Mathematical models for quantitative assessment of bioluminescence resonance energy transfer: application to seven transmembrane receptors oligomerization

Systematic discovery of protein interaction interfaces using AlphaFold and experimental validation

Structural resolution of protein interactions enables mechanistic and functional studies as well as interpretation of disease variants. However, structural data is still missing for most protein interactions because we lack computational and experimental tools at scale. This is particularly true for interactions mediated by short linear motifs occurring in disordered regions of proteins. We find that AlphaFold-Multimer predicts with high sensitivity but limited specificity structures of domain-motif interactions when using small protein fragments as input. Sensitivity decreased substantially when using long protein fragments or full length proteins. We delineated a protein fragmentation strategy particularly suited for the prediction of domain-motif interfaces and applied it to interactions between human proteins associated with neurodevelopmental disorders. This enabled the prediction of highly confident and likely disease-related novel interfaces, which we further experimentally corroborated for FBXO23-STX1B, STX1B-VAMP2, ESRRG-PSMC5, PEX3-PEX19, PEX3-PEX16, and SNRPB-GIGYF1 providing novel molecular insights for diverse biological processes. Our work highlights exciting perspectives, but also reveals clear limitations and the need for future developments to maximize the power of Alphafold-Multimer for interface predictions.

Functional approaches to the study of G-protein-coupled receptors in postmortem brain tissue: [35S]GTPγS binding assays combined with immunoprecipitation

G-protein-coupled receptors (GPCRs) have an enormous biochemical importance as they bind to diverse extracellular ligands and regulate a variety of physiological and pathological responses. G-protein activation measures the functional consequence of receptor occupancy at one of the earliest receptor-mediated events. Receptor coupling to G-proteins promotes the GDP/GTP exchange on Gα subunits. Thus, modulation of the binding of the poorly hydrolysable GTP analog [³⁵S]GTPγS to the Gα-protein subunit can be used as a functional approach to quantify GPCR interaction with agonist, antagonist or inverse agonist drugs. In order to determine receptor-mediated selective activation of the different Gα-proteins, [³⁵S]GTPγS binding assays combined with immunodetection by specific antibodies have been developed and applied to physiological and pathological brain conditions. Currently, immunoprecipitation with magnetic beads and scintillation proximity assays are the most habitual techniques for this purpose. The present review summarizes the different procedures, advantages and limitations of the [³⁵S]GTPγS binding assays combined with selective Gα-protein sequestration methods. Experience of functional coupling of several GPCRs to different Gα-proteins and recommendations for optimal performance in brain membranes are described. One of the biggest opportunities opened by these techniques is that they enable evaluation of biased agonism in the native tissue, which results in high interest in drug discovery. The available results derived from application of these functional methodologies to study GPCR dysfunctions in neuro-psychiatric disorders are also described. In conclusion, [³⁵S]GTPγS binding combined with antibody-mediated immunodetection represents an useful method to separately evaluate the functional activity of drugs acting on GPCRs over each Gα-protein subtype.

Formation of Kiss1R/GPER Heterocomplexes Negatively Regulates Kiss1R-mediated Signalling through Limiting Receptor Cell Surface Expression

Kisspeptin receptor (Kiss1R) is an important receptor that plays central regulatory roles in reproduction by regulating hormone release in the hypothalamus. We hypothesize that the formation of heterocomplexes between Kiss1R and other hypothalamus G protein-coupled receptors (GPCRs) affects their cellular signaling. Through screening of potential interactions between Kiss1R and hypothalamus GPCRs, we identified G protein-coupled estrogen receptor (GPER) as one interaction partner of Kiss1R. Based on the recognised function of kisspeptin and estrogen in regulating the reproductive system, we investigated the Kiss1R/GPER heterocomplex in more detail and revealed that complex formation significantly reduced Kiss1R-mediated signaling. GPER did not directly antagonize Kiss1R conformational changes upon ligand binding, but it rather reduced the cell surface expression of Kiss1R. These results therefore demonstrate a regulatory mechanism of hypothalamic hormone receptors via receptor cooperation in the reproductive system and modulation of receptor sensitivity.

