Application of fluorescence resonance energy transfer in protein studies

Theoretical model of donor-donor and donor-acceptor energy transfer on a nanosphere

In this study, we introduce a novel advancement in the field of theoretical exploration. Specifically, we investigate the transfer and trapping of electronic excitations within a two-component disordered system confined to a finite volume. The implications of our research extend to energy transfer phenomena on spherical nanoparticles, characterized by randomly distributed donors and acceptors on their surface. Utilizing the three-body Padé approximant technique, previously employed in single-component systems, we apply it to address the challenge of trapping within our system. To validate the robustness of our model, we conduct Monte Carlo simulations on a donor-acceptor system positioned on a spherical nanoparticle. In particular, very good agreement between the model and Monte Carlo simulations has been found for donor fluorescence intensity decay.

Ruthenium nitrosyl complexes with NO release capability: the use of fluorene as an antenna

A ruthenium nitrosyl complex of formula [RuII(fluorene(C6)CH2O-terpy)(bipy)(NO)]3+ (AC) in which fluorene(C6) is the 9,9-dihexylfluorene, terpy the 2,2';6',2''-terpyridine, and bipy the 2,2'-bipyridine is presented with its related [RuII(MeO-terpy)(bipy)(NO)]3+ (C) and 9,9-dihexylfluorene 2-hydroxymethylfluorene (A) building blocks. The reference complex C undergoes NO release capabilities under irradiation at λ = 365 nm. The effect of the introduction of the fluorescent A antenna within the resulting AC complex is discussed both experimentally and theoretically. The importance of the encaging parameter defined as ϕAC·IAC, in which IAC is the quantity of light absorbed by AC and ϕAC the quantum yield of NO release is evidenced and found to be concentration dependent. The conditions of optimization of the antenna approach to maximize ϕAC·IAC are discussed. The crystal structure of [RuII(fluorene(C6)CH2O-terpy)(bipy)(NO2)](PF6), the last intermediate in the synthesis of AC is also presented.

Sensing, Imaging, and Therapeutic Strategies Endowing by Conjugate Polymers for Precision Medicine

Conjugated polymers (CPs) have promising applications in biomedical fields, such as disease monitoring, real-time imaging diagnosis, and disease treatment. As a promising luminescent material with tunable emission, high brightness and excellent stability, CPs are widely used as fluorescent probes in biological detection and imaging. Rational molecular design and structural optimization have broadened absorption/emission range of CPs, which are more conductive for disease diagnosis and precision therapy. This review provides a comprehensive overview of recent advances in the application of CPs, aiming to elucidate their structural and functional relationships. The fluorescence properties of CPs and the mechanism of detection signal amplification are first discussed, followed by an elucidation of their emerging applications in biological detection. Subsequently, CPs-based imaging systems and therapeutic strategies are illustrated systematically. Finally, recent advancements in utilizing CPs as electroactive materials for bioelectronic devices are also investigated. Moreover, the challenges and outlooks of CPs for precision medicine are discussed. Through this systematic review, it is hoped to highlight the frontier progress of CPs and promote new breakthroughs in fundamental research and clinical transformation.

Using Förster Resonance Energy Transfer (FRET) to Understand the Ubiquitination Landscape

Förster resonance energy transfer (FRET) is a fluorescence technique that allows quantitative measurement of protein interactions, kinetics and dynamics. This review covers the use of FRET to study the structures and mechanisms of ubiquitination and related proteins. We survey FRET assays that have been developed where donor and acceptor fluorophores are placed on E1, E2 or E3 enzymes and ubiquitin (Ub) to monitor steady-state and real-time transfer of Ub through the ubiquitination cascade. Specialized FRET probes placed on Ub and Ub-like proteins have been developed to monitor Ub removal by deubiquitinating enzymes (DUBs) that result in a loss of a FRET signal upon cleavage of the FRET probes. FRET has also been used to understand conformational changes in large complexes such as multimeric E3 ligases and the proteasome, frequently using sophisticated single molecule methods. Overall, FRET is a powerful tool to help unravel the intricacies of the complex ubiquitination system.

Immunoassay System Based on the Technology of Time-Resolved Fluorescence Resonance Energy Transfer

To enhance the specificity and sensitivity, cut the cost, and realize joint detection of multiple indicators, an immunoassay system based on the technology of time-resolved fluorescence resonance energy transfer (TR-FRET) was studied. Due to the FRET of the reagent, the donor probe and acceptor probe emitted specific fluorescence to enhance specificity. Long-lifetime specific fluorescence from the acceptor probe was combined with time-resolved technology to enhance sensitivity. A xenon flash lamp and a photomultiplier tube (PMT) were selected as the light source and detector, respectively. A filter-switching mechanism was placed in the light path, so the fluorescence signal from the donor and acceptor was measured alternately. The instrument's design is given, and some specificI parts are described in detail. Key technical specifications of the instrument and procalcitonin (PCT), C-reactive protein (CRP), and interleukin-6(IL-6) were tested, and the test results were presented subsequently. The CV value of the self-designed counting module is better than 0.01%, and the instrument noises for 620 nm and 665 nm are 41.44 and 10.59, respectively. When set at 37 °C, the temperature bias (B) is 0.06 °C, and the temperature fluctuation is 0.10 °C. The CV and bias are between ±3% and 5%, respectively, when pipetting volumes are between 10 μL and 100 μL. Within the concentration range of 0.01 nM to 10 nM, the luminescence values exhibit linear regression correlation coefficients greater than 0.999. For PCT detection, when the concentration ranges from 0.02 ng/mL to 50 ng/mL, the correlation coefficient of linear fitting exceeds 0.999, and the limit of quantification is 0.096 ng/mL. For CRP and IL-6, the detection concentration ranges from 0 ng/mL to 500 ng/mL and 0 ng/mL to 20 ng/mL, respectively, with limits of quantification of 2.70 ng/mL and 2.82 ng/mL, respectively. The experimental results confirm the feasibility of the technical and instrumental solutions.

