Fluorescence resonance energy transfer analysis of cell surface receptor interactions and signaling using spectral variants of the green fluorescent protein

Sharks Provide Evidence for a Highly Complex TNFSF Repertoire in the Jawed Vertebrate Ancestor

Cytokines of the TNF superfamily (TNFSF) control many immunological processes and are implicated in the etiology of many immune disorders and diseases. Despite their obvious biological importance, the TNFSF repertoires of many species remain poorly characterized. In this study, we perform detailed bioinformatic, phylogenetic, and syntenic analyses of five cartilaginous fish genomes to identify their TNFSF repertoires. Strikingly, we find that shark genomes harbor ∼30 TNFSF genes, more than any other vertebrate examined to date and substantially more than humans. This is due to better retention of the ancestral jawed vertebrate TNFSF repertoire than any other jawed vertebrate lineage, combined with lineage-specific gene family expansions. All human TNFSFs appear in shark genomes, except for lymphotoxin-α (LTA; TNFSF1) and TNF (TNFSF2), and CD70 (TNFSF7) and 4-1BBL (TNFSF9), which diverged by tandem duplications early in tetrapod and mammalian evolution, respectively. Although lacking one-to-one LTA and TNF orthologs, sharks have evolved lineage-specific clusters of LTA/TNF co-orthologs. Other key findings include the presence of two BAFF (TNFSF13B) genes along with orthologs of APRIL (TNFSF13) and BALM (TNFSF13C) in sharks, and that all cartilaginous fish genomes harbor an ∼400-million-year-old cluster of multiple FASLG (TNFSF6) orthologs. Finally, sharks have retained seven ancestral jawed vertebrate TNFSF genes lost in humans. Taken together, our data indicate that the jawed vertebrate ancestor possessed a much larger and diverse TNFSF repertoire than previously hypothesized and oppose the idea that the cartilaginous fish immune system is “primitive” compared with that of mammals.

The role of polymeric chains as a protective environment for improving the stability and efficiency of fluorogenic peptide substrates

We have faced the preparation of fully water-soluble fluorescent peptide substrate with long-term environmental stability (in solution more than 35 weeks) and, accordingly, with stable results in the use of this probe in determining the activity of enzymes. We have achieved this goal by preparing a co-polymer of the commercial N-vinyl-2-pyrrolidone (99.5% mol) and a fluorescent substrate for trypsin activity determination having a vinylic group (0.5%). The activity of trypsin has been measured in water solutions of this polymer over time, contrasted against the activity of both the commercial substrate Z-L-Arg-7-amido-4-methylcoumarin hydrochloride and its monomeric derivative, prepared ad-hoc. Initially, the activity of the sensory polymer was 74.53 ± 1.72 nmol/min/mg of enzyme, while that of the commercial substrate was 20.44 ± 0.65 nmol/min/mg of enzyme, the former maintained stable along weeks and the latter with a deep decay to zero in three weeks. The ‘protection’ effect exerted by the polymer chain has been studied by solvation studies by UV–Vis spectroscopy, steady-state & time resolved fluorescence, thermogravimetry and isothermal titration calorimetry.

Interaction of human CRX and NRL in live HEK293T cells measured using fluorescence resonance energy transfer (FRET)

CRX and NRL are retina-specific transcription factors that control rod photoreceptor differentiation and synergistically activate rod phototransduction gene expression. Previous experiments showed they interact in vitro and in yeast two-hybrid assays. Here, we examined CRX-NRL interaction in live HEK293T cells using two fluorescence resonance energy transfer (FRET) approaches: confocal microscopy and flow cytometry (FC-FRET). FC-FRET can provide measurements from many cells having wide donor–acceptor expression ranges. FRET efficiencies were calibrated with a series of donor (EGFP)-acceptor (mCherry) fusion proteins separated with linkers between 6–45 amino acids. CRX and NRL were fused at either terminus with EGFP or mCherry to create fluorescent proteins, and all combinations were tested in transiently transfected cells. FRET signals between CRX or NRL homo-pairs were highest with both fluorophores fused to the DNA binding domains (DBD), lower with both fused to the activation domains (AD), and not significant when fused on opposite termini. NRL had stronger FRET signals than CRX. A significant FRET signal between CRX and NRL hetero-pairs was detected when donor was fused to the CRX DNA binding domain and the acceptor fused to the NRL activation domain. FRET signals increased with CRX or NRL expression levels at a rate much higher than expected for collisional FRET alone. Together, our results show the formation of CRX-NRL complexes in live HEK293T cells that are close enough for FRET.