Advanced Bioluminescence System for In Vivo Imaging with Brighter and Red-Shifted Light Emission

In vivo bioluminescence imaging (BLI), which is based on luminescence emitted by the luciferase-luciferin reaction, has enabled continuous monitoring of various biochemical processes in living animals. Bright luminescence with a high signal-to-background ratio, ideally red or near-infrared light as the emission maximum, is necessary for in vivo animal experiments. Various attempts have been undertaken to achieve this goal, including genetic engineering of luciferase, chemical modulation of luciferin, and utilization of bioluminescence resonance energy transfer (BRET). In this review, we overview a recent advance in the development of a bioluminescence system for in vivo BLI. We also specifically examine the improvement in bioluminescence intensity by mutagenic or chemical modulation on several beetle and marine luciferase bioluminescence systems. We further describe that intramolecular BRET enhances luminescence emission, with recent attempts for the development of red-shifted bioluminescence system, showing great potency in in vivo BLI. Perspectives for future improvement of bioluminescence systems are discussed.

Using the novel HiBiT tag to label cell surface relaxin receptors for BRET proximity analysis

Relaxin family peptide 1 (RXFP1) is the receptor for relaxin a peptide hormone with important therapeutic potential. Like many G protein-coupled receptors (GPCRs), RXFP1 has been reported to form homodimers. Given the complex activation mechanism of RXFP1 by relaxin, we wondered whether homodimerization may be explicitly required for receptor activation, and therefore sought to determine if there is any relaxin-dependent change in RXFP1 proximity at the cell surface. Bioluminescence resonance energy transfer (BRET) between recombinantly tagged receptors is often used in GPCR proximity studies. RXFP1 targets poorly to the cell surface when overexpressed in cell lines, with the majority of the receptor proteins sequestered within the cell. Thus, any relaxin-induced changes in RXFP1 proximity at the cell surface may be obscured by BRET signal originating from intracellular compartments. We therefore, utilized the newly developed split luciferase system called HiBiT to specifically label the extracellular terminus of cell surface RXFP1 receptors in combination with mCitrine-tagged receptors, using the GABAB heterodimer as a positive control. This demonstrated that the BRET signal detected from RXFP1-RXFP1 proximity at the cell surface does not appear to be due to stable physical interactions. The fact that there is also no relaxin-mediated change in RXFP1-RXFP1 proximity at the cell surface further supports these conclusions. This work provides a basis by which cell surface GPCR proximity and expression levels can be specifically studied using a facile and homogeneous labeling technique such as HiBiT.

Functional characterization of β2-adrenergic and insulin receptor heteromers

This study aimed to functionally characterize β₂-adrenergic (β₂AR) and insulin receptor (IR) heteromers in regard to β-arrestin 2 (βarr2) recruitment and cAMP signaling and to examine the involvement of the cytoplasmic portion of the IR β chain in heteromerization with β₂AR. Evidence for β₂AR:IR:βarr2 complex formation and the specificity of the IR:βarr2 interaction was first provided by bioinfomatics analysis. Receptor-heteromer investigation technology (HIT) then provided functional evidence of β₂AR:IR heterodimerization by showing isoproterenol-induced but not insulin-induced GFP²-βarr2 recruitment to the β₂AR:IR complex; the IR:βarr2 interaction was found to only be constitutive. The constitutive IR:βarr2 BRET signal (BRET_const) was significantly smaller in cells coexpressing IR-RLuc8 and a GFP²-βarr2 1-185 mutant lacking the proposed IR binding domain. β₂AR:IR heteromerization also influenced the pharmacological phenotype of β₂AR, i.e., its efficacy in recruiting βarr2 and activating cAMP signaling. Evidence suggesting involvement of the cytoplasmic portion of the IR β chain in the interaction with β₂AR was provided by BRET² saturation and HIT assays using an IR 1-1271 stop mutant lacking the IR C-terminal tail region. For the complex consisting of IR 1-1271-RLuc8:β₂AR-GFP², saturation was not reached, most likely reflecting random collisions between IR 1-1271 and β₂AR. Furthermore, in the HIT assay, no substantial agonist-induced increase in the BRET² signal was detected that would be indicative of βarr2 recruitment to the IR 1-1271:β₂AR heteromer. Complementary 3D visualization of β₂AR:IR provided supporting evidence for stability of the heterotetramer complex and identified amino acid residues involved in β₂AR:IR heteromerization. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.