Extending Infrared Emission via Energy Transfer in a CsPbI3-Cyanine Dye Hybrid

Directing energy flow in light harvesting assemblies of nanocrystal-chromophore hybrid systems requires a better understanding of factors that dictate excited-state processes. In this study, we explore excited-state interactions within the CsPbI3-cyanine dye (IR125) hybrid assembly through a comprehensive set of steady-state and time-resolved absorption and photoluminescence (PL) experiments. Our photoluminescence investigations reveal the quenching of CsPbI3 emission alongside the simultaneous enhancement of IR125 fluorescence, providing evidence for a singlet energy transfer. The evaluation of both photoluminescence (PL) quenching and PL decay measurements yield ∼94% energy transfer efficiency for the CsPbI3-IR125 hybrid assembly. Transient absorption spectroscopy further unveils that this singlet energy transfer process operates on an ultrafast time scale, occurring within 400 ps with a rate constant of energy transfer of 1.4 × 1010 s-1. Our findings highlight the potential of the CsPbI3-IR125 hybrid assembly to extend the emission of halide perovskites into the infrared region, paving the way for light energy harvesting and display applications.

Target-based drug discovery: Applications of fluorescence techniques in high throughput and fragment-based screening

Target-based discovery of first-in-class therapeutics demands an in-depth understanding of the molecular mechanisms underlying human diseases. Precise measurements of cellular and biochemical activities are critical to gain mechanistic knowledge of biomolecules and their altered function in disease conditions. Such measurements enable the development of intervention strategies for preventing or treating diseases by modulation of desired molecular processes. Fluorescence-based techniques are routinely employed for accurate and robust measurements of in-vitro activity of molecular targets and for discovering novel chemical molecules that modulate the activity of molecular targets. In the current review, the authors focus on the applications of fluorescence-based high throughput screening (HTS) and fragment-based ligand discovery (FBLD) techniques such as fluorescence polarization (FP), Förster resonance energy transfer (FRET), fluorescence thermal shift assay (FTSA) and microscale thermophoresis (MST) for the discovery of chemical probe to exploring target's role in disease biology and ultimately, serve as a foundation for drug discovery. Some recent advancements in these techniques for compound library screening against important classes of drug targets, such as G-protein-coupled receptors (GPCRs) and GTPases, as well as phosphorylation- and acetylation-mediated protein-protein interactions, are discussed. Overall, this review presents a landscape of how these techniques paved the way for the discovery of small-molecule modulators and biologics against these targets for therapeutic benefits.

Single-molecule analysis reveals that a glucagon-bound extracellular domain of the glucagon receptor is dynamic

Dynamic information is vital to understanding the activation mechanism of G protein-coupled receptors (GPCRs). Despite the availability of high-resolution structures of different conformational states, the dynamics of those states at the molecular level are poorly understood. Here, we used total internal reflection fluorescence microscopy to study the extracellular domain (ECD) of the glucagon receptor (GCGR), a class B family GPCR that controls glucose homeostasis. Single-molecule fluorescence resonance energy transfer was used to observe the ECD dynamics of GCGR molecules expressed and purified from mammalian cells. We observed that for apo-GCGR, the ECD is dynamic and spent time predominantly in a closed conformation. In the presence of glucagon, the ECD is wide open and also shows more dynamic behavior than apo-GCGR, a finding that was not previously reported. These results suggest that both apo-GCGR and glucagon-bound GCGRs show reversible opening and closing of the ECD with respect to the seven-transmembrane (7TM) domain. This work demonstrates a molecular approach to visualizing the dynamics of the GCGR ECD and provides a foundation for understanding the conformational changes underlying GPCR activation, which is critical in the development of new therapeutics.

The effect of human serum albumin glycation on the binding of antidiabetic agent-exenatide; the multispectroscopic studies. Part II

The glycated human serum albumin (gHSA) interaction with antidiabetic agent - exenatide (exe) was investigated by spectroscopic techniques. The fluorescence spectroscopy showed an association between exe and protein as 4.57 × 10⁴ M^-1 (290 K), 3.33 × 10⁴ M^-1 (300K) and 2.54 × 10⁴ M^-1 (310K) and the number of binding sites were 1.15, 1.09, and 1.02, respectively. The binding process occurred spontaneously and the electrostatic bonds play a predominant role in the gHSA-exe interaction. The affinity drug to gHSA was studied in the presence of metal ions (Ca²⁺, Cr³⁺, Zn²⁺), which are often given as a supplement to the diet. Our study indicates that metal ions have an effect on the increase of the affinity of exenatide to glycated albumin and it is the result of the metal ion-exenatide-albumin interaction. Based on CD spectroscopy it is evident that exenatide has nonsignificant effect on the protein secondary structure, but the changes are visible in the presence of metal ions. Zeta potential measurements indicate instability of the system at lower concentration of the ligand - the effect is desired in point of view of pharmacology.

Interaction of Ibuprofen with Partially Unfolded Bovine Serum Albumin in the Presence of Ionic Micelles and Oligosaccharides at Different λex and pH: A Spectroscopic Analysis

The interaction between the plasma protein bovine serum albumin (BSA) and the drug ibuprofen (IBU) has been investigated at three different pH values (7.4, 6.5, and 8.0) in the presence of oligosaccharides and surfactants. The interaction analysis of BSA with oligosaccharides and surfactants has also been studied in the absence of the drug ibuprofen. The results obtained give convenient and efficient access to understand the mechanism of binding of ibuprofen to BSA, and the major forces involved are found to be hydrophobic forces, hydrogen bonding and ionic interactions. In addition to that, the formation of inclusion complexes of ibuprofen with oligosaccharides (β-CD and 2-HP-β-CD) has been observed, which has depicted that due to the hydrophobic nature of ibuprofen, it becomes more soluble in the presence of oligosaccharides, but due to the larger size of the inclusion complexes, these could not be able to access the hydrophobic pocket of BSA where tryptophan-212 (Trp-212) resides. The binding interaction between BSA and ibuprofen is observed in the presence of surfactants (SDS and CTAB), which partially unfold the protein. Non-radiative fluorescence resonance energy transfer (FRET) from Trp and Tyr residues of BSA in the presence of an anionic surfactant SDS to ibuprofen has depicted that there is a possibility of drug binding even in the partially unfolded state of BSA protein. Furthermore, the distance between the protein and the drug has been calculated from the FRET efficiency, which gives a comprehensive overview of ibuprofen binding to BSA even in its partially denatured state. The hydrophobic drug binding to the partially unfolded serum albumin protein (BSA) supports the "necklace and bead structures" model and opens up a new direction of drug loading and delivery system, which will have critical therapeutic applications in the efficient delivery of pharmacologically prominent drugs.