Flow cytometry based-FRET: basics, novel developments and future perspectives

Förster resonance energy transfer (FRET) is a widespread technology used to analyze and quantify protein interactions in multiple settings. While FRET is traditionally measured by microscopy, flow cytometry based-FRET is becoming popular within the last decade and more commonly used. Flow cytometry based-FRET offers the possibility to assess FRET in a short time-frame in a high number of cells thereby allowing stringent and statistically robust quantification of FRET in multiple samples. Furthermore, established, simple and easy to implement gating strategies facilitate the adaptation of flow cytometry based-FRET measurements to most common flow cytometers. We here summarize the basics of flow cytometry based-FRET, highlight recent novel developments in this field and emphasize on exciting future perspectives.

Illuminating the Path to Target GPCR Structures and Functions

G-Protein-coupled receptors (GPCRs) are ubiquitous within eukaryotes, responsible for a wide array of physiological and pathological processes. Indeed, the fact that they are the most drugged target in the human genome is indicative of their importance. Despite the clear interest in GPCRs, most information regarding their activity has been so far obtained by analyzing the response from a "bulk medium". As such, this Perspective summarizes some of the common methods for this indirect observation. Nonetheless, by inspecting approaches applying super-resolution imaging, we argue that imaging is perfectly situated to obtain more detailed structural and spatial information, assisting in the development of new GPCR-targeted drugs and clinical strategies. The benefits of direct optical visualization of GPCRs are analyzed in the context of potential future directions in the field.

Milestones in the development and implementation of FRET-based sensors of intracellular signals: A biological perspective of the history of FRET

Fӧrster resonance energy transfer (FRET) has been described for more than a century. FRET has become a mainstay for the study of protein localization in living cells and tissues. It has also become widely used in the fields that comprise cellular signaling. FRET-based probes have been developed to monitor second messenger signals, the phosphorylation state of peptides and proteins, and subsequent cellular responses. Here, we discuss the milestones that led to FRET becoming a widely used tool for the study of biological systems: the theoretical description of FRET, the insight to use FRET as a molecular ruler, and the isolation and genetic modification of green fluorescent protein (GFP). Each of these milestones were critical to the development of a myriad of FRET-based probes and reporters in common use today. FRET-probes offer a unique opportunity to interrogate second messenger signals and subsequent protein phosphorylation - and perhaps the most effective approach for study of cAMP/PKA pathways. As such, FRET probes are widely used in the study of intracellular signaling pathways. Yet, somehow, the potential of FRET-based probes to provide windows through which we can visualize complex cellular signaling systems has not been fully reached. Hence we conclude by discussing the technical challenges to be overcome if FRET-based probes are to live up to their potential for the study of complex signaling networks.

Molecular Imaging with Reporter Genes: Has Its Promise Been Delivered?

The first reporter systems were developed in the early 1980s and were based on measuring the activity of an enzyme-as a surrogate measure of promoter-driven transcriptional activity-which is now known as a reporter gene system. The initial objective and application of reporter techniques was to analyze the activity of a specific promoter (namely, the expression of a gene that is under the regulation of the specific promoter that is linked to the reporter gene). This system allows visualization of specific promoter activity with great sensitivity. In general, there are 2 classes of reporter systems: constitutively expressed (always-on) reporter constructs used for cell tracking, and inducible reporter systems sensitive to endogenous signaling molecules and transcription factors that characterize specific tissues, tumors, or signaling pathways.This review traces the development of different reporter systems, using fluorescent and bioluminescent proteins as well as radionuclide-based reporter systems. The development and application of radionuclide-based reporter systems is the focus of this review. The question at the end of the review is whether the "promise" of reporter gene imaging has been realized. What is required for moving forward with radionuclide-based reporter systems, and what is required for successful translation to clinical applications?

