Creation of stable and strictly regulated enzyme switch for signal-on immunodetection of various small antigens

Customizable OpenGUS immunoassay: A homogeneous detection system using β-glucuronidase switch and label-free antibody

We developed a customizable OpenGUS immunoassay that enables rapid and sensitive detection of analytes without requiring antibody modification. This immunoassay employs label-free whole antibodies, an antibody-binding Z domain (ZD) derived from Staphylococcal protein A, and a β-glucuronidase (GUS) switch mutant, allowing for easy replacement of antibodies to tailor the immunoassays for various targeted antigens. The working principle is that the OpenGUS probe, the fusion protein of ZD and a GUS switch, converts the antibody-antigen interaction into GUS activation in a one-pot reaction. To enhance the signal-to-background ratio of the immunoassay, a GUS switch mutant that exhibits reduced background activation was developed by screening several additional mutations at the diagonal interface residue H514. Moreover, we optimized the composition of the reaction buffer, including organic solvents, salt, and surfactant. Under optimal conditions, we customized OpenGUS immunoassays for Cry j 1, human C-reactive protein, and human lactoferrin, achieving around 10-20-fold maximum fluorescence (15 min) or colorimetric (2 h) responses with picomolar to low nanomolar level detection limit, simply by using commercially available IgGs. Additionally, the three analytes were successfully detected in complex matrices similar to those used in practical applications. We believe that this customizable OpenGUS immunoassay will pave the way for the prompt development of rapid and sensitive homogeneous immunoassays for point-of-care diagnostics, high-throughput testing, and onsite environmental assessments.

A breakthrough of immunoassay format for hapten: recent insights into noncompetitive immunoassays to detect small molecules

Immunoassay based on the antibodies specific for targets has advantages of high sensitivity, simplicity and low cost, therefore it has received more attention in recent years, especially for the rapid detection of small molecule chemicals present in foods, diagnostics and environments. However, limited by low molecular weight and only one antigenic determinant existed, immunoassays for these small molecule chemicals, namely hapten substances, were commonly performed in a competitive immunoassay format, whose sensitivities were obviously lower than the sandwich enzyme-linked immunosorbent assay generally adaptable for the protein targets. In order to break through the bottleneck of detection format, researchers have designed and established several novel noncompetitive immunoassays for the haptens in the past few years. In this review, we focused on the four representative types of noncompetitive immunoassay formats and described their characteristics and applications in rapid detection of small molecules. Meanwhile, a systematic discussion on the current technologies challenges and the possible solutions were also summarized. This review aims to provide an updated overview of the current state-of-the-art in noncompetitive immunoassay for small molecules, and inspire the development of novel designs for small molecule detection.

Chimeric Protein Switch Biosensors

Rapid detection of protein and small-molecule analytes is a valuable technique across multiple disciplines, but most in vitro testing of biological or environmental samples requires long, laborious processes and trained personnel in laboratory settings, leading to long wait times for results and high expenses. Fusion of recognition with reporter elements has been introduced to detection methods such as enzyme-linked immunoassays (ELISA), with enzyme-conjugated secondary antibodies removing one of the many incubation and wash steps. Chimeric protein switch biosensors go further and provide a platform for homogenous mix-and-read assays where long wash and incubation steps are eradicated from the process. Chimeric protein switch biosensors consist of an enzyme switch (the reporter) coupled to a recognition element, where binding of the analyte results in switching the activity of the reporter enzyme on or off. Several chimeric protein switch biosensors have successfully been developed for analytes ranging from small molecule drugs to large protein biomarkers. There are two main formats of chimeric protein switch biosensor developed, one-component and multi-component, and these formats exhibit unique advantages and disadvantages. Genetically fusing a recognition protein to the enzyme switch has many advantages in the production and performance of the biosensor. A range of immune and synthetic binding proteins have been developed as alternatives to antibodies, including antibody mimetics or antibody fragments. These are mainly small, easily manipulated proteins and can be genetically fused to a reporter for recombinant expression or manipulated to allow chemical fusion. Here, aspects of chimeric protein switch biosensors will be reviewed with a comparison of different classes of recognition elements and switching mechanisms.

Creating a Thermostable β-Glucuronidase Switch for Homogeneous Immunoassay by Disruption of Conserved Salt Bridges at Diagonal Interfaces

Escherichia coli β-glucuronidase (GUS) has been used as a reporter enzyme in molecular biology and engineered as an enzyme switch for the development of homogeneous biosensors. In this study, we developed a thermostable GUS enzyme switch based on the thermostable GUS mutant TR3337 by disrupting a conserved salt bridge (H514-E523) between the diagonal subunits of its homotetramer. A combinatorial library (240 variants) was screened using a novel high-throughput strategy, which led to the identification of mutant DLW (H514D/M516L/Y517W) as a functional enzyme switch in a caffeine-recognizing immunosensor. Molecular dynamics simulations were performed to predict the topology change around position 514, and a side-chain flip of D514 (repulsion with E523) was observed in the DLW mutant. Up to 1.8-fold of signal-to-background ratio was confirmed when measured at up to 45 °C, thereby highlighting the DLW mutant as a versatile tool for developing thermostable immunosensors for in vitro and in cellulo applications.