Elucidating the Oligomerization and Cellular Interactions of a Trimer Derived from Aβ through Fluorescence and Mass Spectrometric Studies

Aβ oligomers play a central role in the neurodegeneration observed with Alzheimer's disease. Our laboratory has developed covalently stabilized trimers derived from residues 17-36 of Aβ as model systems for studying Aβ oligomers. In the current study, we apply the emerging techniques of fluorescence lifetime imaging microscopy (FLIM) and native mass spectrometry (native MS) to better understand the assembly and interactions of the oligomer model system 2AT-L in aqueous solutions and with cells. 2AT-L and fluorescently labeled 2AT-L analogues assemble in the membrane-like environment of SDS-PAGE, showing diffuse bands of oligomers in equilibrium. Native ion mobility-mass spectrometry (native IM-MS) of 2AT-L allows for the identification of discrete oligomers in solution and shows similar patterns of oligomer formation between 2AT-L and fluorescently labeled analogues. Fluorescence microscopy with SH-SY5Y cells reveals that fluorescently labeled 2AT-L analogues colocalize within lysosomes. FLIM studies with phasor analysis further elucidate the assembly of 2AT-L within cells and establish the occurrence of FRET, indicating the presence of oligomers within cells. Collectively, these multiple complementary techniques help better understand the complex behavior of the 2AT-L model system.

Dominating Interlayer Resonant Energy Transfer in Type-II 2D Heterostructure

Type-II heterostructures (HSs) are essential components of modern electronic and optoelectronic devices. Earlier studies have found that in type-II transition metal dichalcogenide (TMD) HSs, the dominating carrier relaxation pathway is the interlayer charge transfer (CT) mechanism. Here, this report shows that, in a type-II HS formed between monolayers of MoSe₂ and ReS₂, nonradiative energy transfer (ET) from higher to lower work function material (ReS₂ to MoSe₂) dominates over the traditional CT process with and without a charge-blocking interlayer. Without a charge-blocking interlayer, the HS area shows 3.6 times MoSe₂ photoluminescence (PL) enhancement as compared to the MoSe₂ area alone. In a completely encapsulated sample, the HS PL emission further increases by a factor of 6.4. After completely blocking the CT process, more than 1 order of magnitude higher MoSe₂ PL emission was achieved from the HS area. This work reveals that the nature of this ET is truly a resonant effect by showing that in a similar type-II HS formed by ReS₂ and WSe₂, CT dominates over ET, resulting in a severely quenched WSe₂ PL. This study not only provides significant insight into the competing interlayer processes but also shows an innovative way to increase the PL emission intensity of the desired TMD material using the ET process by carefully choosing the right material combination for HS.

Proximity-dependent biotinylation approaches to study apicomplexan biology

In the last 10 years, proximity-dependent biotinylation (PDB) techniques greatly expanded the ability to study protein environments in the living cell that range from specific protein complexes to entire compartments. This is achieved by using enzymes such as BirA* and APEX that are fused to proteins of interest and biotinylate proteins in their proximity. PDB techniques are now also increasingly used in apicomplexan parasites. In this review, we first give an overview of the main PDB approaches and how they compare with other techniques that address similar questions. PDB is particularly valuable to detect weak or transient protein associations under physiological conditions and to study cellular structures that are difficult to purify or have a poorly understood protein composition. We also highlight new developments such as novel smaller or faster-acting enzyme variants and conditional PDB approaches, providing improvements in both temporal and spatial resolution which may offer broader application possibilities useful in apicomplexan research. In the second part, we review work using PDB techniques in apicomplexan parasites and how this expanded our knowledge about these medically important parasites.

Distance-based measurement determines the coexistence of B protein hetero- and homodimers in lily tepal and stamen tetrameric complexes

The floral quartet model proposes that plant MADS box proteins function as higher order tetrameric complexes. However, in planta evidence for MADS box tetramers remains scarce. Here, we applied a strategy using in vivo fluorescence resonance energy transfer (FRET) based on the distance change and distance symmetry of stable tetrameric complexes in tobacco (Nicotiana benthamiana) leaf cells to improve the accuracy of the estimation of heterotetrameric complex formation. This measuring system precisely verified the stable state of Arabidopsis petal (AP3/PI/SEP3/AP1) and stamen (AP3/PI/SEP3/AG) complexes and showed that the lily (Lilium longiflorum) PI co-orthologs LMADS8 and LMADS9 likely formed heterotetrameric petal complexes with Arabidopsis AP3/SEP3/AP1, which rescued petal defects of pi mutants. However, L8/L9 did not form heterotetrameric stamen complexes with Arabidopsis AP3/SEP3/AG to rescue the stamen defects of the pi mutants. Importantly, this system was applied successfully to find complicated tepal and stamen heterotetrameric complexes in lily. We found that heterodimers of B function AP3/PI orthologs (L1/L8) likely coexist with the homodimers of PI orthologs (L8/L8, L9/L9) to form five (two most stable and three stable) tepal- and four (one most stable and three stable) stamen-related heterotetrameric complexes with A/E and C/E function proteins in lily. Among these combinations, L1 preferentially interacted with L8 to form the most stable heterotetrameric complexes, and the importance of the L8/L8 and L9/L9 homodimers in tepal/stamen formation in lily likely decreased to a minor part during evolution. The system provides substantial improvements for successfully estimating the existence of unknown tetrameric complexes in plants.

Anti-Platelet Effect Induced by Iron Oxide Nanoparticles: Correlation with Conformational Change in Fibrinogen

Iron oxide nanoparticles are developed for various biomedical applications, however, there is limited understanding regarding their effects and toxicity on blood components. The particles traveling in circulation inevitably interact with blood cells and plasma proteins and may interfere with hemostasis. Specifically, this study focuses on the influence of superparamagnetic iron oxide nanoparticles (SPIONs) coated with a biocompatible polymer, polyvinyl alcohol (PVA), on platelet function. Here, engineered SPIONs that are functionalized with various PVA coatings to provide these particles with different surface charges and polymer packing are described. These formulations are assessed for any interference with human platelet functions and coagulation, ex vivo. Positively charged SPIONs induce a significant change in platelet GPIIb-IIIa conformation, indicative of platelet activation at the dose of 500 µg mL^-1 . Remarkably, engineered PVA(polyvinyl alcohol)-SPIONs all display a robust dose-dependent anti-platelet effect on platelet aggregation, regardless of the PVA charge and molecular weight. After assessing hypotheses involving SPION-induced steric hindrance in platelet-platelet bridging, as well as protein corona involvement in the antiplatelet effect, the study concludes that the presence of PVA-SPIONs induces fibrinogen conformational change, which correlates with the observed dose-dependent anti-platelet effect.