Conformational states of TNFR1 as a molecular switch for receptor function

Tumor necrosis factor receptor 1 (TNFR1) is a transmembrane receptor that plays a key role in the regulation of the inflammatory pathway. While inhibition of TNFR1 has been the focus of many studies for the treatment of autoimmune diseases such as rheumatoid arthritis, activation of the receptor is important for the treatment of immunodeficiency diseases such as HIV and neurodegenerative diseases such as Alzheimer's disease where a boost in immune signaling is required. In addition, activation of other TNF receptors such as death receptor 5 or FAS receptor is important for cancer therapy. Here, we used a previously established TNFR1 fluorescence resonance energy transfer (FRET) biosensor together with a fluorescence lifetime technology as a high-throughput screening platform to identify a novel small molecule that activates TNFR1 by increasing inter-monomeric spacing in a ligand-independent manner. This shows that the conformational rearrangement of pre-ligand assembled receptor dimers can determine the activity of the receptor. By probing the interaction between the receptor and its downstream signaling molecule (TRADD) our findings support a new model of TNFR1 activation in which varying conformational states of the receptor act as a molecular switch in determining receptor function.

Noncompetitive inhibitors of TNFR1 probe conformational activation states

Tumor necrosis factor receptor 1 (TNFR1) is a central mediator of the inflammatory pathway and is associated with several autoimmune diseases such as rheumatoid arthritis. A revision to the canonical model of TNFR1 activation suggests that activation involves conformational rearrangements of preassembled receptor dimers. Here, we identified small-molecule allosteric inhibitors of TNFR1 activation and probed receptor dimerization and function. Specifically, we used a fluorescence lifetime-based high-throughput screen and biochemical, biophysical, and cellular assays to identify small molecules that noncompetitively inhibited the receptor without reducing ligand affinity or disrupting receptor dimerization. We also found that residues in the ligand-binding loop that are critical to the dynamic coupling between the extracellular and the transmembrane domains played a key gatekeeper role in the conformational dynamics associated with signal propagation. Last, using a simple structure-activity relationship analysis, we demonstrated that these newly found molecules could be further optimized for improved potency and specificity. Together, these data solidify and deepen the new model for TNFR1 activation.

Clinical or ATPase domain mutations in ABCD4 disrupt the interaction between the vitamin B12-trafficking proteins ABCD4 and LMBD1

Vitamin B₁₂ (cobalamin (Cbl)), in the cofactor forms methyl-Cbl and adenosyl-Cbl, is required for the function of the essential enzymes methionine synthase and methylmalonyl-CoA mutase, respectively. Cbl enters mammalian cells by receptor-mediated endocytosis of protein-bound Cbl followed by lysosomal export of free Cbl to the cytosol and further processing to these cofactor forms. The integral membrane proteins LMBD1 and ABCD4 are required for lysosomal release of Cbl, and mutations in the genes LMBRD1 and ABCD4 result in the cobalamin metabolism disorders cblF and cblJ. We report a new (fifth) patient with the cblJ disorder who presented at 7 days of age with poor feeding, hypotonia, methylmalonic aciduria, and elevated plasma homocysteine and harbored the mutations c.1667_1668delAG [p.Glu556Glyfs*27] and c.1295G>A [p.Arg432Gln] in the ABCD4 gene. Cbl cofactor forms are decreased in fibroblasts from this patient but could be rescued by overexpression of either ABCD4 or, unexpectedly, LMBD1. Using a sensitive live-cell FRET assay, we demonstrated selective interaction between ABCD4 and LMBD1 and decreased interaction when ABCD4 harbored the patient mutations p.Arg432Gln or p.Asn141Lys or when artificial mutations disrupted the ATPase domain. Finally, we showed that ABCD4 lysosomal targeting depends on co-expression of, and interaction with, LMBD1. These data broaden the patient and mutation spectrum of cblJ deficiency, establish a sensitive live-cell assay to detect the LMBD1-ABCD4 interaction, and confirm the importance of this interaction for proper intracellular targeting of ABCD4 and cobalamin cofactor synthesis.

