field of fluorescent biosensors and fluorescence imaging techniques, and eventually guide the reader through the choice of the best combination of fluorescent tools and techniques to answer specific biological questions. We particularly focus on sensors for probing the bioenergetics and physicochemical status of the cell.

In Situ Formation of Au-Glycopolymer Nanoparticles for Surface-Enhanced Raman Scattering-Based Biosensing and Single-Cell Immunity

Successful synthesis of glyconanoparticles has attracted much attention due to their various biointeractive capabilities, but it is still a challenge to understand different single-cell responses to exogenous particles among cell populations. Herein, we designed polyaniline-containing galactosylated gold nanoparticles (Au@PGlyco NPs) via in situ polymerization of ortho-nitrophenyl-β-galactoside assisted by Au nucleation. The nanogold-carrying polyaniline block produced electromagnetic enhancement in surface-enhanced Raman scattering (SERS). The underlying polymerization mechanism of ortho-nitrophenyl compounds via the formation of Au nanoparticles was investigated. Depending on how the galactoside moiety reacted with β-galactosidase derived from bacteria, the Au@PGlyco NPs-mediated SERS biosensor could detect low amounts of bacteria (∼1 × 102 CFU/mL). In addition, a high accumulation of Au@PGlyco NPs mediated the immune response of tumor-associated M2 macrophages to the immunogenic M1 macrophage transition, which was elicited by reactive oxygen levels biostimulation using single-cell SERS-combined fluorescence imaging. Our study suggested that Au@PGlyco NPs may serve as a biosensing platform with the labeling capacity on galactose-binding receptors expressed cell and immune regulation.

Ribose-Binding Protein Mutants With Improved Interaction Towards the Non-natural Ligand 1,3-Cyclohexanediol

Bioreporters consist of genetically modified living organisms that respond to the presence of target chemical compounds by production of an easily measurable signal. The central element in a bioreporter is a sensory protein or aptamer, which, upon ligand binding, modifies expression of the reporter signal protein. A variety of naturally occurring or modified versions of sensory elements has been exploited, but it has proven to be challenging to generate elements that recognize non-natural ligands. Bacterial periplasmic binding proteins have been proposed as a general scaffold to design receptor proteins for non-natural ligands, but despite various efforts, with only limited success. Here, we show how combinations of randomized mutagenesis and reporter screening improved the performance of a set of mutants in the ribose binding protein (RbsB) of Escherichia coli, which had been designed based on computational simulations to bind the non-natural ligand 1,3-cyclohexanediol (13CHD). Randomized mutant libraries were constructed that used the initially designed mutants as scaffolds, which were cloned in an appropriate E. coli bioreporter system and screened for improved induction of the GFPmut2 reporter fluorescence in presence of 1,3-cyclohexanediol. Multiple rounds of library screening, sorting, renewed mutagenesis and screening resulted in 4.5-fold improvement of the response to 1,3-cyclohexanediol and a lower detection limit of 0.25 mM. All observed mutations except one were located outside the direct ligand-binding pocket, suggesting they were compensatory and helping protein folding or functional behavior other than interaction with the ligand. Our results thus demonstrate that combinations of ligand-binding-pocket redesign and randomized mutagenesis can indeed lead to the selection and recovery of periplasmic-binding protein mutants with non-natural compound recognition. However, current lack of understanding of the intermolecular movement and ligand-binding in periplasmic binding proteins such as RbsB are limiting the rational production of further and better sensory mutants.