The Patrol Yeast: A new biosensor armed with antibody-receptor chimera detecting a range of toxic substances associated with food poisoning

Baker's yeast is an attractive host with established safety and stability characteristics. Many yeast-based biosensors have been developed, but transmembrane signal transduction has not been used to detect membrane-impermeable substances using antigen-antibody interactions. Therefore, we created Patrol Yeast, a novel yeast-based immunosensor of various targets, particularly toxic substances in food. A membrane-based yeast two-hybrid system using split-ubiquitin was successfully used to detect practically important concentration ranges of caffeine and aflatoxins using separated variable regions of an antibody. Moreover, enterohemorrhagic Escherichia coli O157 was detected using a specific single-chain antibody, in which Zymolyase was added to partially destroy the cell wall. The incorporation of secreted Cypridina luciferase reporter further simplified the signal detection procedures without cell lysis. The methodology is more cost-effective and faster than using mammalian cells. The ability to detect various targets renders Patrol Yeast a valuable tool for ensuring food and beverage safety and addressing other environmental and technological issues.

Structural Biology of Nanobodies against the Spike Protein of SARS-CoV-2

Nanobodies are 130 amino acid single-domain antibodies (V_HH) derived from the unique heavy-chain-only subclass of Camelid immunogloblins. Their small molecular size, facile expression, high affinity and stability have combined to make them unique targeting reagents with numerous applications in the biomedical sciences. The first nanobody agent has now entered the clinic as a treatment against a blood disorder. The spread of the SARS-CoV-2 virus has seen the global scientific endeavour work to accelerate the development of technologies to try to defeat a pandemic that has now killed over four million people. In a remarkably short period of time, multiple studies have reported nanobodies directed against the viral Spike protein. Several agents have been tested in culture and demonstrate potent neutralisation of the virus or pseudovirus. A few agents have completed animal trials with very encouraging results showing their potential for treating infection. Here, we discuss the structural features that guide the nanobody recognition of the receptor binding domain of the Spike protein of SARS-CoV-2.

Engineering Protein Switches for Rapid Diagnostic Tests

Biological signaling pathways are underpinned by protein switches that sense and respond to molecular inputs. Inspired by nature, engineered protein switches have been designed to directly transduce analyte binding into a quantitative signal in a simple, wash-free, homogeneous assay format. As such, they offer great potential to underpin point-of-need diagnostics that are needed across broad sectors to improve access, costs, and speed compared to laboratory assays. Despite this, protein switch assays are not yet in routine diagnostic use, and a number of barriers to uptake must be overcome to realize this potential. Here, we review the opportunities and challenges in engineering protein switches for rapid diagnostic tests. We evaluate how their design, comprising a recognition element, reporter, and switching mechanism, relates to performance and identify areas for improvement to guide further optimization. Recent modular switches that enable new analytes to be targeted without redesign are crucial to ensure robust and efficient development processes. The importance of translational steps toward practical implementation, including integration into a user-friendly device and thorough assay validation, is also discussed.

Transmembrane signaling on a protocell: Creation of receptor-enzyme chimeras for immunodetection of specific antibodies and antigens

It is known that digital counting of fluorescent signals generated in many small compartments can significantly improve the detection sensitivity of the enzyme-linked immunosorbent assay (ELISA). However, the reported digital ELISA systems need extensive washing steps to remove background signal, which hampers their performance. To tackle this problem, we developed a vesicle (Protocell) array wherein binding of an external protein analyte is coupled to signal amplification and intra-vesicular fluorescence readout. We chose β-glucuronidase (GUS) as a reporter enzyme as its function requires assembly of four subunits through dimerization of a pair of dimers that can be inhibited by a set of interface mutations. Using a thermostabilized GUS mutant IV-5, we screened out an interface mutant (M516K, F517W) to create IV5_m - a mutant with high thermostability and activity conditional on induced dimerization. After tethering a short N-terminal tag and transmembrane (TM) sequences, the fusion protein was expressed by cell-free protein synthesis inside protocells. When a corresponding tag-specific antibody was applied outside of the protocells, a clear increase in GUS activity was observed inside vesicles by adding fluorescent substrate, probably due to spontaneous integration of the tagged TM protein into the vesicles and dimerization by the antibody bound to the displayed tag. Furthermore, using flow cytometry, quantitative digital read out was obtained by counting fluorescent protocells exposed to varying concentrations of external antibodies that included Trastuzumab. Additionally, through use of an anti-caffeine V_HH-SpyCatcher fusion protein, caffeine could be detected using SpyTag-fused TM-IV5_m protein expressed in protocells, suggesting utility of this platform for detection of diverse antigen types.