The Bromodomain and Extra-Terminal Domain (BET) Family: Functional Anatomy of BET Paralogous Proteins

The Bromodomain and Extra-Terminal Domain (BET) family of proteins is characterized by the presence of two tandem bromodomains and an extra-terminal domain. The mammalian BET family of proteins comprises BRD2, BRD3, BRD4, and BRDT, which are encoded by paralogous genes that may have been generated by repeated duplication of an ancestral gene during evolution. Bromodomains that can specifically bind acetylated lysine residues in histones serve as chromatin-targeting modules that decipher the histone acetylation code. BET proteins play a crucial role in regulating gene transcription through epigenetic interactions between bromodomains and acetylated histones during cellular proliferation and differentiation processes. On the other hand, BET proteins have been reported to mediate latent viral infection in host cells and be involved in oncogenesis. Human BRD4 is involved in multiple processes of the DNA virus life cycle, including viral replication, genome maintenance, and gene transcription through interaction with viral proteins. Aberrant BRD4 expression contributes to carcinogenesis by mediating hyperacetylation of the chromatin containing the cell proliferation-promoting genes. BET bromodomain blockade using small-molecule inhibitors gives rise to selective repression of the transcriptional network driven by c-MYC These inhibitors are expected to be potential therapeutic drugs for a wide range of cancers. This review presents an overview of the basic roles of BET proteins and highlights the pathological functions of BET and the recent developments in cancer therapy targeting BET proteins in animal models.

Current Experimental Methods for Characterizing Protein-Protein Interactions

Protein molecules often interact with other partner protein molecules in order to execute their vital functions in living organisms. Characterization of protein-protein interactions thus plays a central role in understanding the molecular mechanism of relevant protein molecules, elucidating the cellular processes and pathways relevant to health or disease for drug discovery, and charting large-scale interaction networks in systems biology research. A whole spectrum of methods, based on biophysical, biochemical, or genetic principles, have been developed to detect the time, space, and functional relevance of protein-protein interactions at various degrees of affinity and specificity. This article presents an overview of these experimental methods, outlining the principles, strengths and limitations, and recent developments of each type of method.

Current Experimental Methods for Characterizing Protein-Protein Interactions

Protein molecules often interact with other partner protein molecules in order to execute their vital functions in living organisms. Characterization of protein-protein interactions thus plays a central role in understanding the molecular mechanism of relevant protein molecules, elucidating the cellular processes and pathways relevant to health or disease for drug discovery, and charting large-scale interaction networks in systems biology research. A whole spectrum of methods, based on biophysical, biochemical, or genetic principles, have been developed to detect the time, space, and functional relevance of protein-protein interactions at various degrees of affinity and specificity. This article presents an overview of these experimental methods, outlining the principles, strengths and limitations, and recent developments of each type of method.

Flow Cytometry Enables Multiplexed Measurements of Genetically Encoded Intramolecular FRET Sensors Suitable for Screening

Genetically encoded sensors based on intramolecular FRET between CFP and YFP are used extensively in cell biology research. Flow cytometry has been shown to offer a means to measure CFP-YFP FRET; we suspected it would provide a unique way to conduct multiplexed measurements from cells expressing different FRET sensors, which is difficult to do with microscopy, and that this could be used for screening. We confirmed that flow cytometry accurately measures FRET signals using cells transiently transfected with an ERK activity reporter, comparing responses measured with imaging and cytometry. We created polyclonal long-term transfectant lines, each expressing a different intramolecular FRET sensor, and devised a way to bar-code four distinct populations of cells. We demonstrated the feasibility of multiplexed measurements and determined that robust multiplexed measurements can be conducted in plate format. To validate the suitability of the method for screening, we measured responses from a plate of bacterial extracts that in unrelated experiments we had determined contained the protein kinase C (PKC)–activating compound teleocidin A-1. The multiplexed assay correctly identifying the teleocidin A-1-containing well. We propose that multiplexed cytometric FRET measurements will be useful for analyzing cellular function and for screening compound collections.