Isoform-specific domain organization determines conformation and function of the peroxisomal biogenesis factor PEX26

Peroxisomal biogenesis factor PEX26 is a membrane anchor for the multi-subunit PEX1-PEX6 protein complex that controls ubiquitination and dislocation of PEX5 cargo receptors for peroxisomal matrix protein import. PEX26 associates with the peroxisomal translocation pore via PEX14 and a splice variant (PEX26Δex5) of unknown function has been reported. Here, we demonstrate PEX26 homooligomerization mediated by two heptad repeat domains adjacent to the transmembrane domain. We show that isoform-specific domain organization determines PEX26 oligomerization and impacts peroxisomal β-oxidation and proliferation. PEX26 and PEX26Δex5 displayed different patterns of interaction with PEX2-PEX10 or PEX13-PEX14 complexes, which relate to distinct pre-peroxisomes in the de novo synthesis pathway. Our data support an alternative PEX14-dependent mechanism of peroxisomal membrane association for the splice variant, which lacks a transmembrane domain. Structure-function relationships of PEX26 isoforms explain an extended function in peroxisomal homeostasis and these findings may improve our understanding of the broad phenotype of PEX26-associated human disorders.

Wild-type p53 oligomerizes more efficiently than p53 hot-spot mutants and overcomes mutant p53 gain-of-function via a "dominant-positive" mechanism

Human p53 protein acts as a transcription factor predominantly in a tetrameric form. Single residue changes, caused by hot-spot mutations of the TP53 gene in human cancer, transform wild-type (wt) p53 tumor suppressor proteins into potent oncoproteins - with gain-of-function, tumor-promoting activity. Oligomerization of p53 allows for a direct interplay between wt and mutant p53 proteins if both are present in the same cells - where a mutant p53's dominant-negative effect known to inactivate wt p53, co-exists with an opposite mechanism - a "dominant-positive" suppression of the mutant p53's gain-of-function activity by wt p53. In this study we determine the oligomerization efficiency of wt and mutant p53 in living cells using FRET-based assays and describe wt p53 to be more efficient than mutant p53 in entering p53 oligomers. The biased p53 oligomerization helps to interpret earlier reports of a low efficiency of the wt p53 inactivation via the dominant-negative effect, while it also implies that the "dominant-positive" effect may be more pronounced. Indeed, we show that at similar wt:mutant p53 concentrations in cells - the mutant p53 gain-of-function stimulation of gene transcription and cell migration is more efficiently inhibited than the wt p53's tumor-suppressive transactivation and suppression of cell migration. These results suggest that the frequent mutant p53 accumulation in human tumor cells does not only directly strengthen its gain-of-function activity, but also protects the oncogenic p53 mutants from the functional dominance of wt p53.