Review of Integrated Optical Biosensors for Point-Of-Care Applications

This article reviews optical biosensors and their integration with microfluidic channels. The integrated biosensors have the advantages of higher accuracy and sensitivity because they can simultaneously monitor two or more parameters. They can further incorporate many functionalities such as electrical control and signal readout monolithically in a single semiconductor chip, making them ideal candidates for point-of-care testing. In this article, we discuss the applications by specifically looking into point-of-care testing (POCT) using integrated optical sensors. The requirement and future perspective of integrated optical biosensors for POC is addressed.

Bacterial Vivisection: How Fluorescence-Based Imaging Techniques Shed a Light on the Inner Workings of Bacteria

The rise in fluorescence-based imaging techniques over the past 3 decades has improved the ability of researchers to scrutinize live cell biology at increased spatial and temporal resolution. In microbiology, these real-time vivisections structurally changed the view on the bacterial cell away from the "watery bag of enzymes" paradigm toward the perspective that these organisms are as complex as their eukaryotic counterparts. Capitalizing on the enormous potential of (time-lapse) fluorescence microscopy and the ever-extending pallet of corresponding probes, initial breakthroughs were made in unraveling the localization of proteins and monitoring real-time gene expression. However, later it became clear that the potential of this technique extends much further, paving the way for a focus-shift from observing single events within bacterial cells or populations to obtaining a more global picture at the intra- and intercellular level. In this review, we outline the current state of the art in fluorescence-based vivisection of bacteria and provide an overview of important case studies to exemplify how to use or combine different strategies to gain detailed information on the cell's physiology. The manuscript therefore consists of two separate (but interconnected) parts that can be read and consulted individually. The first part focuses on the fluorescent probe pallet and provides a perspective on modern methodologies for microscopy using these tools. The second section of the review takes the reader on a tour through the bacterial cell from cytoplasm to outer shell, describing strategies and methods to highlight architectural features and overall dynamics within cells.

A novel nucleic acid amplification system based on nano-gap embedded active disk resonators

Recent advances in nucleic acid based testing using bio-optical sensor approaches have been introduced but most are based on hybridization between the optical sensor and the bio-molecule and not on an amplification mechanism. Direct nucleic acid amplification on an optical sensor has several technical limitations, such as the sensitivity of the temperature sensor, instrument complexity, and high background signal. We here describe a novel nucleic acid amplification method based on a whispering gallery mode active resonator and discuss its potential molecular diagnostic application. By implanting nanoclusters as active compounds, this active resonator operates without tapered fiber coupling and emits a strong photoluminescence signal with low background in the wavelength of low absorption in an aqueous environment that is typical of biosensors. Our method also offers an extremely low detection threshold down to a single copy within 10 min due to the strong light-matter interaction in a nano-gap structure. We envision that this active resonator provides a high refractive index contrast for tight mode confinement with simple alignment as well as the possibility of reducing the device size so that a point-of-care system with low-cost, high-sensitivity and simplicity.

Native Cell Membrane Nanoparticles System for Membrane Protein-Protein Interaction Analysis

Protein-protein interactions in cell membrane systems play crucial roles in a wide range of biological processes- from cell-to-cell interactions to signal transduction; from sensing environmental signals to biological response; from metabolic regulation to developmental control. Accurate structural information of protein-protein interactions is crucial for understanding the molecular mechanisms of membrane protein complexes and for the design of highly specific molecules to modulate these proteins. Many in vivo and in vitro approaches have been developed for the detection and analysis of protein-protein interactions. Among them the structural biology approach is unique in that it can provide direct structural information of protein-protein interactions at the atomic level. However, current membrane protein structural biology is still largely limited to detergent-based methods. The major drawback of detergent-based methods is that they often dissociate or denature membrane protein complexes once their native lipid bilayer environment is removed by detergent molecules. We have been developing a native cell membrane nanoparticle system for membrane protein structural biology. Here, we demonstrate the use of this system in the analysis of protein-protein interactions on the cell membrane with a case study of the oligomeric state of AcrB.

A manganese (II) complex tethered with S-benzyldithiocarbazate Schiff base: Synthesis, characterization, in-vitro therapeutic activity and protein interaction studies

There is an urgent need to eliminate the era of superbugs through design and development of novel and sustainable drugs. Transition metal complexes can be one of the hopes for tackling drug resistant pathogens. In this view, we have developed a manganese complex appended with an ON donor ligand which has shown excellent activity against one of the prominent fungal species. The Mn (II) complex, [Mn^II(OH₂)₂(Hhpdbal-sbdt)₂] (1) was synthesized using a Schiff base ligand derived from an azo aldehyde and S-benzyldithiocarbazate. The complex was characterized with the help of analytical techniques such as elemental analysis, FT-IR, EDAX, EPR and TGA. The solution behavior in physiological conditions and in biological media was preliminarily evaluated by studying the behavior of complex in varied pH conditions and in the presence of protein, BSA. The effect of the compound on few drug resistant pathogenic species of bacteria and fungi and on the uptake of glucose by insulin resistant cells was evaluated using whole cell inhibition assay and NBDG assay respectively. The study gave a noteworthy result with respect to the manganese compound's biological activity, with an inhibitory activity of 93% towards a fungi species, Cryptococcus neoformans and with a relatively good glucose uptake inducing capacity. The manganese complex, which maintains its stability over a wide range of pH conditions and interacts with serum protein, BSA in a facile manner can be an excellent drug candidate and eventually be added to the library of compounds being screened for in vivo activity studies.