Real-time detection of cellular death receptor-4 activation by fluorescence resonance energy transfer

Targeted therapy involving the activation of death receptors DR4 and/or DR5 by its ligand, TRAIL, can selectively induce apoptosis in certain tumor cells. In order to profile the dynamic activation or trimerization of TRAIL-DR4 in live cells in real-time, the development of an apoptosis reporter cell line is essential. Fluorescence resonance energy transfer (FRET) technology via a FRET pair, cyan fluorescence protein (CFP) and yellow fluorescence protein (YFP), was used in this study. DR4-CFP and DR4-YFP were stably expressed in human lung cancer PC9 cells. Flow cytometer sorting and limited dilution coupled with fluorescence microscopy were used to select a monoclonal reporter cell line with high and compatible expression levels of DR4-CFP and DR4-YFP. FRET experiments were conducted and FRET efficiencies were monitored according to the Siegel's YFP photobleaching FRET protocol. Upon TRAIL induction a significant increase in FRET efficiencies from 5% to 9% demonstrated the ability of the DR4-CFP/YFP reporter cell line in monitoring the dynamic activation of TRAIL pathways. 3D reconstructed confocal images of DR4-CFP/YFP reporter cells exhibited a colocalized expression of DR4-CFP and DR4-YFP mainly on cell membranes. FRET results obtained during this study complements the use of epi-fluorescence microscopy for FRET analysis. The real-time FRET analysis allows the dynamic profiling of the activation of TRAIL pathways by using the time-lapse fluorescence microscopy. Therefore, DR4-CFP/YFP PC9 reporter cells along with FRET technology can be used as a tool for anti-cancer drug screening to identify compounds that are capable of activating TRAIL pathways.

Partial acceptor photobleaching-based quantitative FRET method completely overcoming emission spectral crosstalks

Based on the quantitative fluorescence resonance energy transfer (FRET) method named PbFRET we reported recently, we herein developed a partial acceptor photobleaching-based quantitative FRET algorithm named B-PbFRET method. B-PbFRET overcomes not only the acceptor excitation crosstalk and donor emission spectral crosstalk but also the acceptor emission spectral crosstalk that harasses previous methods including fluorescence lifetime (FLIM), fluorescence recovery of donor after acceptor photobleaching, and acceptor sensitized emission (SE)-based methods. B-PbFRET method is implemented by simultaneously measuring the fluorescence intensity of both donor and acceptor channels at donor excitation before and after partial acceptor photobleaching, and it can directly measure the FRET efficiency (E) without any verified references. Based on the theoretical analysis of B-PbFRET, we also developed a more straightforward correction method named C-PbFRET to obtain the absolute E from the value measured by PbFRET for a given donor-acceptor pair. We validated both B-PbFRET and C-PbFRET methods by measuring the E of two linked constructs, 18AA and SCAT3 proteins, in single living cells, and our data demonstrated that both B-PbFRET and C-PbFRET methods can directly measure the absolute E of the linked constructs inside living cells under different degrees of acceptor emission spectral crosstalk.

Tumor necrosis factor receptors: biology and therapeutic potential in kidney diseases

The major evolutionary advance represented in the human immune system is a mechanism of antigen-directed immunity in which tumor necrosis factor (TNF)-α and TNF receptors (TNFRs) play essential roles. Binding of TNF-α to the 55-kDa type I TNFR (TNFR1, TNFRSF1A, CD120a, p55) or the 75-kDa type II TNFR (TNFR2, TNFRSF1B, CD120b, p75) activates signaling pathways controlling inflammatory, immune and stress responses, as well as host defense and apoptosis. Multiple studies have investigated the role of TNFRs in the development of early and late renal failure (diabetic nephropathy, nephroangiosclerosis, acute kidney transplant rejection, renal cell carcinoma, glomerulonephritis, sepsis and obstructive renal injury). This article reviews the general characteristics, the analytical aspects and the biology of TNFRs in this domain. In addition, the potential therapeutic application of specific TNFR blockers is discussed.