LuTHy: a double-readout bioluminescence-based two-hybrid technology for quantitative mapping of protein-protein interactions in mammalian cells

Information on protein-protein interactions (PPIs) is of critical importance for studying complex biological systems and developing therapeutic strategies. Here, we present a double-readout bioluminescence-based two-hybrid technology, termed LuTHy, which provides two quantitative scores in one experimental procedure when testing binary interactions. PPIs are first monitored in cells by quantification of bioluminescence resonance energy transfer (BRET) and, following cell lysis, are again quantitatively assessed by luminescence-based co-precipitation (LuC). The double-readout procedure detects interactions with higher sensitivity than traditional single-readout methods and is broadly applicable, for example, for detecting the effects of small molecules or disease-causing mutations on PPIs. Applying LuTHy in a focused screen, we identified 42 interactions for the presynaptic chaperone CSPα, causative to adult-onset neuronal ceroid lipofuscinosis (ANCL), a progressive neurodegenerative disease. Nearly 50% of PPIs were found to be affected when studying the effect of the disease-causing missense mutations L115R and ∆L116 in CSPα with LuTHy. Our study presents a robust, sensitive research tool with high utility for investigating the molecular mechanisms by which disease-associated mutations impair protein activity in biological systems.

Bidirectional allosteric interactions between cannabinoid receptor 1 (CB1) and dopamine receptor 2 long (D2L) heterotetramers

Type 1 cannabinoid (CB₁) and dopamine 2 long form (D_2L) receptors can physically interact to form heteromers that display unique pharmacology in vitro compared to homomeric complexes. Co-expression of CB₁ and D_2L and co-application of CB₁ and D₂ agonists increases cAMP levels while administration of either agonist alone decreases cAMP levels. To understand the observed co-agonist response, our first goal of the current study was to define the stoichiometry of CB₁/D_2L/Gα protein complexes. Using bioluminescence resonance energy transfer 2 (BRET²), we confirmed that, CB₁ homodimers, D_2L homodimers, and CB₁/D_2L heteromers are formed. By using sequential resonance energy transfer 2 (SRET²) combined with bimolecular fluorescence complementation (BiFC), we were able to demonstrate that CB₁/D_2L form heterotetramers consisting of CB₁ and D_2L homodimers. We demonstrated that CB₁/D_2L heterotetramers are coupled to at least two Gα proteins. The second aim of the study was to investigate allosteric effects of a D_2L agonist (quinpirole) on CB₁ receptor function and to investigate the effects of a CB₁ agonist [arachidonyl-2-chloroethylamide (ACEA)] on D_2L receptor function within CB₁/D_2L heterotetramers. Treating cells co-expressing CB₁ and D_2L with both ACEA and quinpirole switched CB₁ and D_2L receptor coupling and signaling from Gα_i to Gα_s proteins, enhanced β-arrestin1 recruitment and receptor co-internalization. The concept of bidirectional allosteric interaction within CB₁/D₂ heterotetramers has important implications for understanding the activity of receptor complexes in native tissues and under pathological conditions.

Recent developments of genetically encoded optical sensors for cell biology

Optical sensors are powerful tools for live cell research as they permit to follow the location, concentration changes or activities of key cellular players such as lipids, ions and enzymes. Most of the current sensor probes are based on fluorescence which provides great spatial and temporal precision provided that high-end microscopy is used and that the timescale of the event of interest fits the response time of the sensor. Many of the sensors developed in the past 20 years are genetically encoded. There is a diversity of designs leading to simple or sometimes complicated applications for the use in live cells. Genetically encoded sensors began to emerge after the discovery of fluorescent proteins, engineering of their improved optical properties and the manipulation of their structure through application of circular permutation. In this review, we will describe a variety of genetically encoded biosensor concepts, including those for intensiometric and ratiometric sensors based on single fluorescent proteins, Forster resonance energy transfer-based sensors, sensors utilising bioluminescence, sensors using self-labelling SNAP- and CLIP-tags, and finally tetracysteine-based sensors. We focus on the newer developments and discuss the current approaches and techniques for design and application. This will demonstrate the power of using optical sensors in cell biology and will help opening the field to more systematic applications in the future.