Enhanced sensitivity and detection range of FRET-based vitamin B12 nanosensor

Vitamin B₁₂ (cobalamin) is a cobalt-containing compound that acts as an essential co-factor for various enzymes involved in the metabolic processes of the living cells. The constructed FRET Sensor for Vitamin Anemia Linked (SenVitAL) displayed marginal FRET efficiency. Here, we report the development of a molecular SenVitAL containing enhanced cyan fluorescent protein (ECFP) and venus as FRET pair to improve the FRET efficiency for optical imaging and screening of already developed sensor by our group. The sensor is the improved version of previously reported SenVitAL and consists of ECFP/venus as FRET pair instead of the originally used pair CFP/YFP. To increase the physiological range of vitamin B₁₂ measurement, affinity mutants were created. Compared to the wild type, SenVitAL-5 with W44Q mutation has higher affinity and displayed large dynamic detection range (0.10-480 µM) in response to vitamin B₁₂ binding. For cell-based monitoring and dynamic measurement of vitamin B₁₂ flux rates_, SenVitAL-5 was successfully expressed in cytosol of yeast and mammalian cells. Changes in the emission intensities of the two fluorophores were detected using confocal microscopy in both cell types in response to vitamin B₁₂. With the addition of 50 µM extracellular vitamin B₁₂ to the cells, the emission intensity of venus increased and that of ECFP decreased over the time. Furthermore, the results show that the variant SenVitAL-5 measures the vitamin B₁₂ in a concentration-dependent manner, showing the resulting increase in the FRET ratio and thus confirming its utility as an ideal fluorescent indicator for the detection of vitamin B₁₂ in eukaryotic systems in real time.

Disruption of auto-inhibition underlies conformational signaling of ASIC1a to induce neuronal necroptosis

We reported previously that acid-sensing ion channel 1a (ASIC1a) mediates acidic neuronal necroptosis via recruiting receptor-interacting protein kinase 1 (RIPK1) to its C terminus (CT), independent of its ion-conducting function. Here we show that the N-terminus (NT) of ASIC1a interacts with its CT to form an auto-inhibition that prevents RIPK1 recruitment/activation under resting conditions. The interaction involves glutamate residues at distal NT and is disrupted by acidosis. Expression of mutant ASIC1a bearing truncation or glutamate-to-alanine substitutions at distal NT causes constitutive cell death. The NT-CT interaction is further disrupted by N-ethylmaleimide-sensitive fusion ATPase (NSF), which associates with ASIC1a-NT under acidosis, facilitating RIPK1 interaction with ASIC1a-CT. Importantly, a membrane-penetrating synthetic peptide representing the distal 20 ASIC1a NT residues, NT_1–20, reduced neuronal damage in both in vitro model of acidotoxicity and in vivo mouse model of ischemic stroke, demonstrating the therapeutic potential of targeting the auto-inhibition of ASIC1a for neuroprotection against acidotoxicity.

Acid-sensing ion channel 1a (ASIC1a) mediates acidic neuronal necroptosis via recruiting receptor-interacting protein kinase 1 (RIPK1). Here authors show that auto-inhibition of ASICa prevents RIPK1 recruitment and demonstrate that targeting the auto-inhibition has therapeutic potential to prevent acidotoxicity.

Spatial energy transfer with observation of bimolecular singlet oxygen emission using quantum dots as donors and zinc-phthalocyanine as acceptors

This study reports the influence of CdSe-ZnS core-shell quantum dots (QDs) for formation of singlet oxygen using zinc-phthalocyanine (ZnPc) dyes in colloidal solutions. Using a microluminescence surface scan technique it was possible to measure accurately the photon diffusion length, or photon mean free path, inside the medium. Analyses were performed for a range of QD concentrations. Photon diffusion length was assigned to the bimolecular singlet oxygen emission at 707 nm. Related singlet oxygen emission was predicted by observing quenching of the photon diffusion length measured at the specific oxygen emission as a function of QD concentration, being a nontrivial phenomenon related to the QD donors. Diffusion length measured at 707 nm increased with QD concentration; in the absence of QDs, as in pure ZnPc samples, the emission peak at 707 nm was not observed.

Förster Resonance Energy Transfer-Mediated Globular Protein Sensing Using Polyelectrolyte Complex Nanoparticles

Polyelectrolyte complex nanoparticles (PEC NPs) are synthesized using two oppositely charged polyelectrolytes, i.e., anionic poly(sodium 4-styrene sulfonate) (PSS) and cationic poly(diallyldimethylammoniumchloride), at molar mixing ratios (n ^-/n ⁺) of ≈0.4, 0.67, 0.75, and 1.5 by applying consecutive centrifugation to modify the optical property of PSS. However, for n ^-/n ⁺ ≈ 0.75, PEC NPs exhibit a larger blue shift and a specific emission peak occurs at ≈278 nm for the 225 nm excitation. The mechanism of such modification of PSS emission after complex formation is proposed. This specific emission by PEC NPs nearly matches with the optical absorption wavelength of globular proteins. The emission intensity of PEC NPs is therefore quenched in the presence of globular proteins (bovine serum albumin, human serum albumin, lysozyme, and hemoglobin) through resonance energy transfer between the donor (PEC NPs) and acceptor (globular proteins). The spectral overlap integral and the variation of the separation distance from 1.8 to 2.5 nm between the donor and acceptor confirm the resonance energy transfer. Sensing of proteins by the PEC NPs is possible within the detection limit of 5 nM and therefore such PEC NPs can be used as an efficient and promising protein sensing material.

FRET Microscopy in Yeast

Förster resonance energy transfer (FRET) microscopy is a powerful fluorescence microscopy method to study the nanoscale organization of multiprotein assemblies in vivo. Moreover, many biochemical and biophysical processes can be followed by employing sophisticated FRET biosensors directly in living cells. Here, we summarize existing FRET experiments and biosensors applied in yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, two important models of fundamental biomedical research and efficient platforms for analyses of bioactive molecules. We aim to provide a practical guide on suitable FRET techniques, fluorescent proteins, and experimental setups available for successful FRET experiments in yeasts.

Site-Directed Fluorescence Approaches for Dynamic Structural Biology of Membrane Peptides and Proteins

Membrane proteins mediate a number of cellular functions and are associated with several diseases and also play a crucial role in pathogenicity. Due to their importance in cellular structure and function, they are important drug targets for ~60% of drugs available in the market. Despite the technological advancement and recent successful outcomes in determining the high-resolution structural snapshot of membrane proteins, the mechanistic details underlining the complex functionalities of membrane proteins is least understood. This is largely due to lack of structural dynamics information pertaining to different functional states of membrane proteins in a membrane environment. Fluorescence spectroscopy is a widely used technique in the analysis of functionally-relevant structure and dynamics of membrane protein. This review is focused on various site-directed fluorescence (SDFL) approaches and their applications to explore structural information, conformational changes, hydration dynamics, and lipid-protein interactions of important classes of membrane proteins that include the pore-forming peptides/proteins, ion channels/transporters and G-protein coupled receptors.