The Conserved Arginine Cluster in the Insert of the Third Cytoplasmic Loop of the Long Form of the D₂ Dopamine Receptor (D2L-R) Acts as an Intracellular Retention Signal

This study examined whether the conserved arginine cluster present within the 29-amino acid insert of the long form of the D₂ dopamine receptor (D_2L-R) confers its predominant intracellular localization. We hypothesized that the conserved arginine cluster (RRR) located within the insert could act as an RXR-type endoplasmic reticulum (ER) retention signal. Arginine residues (R) within the cluster at positions 267, 268, and 269 were charge-reserved to glutamic acids (E), either individually or in clusters, thus generating single, double, and triple D_2L-R mutants. Through analyses of cellular localization by confocal microscopy and enzyme-linked immunosorbent assay (ELISA), radioligand binding assay, bioluminescence resonance energy transfer (BRET²) β-arrestin 2 (βarr2) recruitment assay, and cAMP signaling, it was revealed that charge reversal of the R residues at all three positions within the motif impaired their colocalization with ER marker calnexin and led to significantly improved cell surface expression. Additionally, these data demonstrate that an R to glutamic acid (E) substitution at position 2 within the RXR motif is not functionally permissible. Furthermore, all generated D_2L-R mutants preserved their functional integrity regarding ligand binding, agonist-induced βarr2 recruitment and Gα_i-mediated signaling. In summary, our results show that the conserved arginine cluster within the 29-amino acid insert of third cytoplasmic loop (IC3) of the D_2L-R appears to be the ER retention signal.

Determination of GLUT1 Oligomerization Parameters using Bioluminescent Förster Resonance Energy Transfer

The facilitated glucose transporter GLUT1 (SLC2A1) is an important mediator of glucose homeostasis in humans. Though it is found in most cell types to some extent, the level of GLUT1 expression across different cell types can vary dramatically. Prior studies in erythrocytes-which express particularly high levels of GLUT1-have suggested that GLUT1 is able to form tetrameric complexes with enhanced transport activity. Whether dynamic aggregation of GLUT1 also occurs in cell types with more modest expression of GLUT1, however, is unclear. To address this question, we developed a genetically encoded bioluminescent Förster resonance energy transfer (BRET) assay using the luminescent donor Nanoluciferase and fluorescent acceptor mCherry. By tethering these proteins to the N-terminus of GLUT1 and performing saturation BRET analysis, we were able to demonstrate the formation of multimeric complexes in live cells. Parallel use of flow cytometry and immunoblotting further enabled us to estimate the density of GLUT1 proteins required for spontaneous oligomerization. These data provide new insights into the physiological relevance of GLUT1 multimerization as well as a new variant of BRET assay that is useful for measuring the interactions among other cell membrane proteins in live cells.

Use of BRET to Study Protein-Protein Interactions In Vitro and In Vivo

Application of bioluminescence resonance energy transfer (BRET) assay has been of special value in measuring dynamic events such as protein-protein interactions (PPIs) in vitro or in vivo. It was only in the late 1990s the BRET assay using RLuc-YFP was introduced for biological research showing its use in determining interaction of two proteins involved in circadian rhythm. Several inherent attributes such as rapid and fairly sensitive ratiometric measurements, assessment of PPI irrespective of protein location in cellular compartment, and cost-effectiveness consenting to high-throughput assay development make BRET a popular genetic reporter-based assay for PPI studies. In BRET-based screening, within a defined proximity range of 10-100 Å, excited state energy of the luminescence molecule can excite the acceptor fluorophore in the form of resonance energy transfer, causing it to emit at its characteristic emission wavelength. Based on this principle, several such donor-acceptor pairs, using the Renilla luciferase or its mutants as donor and either GFP2, YFP, mOrange, TagRFP, or TurboFP as acceptor, have been reported for use.In recent years, BRET-related research has become significantly versatile in the assay format and its applicability by adopting the assay on multiple detection devices such as small-animal optical imaging platform or bioluminescence microscope. Beyond the scope of quantitative measurement of PPIs and protein dimerization, molecular optical imaging applications based on BRET assays have broadened its scope for screening of pharmacological compounds by unifying in vitro, live cell, and in vivo animal/plant measurement all on one platform. Taking examples from the literature, this chapter contributes to in-depth methodological details on how to perform in vitro and in vivo BRET experiments, and illustrates its advantages as a single-format assay.

BRET evidence that β2 adrenergic receptors do not oligomerize in cells

Bioluminescence resonance energy transfer (BRET) is often used to study association of membrane proteins, and in particular oligomerization of G protein-coupled receptors (GPCRs). Oligomerization of class A GPCRs is controversial, in part because the methods used to study this question are not completely understood. Here we reconsider oligomerization of the class A β₂ adrenergic receptor (β₂AR), and reevaluate BRET titration as a method to study membrane protein association. Using inducible expression of the energy acceptor at multiple levels of donor expression we find that BRET between β₂AR protomers is directly proportional to the density of the acceptor up to ~3,000 acceptors μm⁻², and does not depend on the density of the donor or on the acceptor:donor (A:D) stoichiometry. In contrast, BRET between tightly-associating control proteins does not depend on the density of the acceptor, but does depend on the density of the donor and on the A:D ratio. We also find that the standard frameworks used to interpret BRET titration experiments rely on simplifying assumptions that are frequently invalid. These results suggest that β₂ARs do not oligomerize in cells, and demonstrate a reliable method of assessing membrane protein association with BRET.

Bioluminescence resonance energy transfer (BRET) to detect the interactions between kappa opioid receptor and non visual arrestins

Bioluminescence resonance energy transfer (BRET) is a very sensitive technique employed to study protein-protein interactions, including G-protein-coupled receptors (GPCRs) hetero- and homo-dimerization. Recently, BRET has also been used to investigate the interaction between GPCRs (e.g., β2 adrenergic receptor, muscarinic M2 receptor, dopaminergic D2 receptor) and non-visual arrestins. Here a BRET protocol is described to investigate interactions between the kappa opioid receptor (KOR) and non visual arrestins (arrestin-2 and arrestin-3) in HEK-293 cells, both under basal conditions and after exposure to KOR ligands.

Demonstration of a direct interaction between β2-adrenergic receptor and insulin receptor by BRET and bioinformatics

Glucose metabolism is under the cooperative regulation of both insulin receptor (IR) and β₂-adrenergic receptor (β₂AR), which represent the receptor tyrosine kinases (RTKs) and seven transmembrane receptors (7TMRs), respectively. Studies demonstrating cross-talk between these two receptors and their endogenous coexpression have suggested their possible interactions. To evaluate the effect of IR and prospective heteromerization on β₂AR properties, we showed that IR coexpression had no effect on the ligand binding properties of β₂AR; however, IR reduced β₂AR surface expression and accelerated its internalization. Additionally, both receptors displayed a similar distribution pattern with a high degree of colocalization. To test the possible direct interaction between β₂AR and IR, we employed quantitative BRET² saturation and competition assays. Saturation assay data suggested constitutive β₂AR and IR homo- and heteromerization. Calculated acceptor/donor (AD₅₀) values as a measure of the relative affinity for homo- and heteromer formation differed among the heteromers that could not be explained by a simple dimer model. In heterologous competition assays, a transient increase in the BRET² signal with a subsequent hyperbolical decrease was observed, suggesting higher-order heteromer formation. To complement the BRET² data, we employed the informational spectrum method (ISM), a virtual spectroscopy method to investigate protein-protein interactions. Computational peptide scanning of β₂AR and IR identified intracellular domains encompassing residues at the end of the 7^th TM domain and C-terminal tail of β₂AR and a cytoplasmic part of the IR β chain as prospective interaction domains. ISM further suggested a high probability of heteromer formation and homodimers as basic units engaged in heteromerization. In summary, our data suggest direct interaction and higher-order β₂AR:IR oligomer formation, likely comprising heteromers of homodimers.