Genetically encoded FRET-based optical sensor for Hg2+ detection and intracellular imaging in living cells

Due to the potential toxicity of mercury, there is an immediate need to understand its uptake, transport and flux within living cells. Conventional techniques used to analyze Hg²⁺ are invasive, involve high cost and are less sensitive. In the present study, a highly efficient genetically encoded mercury FRET sensor (MerFS) was developed to measure the cellular dynamics of Hg²⁺ at trace level in real time. To construct MerFS, the periplasmic mercury-binding protein MerP was sandwiched between enhanced cyan fluorescent protein (ECFP) and venus. MerFS is pH stable, offers a measurable fluorescent signal and binds to Hg²⁺ with high sensitivity and selectivity. Mutant MerFS-51 binds with an apparent affinity (K_d) of 5.09 × 10^-7 M, thus providing a detection range for Hg²⁺ quantification between 0.210 µM and 1.196 µM. Furthermore, MerFS-51 was targeted to Escherichia coli (E. coli), yeast and human embryonic kidney (HEK)-293T cells that allowed dynamic measurement of intracellular Hg²⁺ concentration with a highly responsive saturation curve, proving its potential application in cellular systems.

QTR-FRET: Efficient background reduction technology in time-resolved förster resonance energy transfer assays

A novel homogeneous assay system QTR-FRET (Quencher modulated Time-Resolved Förster Resonance Energy Transfer) combining quenching resonance energy transfer (QRET) and time-resolved Förster resonance energy transfer (TR-FRET) was developed to reduce background signal in the conventional energy transfer applications. The TR-FRET functionality is often limited by the lanthanide donor background signal leading to the use of low donor concentration. QTR-FRET reduces this background by introducing soluble quencher molecule, and in this work the concept functionality was proven and compared to previously introduced QRET and TR-FRET technologies. Comparison was performed with three different Eu³⁺-chelates exhibiting different luminescent lifetime and stability. The side-by-side comparison of the three signaling systems and Eu³⁺-chelates was demonstrated in a model assay with Eu³⁺-chelate conjugated biotin and streptavidin (SA) or Cy5-SA conjugate. Comparison of the methodologies showed increased signal-to-background ratios when comparing QTR-FRET to TR-FRET, especially at high Eu³⁺-biotin concentrations. Quenching the non-bound Eu³⁺-biotin improved the assay performance, which suggests that an improved assay performance can be attained with the QTR-FRET method. QTR-FRET is expected to be especially useful for Eu³⁺-labeled ligands with low affinity or assays requiring high Eu³⁺-ligand concentration. The QTR-FRET indicated potential for multi-analyte approaches separately utilizing the direct QRET-type Eu³⁺-chelate signal and energy transfer signal readout in a single-well. This potential was hypothesized with Avi-KRAS nucleotide exchange assay as a second biologically relevant model system.

Fluorescence resonance energy transfer in multifunctional nanofibers designed via block copolymer self-assembly

In the present study, we demonstrate the fabrication of multifunctional nanofibers, loaded with CdSe quantum dots (QDs) and sulforhodamine 101 (S101) dye, via the self-assembly process of a polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer (BCP). The CdSe QDs and S101 dye were simultaneously incorporated in the cylindrical domains, constituted of P4VP blocks, of the self-assembled BCP structure. The cylindrical domains subsequently were isolated as individual nanofibers via the selective-swelling approach. The confinement imposed due to the nano-dimension geometry of the cylindrical domains enabled the QDs and S101 dye to localize within their Förster radius enabling an efficient fluorescence resonance energy transfer (FRET) between them. The mean lifetime of donor emission varied from 4.56 to 3.38 ns with the change in the ratio of S101 dye and CdSe QDs within the nanofibers. Furthermore, using efficiency measurements and the corresponding Förster distances, donor-acceptor distances were determined. Moreover, the kinetics of energy transfer from CdSe QDs to S101 was studied by the Poisson binding model, to understand the interactions between CdSe QDs and S101 dye molecules. The numbers of dye molecules per CdSe QD were determined, by assuming random distribution of S101 dye molecules around the CdSe QDs in the nanofibers. The results showed that the number of dye molecules per QD increased with increasing concentration of dye molecules in the nanofibers. The resulting multifunctional nanofibers could have potential applications in optoelectronics, photonics and sensors which utilize the FRET process.

The interaction between human serum albumin and antidiabetic agent - exenatide: determination of the mechanism binding and effect on the protein conformation by fluorescence and circular dichroism techniques - Part I

The interactions between transport proteins and drugs are very important from the pharmacological point of view. In this study, using fluorescence and circular dichroism (CD) techniques, we investigated the interaction between human serum albumin (HSA) and incretin antidiabetic drug - exenatide. Moreover, the effect of common metal ions (Ca²⁺, Zn²⁺, Cr³⁺) on the exenatide-HSA binding - was also described. Based on the experimental data under pseudophysiological conditions, the calculated binding constant values are on the order of 10⁴ M^-1, and the constants are lower in the presence of metal ions. We observed the increase of the hydrophobicity near the tryptophan-214 residue in subdomain IIA, but almost no change in the hydrophobicity surrounding tyrosine residues. A similar effect on the tryptophan microenvironment is influenced by metal ions. The calculated thermodynamic parameters indicated that the characteristic electrostatic and hydrophobic interactions play an important role in the albumin-exenatide complexes. The CD studies showed that exenatide does not change the secondary structure of the protein but used metal ions have some impact on albumin α-helical content.Communicated by Ramaswamy H. Sarma.

Maslinic acid conjugate with 7-amino-4-methylcoumarin as probe to monitor the temperature dependent conformational changes of human serum albumin by FRET

Synthesis, characterization and spectroscopic investigation of maslinic acid labeled with fluorescent 7-amino-4-methylcoumarin is reported. It was found that the coumarin-maslinic derivative (MaCo) forms an excellent fluorescence resonance energy transfer (FRET) pair with the tryptophan (Trp) residue of human serum albumin (HSA). This feature allowed for monitoring HSA conformational alterations by measuring the distance between donor (Trp) and acceptor (MaCo) through Förster energy transfer mechanism. Displacement experiments confirmed that MaCo binds to subdomain IIA of HSA with independence of temperature. It was observed that, in the temperature range 35-45 °C, the fluorescence emission maximum of HSA-MaCo complex decreased, whereas in the range 45 °C-65 °C, an increment was detected. The concomitant change in the polarity of environment surrounding Trp was confirmed by red edge excitation shift experiments. Thermal denaturation of HSA was followed by time-resolved fluorescence spectroscopy. Average lifetime of Trp residue decreased with temperature due to the increment of solvent collisions and changes in the solvent exposure of Trp. To discriminate the importance of each effect, lifetime of N-Acetyl-L-tryptophanamide (NATA) at different temperatures was measured. Circular dichroism (CD) studies confirmed the loss of secondary structure of HSA with increasing temperature and showed a different trend in the conformational transformation below and above 45 °C, in agreement with steady-state and time-resolved fluorescence experiments.

Directed Energy Transfer through DNA-Templated J-Aggregates

Strongly coupled molecular dye aggregates have unique optoelectronic properties that often resemble those of light harvesting complexes found in Nature. The exciton dynamics in coupled dye aggregates could enhance the long-range transfer of optical excitation energy with high efficiency. In principle, dye aggregates could serve as important components in molecular-scale photonic devices; however, rational design of these coupled dye aggregates with precise control over their organization, interactions, and dynamics remains a challenge. DNA nanotechnology has recently been used to build an excitonic circuit by organizing pseudoisocyanine (PIC) dyes forming J-aggregates on the templates of poly(dA)-poly(dT) DNA duplexes. Here, the excitonic properties of the PIC J-aggregates on DNA are characterized spectroscopically in detail using poly(dA)-poly(dT) tract lengths of 24 and 48 base pairs. The excitonic properties of these DNA templated dye assemblies depend on the length and sequence of the DNA template. The incorporation of a gap of two GC base pairs between two segments of poly(dA)-poly(dT) DNA markedly reduces the delocalization of excitation in the J-aggregates. With a quantum dot (QD) as the light absorber and energy donor and using Alexa Fluor 647 (AF647) as the energy acceptor, with a DNA-templated J-aggregate in between, significant energy transfer from QD to AF647 is observed over a distance far longer than possible without the aggregate bridge. By comparing the efficiency of energy transfer through a continuous J-aggregate with the efficiency when the aggregate has a discontinuity in the middle, the effects of energy transfer within the aggregate bridge between the donor and acceptor are evaluated.

Potent drug candidature of an ONS donor tethered copper (II) complex: Anticancer activity, cytotoxicity and spectroscopically approached BSA binding studies

In our continued efforts to develop metal based therapeutic agents, we have synthesized a novel copper(II) complex, [{Cu(hpdbal-sbdt)}₂] (2) tethered with a biocompatible ONS^2- donor backbone [H₂hpdbal-sbdt] (1) [H₂hpdbal-sbdt is a tridentate ligand derived from S-benzyldithiocarbazate (Hsbdt) and 2-hydroxy-5-(phenyldiazenyl)benzaldehyde (Hhpdbal)]. The metal complex (2) was characterized using attenuated total reflection-infrared (ATR-IR) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, thermogravimetry and differential scanning calorimetric (TG-DSC) analysis, field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS) and elemental (CHNS) analysis. The antineoplastic ability of copper complex was evaluated in vitro against human cervical cancer (HeLa) cells. MTT assay results showed that the copper complex exhibited significant growth inhibition of HeLa cells with an IC₅₀ value of 4.46 μM and this value was compared with reported standards. Cytotoxicity of the copper complex towards human embryonic kidney cells (HEK-293) was also evaluated. The potentially active copper complex was studied for its solution state stability at a pH range of 3-9. Following this, the interactive behaviour of the bioactive copper complex with a drug transporter protein (BSA) was deciphered through multi-spectrosopic investigations like steady-state fluorescence, three-dimensional fluorescence, deconvoluted-IR and UV-Visible techniques.

Application of Förster Resonance Energy Transfer (FRET) technique to elucidate intracellular and In Vivo biofate of nanomedicines

Extensive studies on nanomedicines have been conducted for drug delivery and disease diagnosis (especially for cancer therapy). However, the intracellular and in vivo biofate of nanomedicines, which is significantly associated with their clinical therapeutic effect, is poorly understood at present. This is because of the technical challenges to quantify the disassembly and behaviour of nanomedicines. As a fluorescence- and distance-based approach, the Förster Resonance Energy Transfer (FRET) technique is very successful to study the interaction of nanomedicines with biological systems. In this review, principles on how to select a FRET pair and construct FRET-based nanomedicines have been described first, followed by their application to study structural integrity, biodistribution, disassembly kinetics, and elimination of nanomedicines at intracellular and in vivo levels, especially with drug nanocarriers including polymeric micelles, polymeric nanoparticles, and lipid-based nanoparticles. FRET is a powerful tool to reveal changes and interaction of nanoparticles after delivery, which will be very useful to guide future developments of nanomedicine.

Current Trends in Biotherapeutic Higher Order Structure Characterization by Irreversible Covalent Footprinting Mass Spectrometry

BACKGROUND

Biotherapeutics, particularly monoclonal antibodies (mAbs), are a maturing class of drugs capable of treating a wide range of diseases. Therapeutic function and solutionstability are linked to the proper three-dimensional organization of the primary sequence into Higher Order Structure (HOS) as well as the timescales of protein motions (dynamics). Methods that directly monitor protein HOS and dynamics are important for mapping therapeutically relevant protein-protein interactions and assessing properly folded structures. Irreversible covalent protein footprinting Mass Spectrometry (MS) tools, such as site-specific amino acid labeling and hydroxyl radical footprinting are analytical techniques capable of monitoring the side chain solvent accessibility influenced by tertiary and quaternary structure. Here we discuss the methodology, examples of biotherapeutic applications, and the future directions of irreversible covalent protein footprinting MS in biotherapeutic research and development.

CONCLUSION

Bottom-up mass spectrometry using irreversible labeling techniques provide valuable information for characterizing solution-phase protein structure. Examples range from epitope mapping and protein-ligand interactions, to probing challenging structures of membrane proteins. By paring these techniques with hydrogen-deuterium exchange, spectroscopic analysis, or static-phase structural data such as crystallography or electron microscopy, a comprehensive understanding of protein structure can be obtained.