Improved methodical approach for quantitative BRET analysis of G Protein Coupled Receptor dimerization

G Protein Coupled Receptors (GPCR) can form dimers or higher ordered oligomers, the process of which can remarkably influence the physiological and pharmacological function of these receptors. Quantitative Bioluminescence Resonance Energy Transfer (qBRET) measurements are the gold standards to prove the direct physical interaction between the protomers of presumed GPCR dimers. For the correct interpretation of these experiments, the expression of the energy donor Renilla luciferase labeled receptor has to be maintained constant, which is hard to achieve in expression systems. To analyze the effects of non-constant donor expression on qBRET curves, we performed Monte Carlo simulations. Our results show that the decrease of donor expression can lead to saturation qBRET curves even if the interaction between donor and acceptor labeled receptors is non-specific leading to false interpretation of the dimerization state. We suggest here a new approach to the analysis of qBRET data, when the BRET ratio is plotted as a function of the acceptor labeled receptor expression at various donor receptor expression levels. With this method, we were able to distinguish between dimerization and non-specific interaction when the results of classical qBRET experiments were ambiguous. The simulation results were confirmed experimentally using rapamycin inducible heterodimerization system. We used this new method to investigate the dimerization of various GPCRs, and our data have confirmed the homodimerization of V2 vasopressin and CaSR calcium sensing receptors, whereas our data argue against the heterodimerization of these receptors with other studied GPCRs, including type I and II angiotensin, β2 adrenergic and CB1 cannabinoid receptors.

Evidence for aggregation of protein kinase CK2 in the cell: a novel strategy for studying CK2 holoenzyme interaction by BRET(2)

Protein kinase CK2 is a ubiquitous pro-survival kinase whose substrate targets are involved in various cellular processes. Crystal structure analysis confirmed constitutive activity of the kinase, yet CK2 activity regulation in the cell is still obscure. In-vitro studies suggest autoinhibitory aggregation of the hetero-tetrameric CK2 holoenzyme as a basis for CK2 regulation. In this study, we applied bioluminescent resonance energy transfer (BRET) technology to investigate CK2 holoenzyme aggregation in living cells. We designed a BRET(2) pair consisting of the fusion proteins CK2α-Rluc8 and CK2α-GFP(2). This BRET(2) sensor reported specific interaction of CK2 holoenzyme complexes. Furthermore, the BRET(2) sensor was applied to study modulators of CK2 aggregation. We found that CK2 aggregation is not static and can be influenced by the CK2-binding protein alpha subunit of the heterotrimeric G-protein that stimulates adenylyl cyclase (Gαs) and the polycationic compound polylysine. Gαs, but not the CK2 substrate β-arrestin2, decreased the BRET(2) signal by up to 50%. Likewise polylysine, but not the CK2 inhibitor DRB, decreased the signal in a dose-dependent manner up to 50%. For the first time, we present direct experimental evidence for CK2 holoenzyme aggregates in the cell. Our data suggest that CK2 activity may be controlled by holoenzyme aggregation, to our knowledge a novel mechanism for protein kinase regulation. Moreover, the BRET(2) sensor used in our study is a novel tool for studying CK2 regulation by aggregation and pharmacological screening for novel allosteric CK2 effectors.

Evolution of BRET Biosensors from Live Cell to Tissue-Scale In vivo Imaging

Development of bioluminescence resonance energy transfer (BRET) based genetic sensors for sensing biological functions such as protein-protein interactions (PPIs) in vivo has a special value in measuring such dynamic events at their native environment. Since its inception in the late nineties, BRET related research has gained significant momentum in terms of adding versatility to the assay format and wider applicability where it has been suitably used. Beyond the scope of quantitative measurement of PPIs and protein dimerization, molecular imaging applications based on BRET assays have broadened its scope for screening pharmacologically important compounds by in vivo imaging as well. In this mini-review we focus on an in-depth analysis of engineered BRET systems developed and their successful application to cell-based assays as well as in vivo non-invasive imaging in live subjects.