The Methods Employed in Mass Spectrometric Analysis of Posttranslational Modifications (PTMs) and Protein-Protein Interactions (PPIs)

Mass Spectrometry (MS) has revolutionized the way we study biomolecules, especially proteins, their interactions and posttranslational modifications (PTM). As such MS has established itself as the leading tool for the analysis of PTMs mainly because this approach is highly sensitive, amenable to high throughput and is capable of assigning PTMs to specific sites in the amino acid sequence of proteins and peptides. Along with the advances in MS methodology there have been improvements in biochemical, genetic and cell biological approaches to mapping the interactome which are discussed with consideration for both the practical and technical considerations of these techniques. The interactome of a species is generally understood to represent the sum of all potential protein-protein interactions. There are still a number of barriers to the elucidation of the human interactome or any other species as physical contact between protein pairs that occur by selective molecular docking in a particular spatiotemporal biological context are not easily captured and measured.PTMs massively increase the complexity of organismal proteomes and play a role in almost all aspects of cell biology, allowing for fine-tuning of protein structure, function and localization. There are an estimated 300 PTMS with a predicted 5% of the eukaryotic genome coding for enzymes involved in protein modification, however we have not yet been able to reliably map PTM proteomes due to limitations in sample preparation, analytical techniques, data analysis, and the substoichiometric and transient nature of some PTMs. Improvements in proteomic and mass spectrometry methods, as well as sample preparation, have been exploited in a large number of proteome-wide surveys of PTMs in many different organisms. Here we focus on previously published global PTM proteome studies in the Apicomplexan parasites T. gondii and P. falciparum which offer numerous insights into the abundance and function of each of the studied PTM in the Apicomplexa. Integration of these datasets provide a more complete picture of the relative importance of PTM and crosstalk between them and how together PTM globally change the cellular biology of the Apicomplexan protozoa. A multitude of techniques used to investigate PTMs, mostly techniques in MS-based proteomics, are discussed for their ability to uncover relevant biological function.

Discovering cellular protein-protein interactions: Technological strategies and opportunities

The analysis of protein interaction networks is one of the key challenges in the study of biology. It connects genotypes to phenotypes, and disruption often leads to diseases. Hence, many technologies have been developed to study protein-protein interactions (PPIs) in a cellular context. The expansion of the PPI technology toolbox however complicates the selection of optimal approaches for diverse biological questions. This review gives an overview of the binary and co-complex technologies, with the former evaluating the interaction of two co-expressed genetically tagged proteins, and the latter only needing the expression of a single tagged protein or no tagged proteins at all. Mass spectrometry is crucial for some binary and all co-complex technologies. After the detailed description of the different technologies, the review compares their unique specifications, advantages, disadvantages, and applicability, while highlighting opportunities for further advancements.

A novel biocompatible NiII tethered moiety as a glucose uptake agent and a hit against methicillin-resistant Staphylococcus aureus

In the efforts to develop a biocompatible transition metal complex as a drug alike for some of the prevailing non-communicable diseases (NCDs) and communicable diseases (CDs), a novel binuclear Ni^II compound [{Ni^II(hpdbal-sbdt)}₂] (2) has been synthesized by the reaction of Ni(OAc)₂.4H₂O and H₂hpdbal-sbdt (1) [1 is a dibasic tridentate ONS^2- donor Schiff base ligand obtained by the condensation of 2-hydroxy-5-(phenyldiazenyl)benzaldehyde (Hhpdbal) and S-benzyldithiocarbazate (Hsbdt)]. Both ligand 1 and compound 2 were structurally characterized in the solid and solution state using various spectroscopic techniques like ATIR, ¹H NMR, ¹³C NMR, TGA, FESEM, EDS and CHNS analysis. The antidiabetic activity of H₂hpdbal-sbdt (1) and [{Ni^II(hpdbal-sbdt)}₂] (2) were assessed using 2-NBDG uptake assay. The assay results showed 85% and 95% of fluorescent glucose uptake by insulin resistant HePG2 cells treated with compounds 1 and 2 respectively. The 2-NBDG uptake by the cells treated with the compound 2 was observed to be comparable to the standard antidiabetic drug metformin. Compounds 1 and 2 were also tested against five bacterial and two fungi strains in order to evaluate pathogen killing activity. Compound 2 showed significant inhibitory action towards the methicillin-resistant Staphylococcus aureus (MRSA) strain with an MIC value of 2 μg/mL whereas the ligand 1 was found to be inactive. Furthermore, the interactive nature of compound 2 with a model serum carrier protein bovine serum albumin (BSA) was studied using a multi-spectroscopic approach which provided an insight into the nature and extent of binding, conformational changes and the quenching of amino acid residues of the protein.

Interaction of Purinergic P2X4 and P2X7 Receptor Subunits

P2X4 and P2X7 are members of the P2X receptor family, comprising seven isoforms (P2X1–P2X7) that form homo- and heterotrimeric non-specific cation channels gated by extracellular ATP. P2X4 and P2X7 are widely coexpressed, particularly in secretory epithelial cells and immune and inflammatory cells, and regulate inflammation and nociception. Although functional heteromerization has been established for P2X2 and P2X3 subunits expressed in sensory neurons, there are contradictory reports regarding a functional interaction between P2X4 and P2X7 subunits. To resolve this issue, we coexpressed P2X4 and P2X7 receptor subunits labeled with green (EGFP) and red (TagRFP) fluorescent proteins in Xenopus laevis oocytes and investigated a putative physical interaction between the fusion proteins by Förster resonance energy transfer (FRET). Coexpression of P2X4 and P2X7 subunits with EGFP and TagRFP located in the extracellular receptor domains led to significant FRET signals. Significant FRET signals were also measured between C-terminally fluorophore-labeled full-length P2X4^1-384 and C-terminally truncated fluorescent P2X7^1-408 subunits. We furthermore used the two-electrode voltage clamp technique to investigate whether human P2X4 and P2X7 receptors (hP2X4, hP2X7) functionally interact at the level of ATP-induced whole-cell currents. Concentration–response curves and effects of ivermectin (P2X4-potentiating drug) or BzATP (P2X7-specific agonist) were consistent with a model in which coexpressed hP2X4 and hP2X7 do not interact. Similarly, the effect of adding specific inhibitors of P2X4 (PSB-15417) or P2X7 (oATP, A438079) could be explained by a model in which only homomers exist, and that these are blocked by the respective antagonist. In conclusion, we show that P2X4 and P2X7 subunits can form heterotrimeric P2X4/P2X7 receptors. However, unlike observations for P2X2 and P2X3, coexpression of P2X4 and P2X7 subunits does not result in a novel electrophysiologically discriminable P2X receptor phenotype.