TNF receptor 1 genetic risk mirrors outcome of anti-TNF therapy in multiple sclerosis

Although there has been much success in identifying genetic variants associated with common diseases using genome-wide association studies (GWAS), it has been difficult to demonstrate which variants are causal and what role they have in disease. Moreover, the modest contribution that these variants make to disease risk has raised questions regarding their medical relevance. Here we have investigated a single nucleotide polymorphism (SNP) in the TNFRSF1A gene, that encodes tumour necrosis factor receptor 1 (TNFR1), which was discovered through GWAS to be associated with multiple sclerosis (MS), but not with other autoimmune conditions such as rheumatoid arthritis, psoriasis and Crohn’s disease. By analysing MS GWAS data in conjunction with the 1000 Genomes Project data we provide genetic evidence that strongly implicates this SNP, rs1800693, as the causal variant in the TNFRSF1A region. We further substantiate this through functional studies showing that the MS risk allele directs expression of a novel, soluble form of TNFR1 that can block TNF. Importantly, TNF-blocking drugs can promote onset or exacerbation of MS, but they have proven highly efficacious in the treatment of autoimmune diseases for which there is no association with rs1800693. This indicates that the clinical experience with these drugs parallels the disease association of rs1800693, and that the MS-associated TNFR1 variant mimics the effect of TNF-blocking drugs. Hence, our study demonstrates that clinical practice can be informed by comparing GWAS across common autoimmune diseases and by investigating the functional consequences of the disease-associated genetic variation.

Sequential inter- and intrasubunit rearrangements during activation of dimeric metabotropic glutamate receptor 1

The metabotropic glutamate receptor 1 (mGluR1), a class C member of the heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptor family, is a constitutive dimer that regulates excitatory neurotransmission. We investigated the role of homodimer formation in mGluR1 activation by examining activation-dependent inter- and intrasubunit conformational changes by fluorescence resonance energy transfer (FRET). We inserted yellow and cyan fluorescent proteins in the second intracellular loop and at the carboxyl terminus of mGluR1 to act as FRET sensors and expressed these proteins in human embryonic kidney 293 cells. Agonist-dependent activation of these mGluR1 chimeras rapidly increased the intersubunit FRET, suggesting rapid movement of the subunits relative to each other. After intersubunit movement, the intrasubunit FRET decreased, reflecting conformational changes within a subunit. Cotransfection of chimeric receptor subunits that were capable or incapable of G protein coupling revealed that only a single subunit assumes an active state in an mGluR1 receptor dimer.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces death receptor 5 networks that are highly organized

Recent evidence suggests that TNF-related apoptosis-inducing ligand (TRAIL), a death-inducing cytokine with anti-tumor potential, initiates apoptosis by re-organizing TRAIL receptors into large clusters, although the structure of these clusters and the mechanism by which they assemble are unknown. Here, we demonstrate that TRAIL receptor 2 (DR5) forms receptor dimers in a ligand-dependent manner at endogenous receptor levels, and these receptor dimers exist within high molecular weight networks. Using mutational analysis, FRET, fluorescence microscopy, synthetic biochemistry, and molecular modeling, we find that receptor dimerization relies upon covalent and noncovalent interactions between membrane-proximal residues. Additionally, by using FRET, we show that the oligomeric structure of two functional isoforms of DR5 is indistinguishable. The resulting model of DR5 activation should revise the accepted architecture of the functioning units of DR5 and the structurally homologous TNF receptor superfamily members.

A comparative analysis of resonance energy transfer methods for Alzheimer related protein-protein interactions in living cells

Fluorescence resonance energy transfer (FRET) and bioluminescence resonance energy transfer (BRET) are extensively used to analyze protein interactions occurring in living cells. Although these two techniques are broadly applied in cellular biology, comparative analysis of their strengths and limitations is lacking. To this end, we analyzed a small network of proteins involved in the amyloidogenic processing of the Alzheimer β-amyloid precursor using FRET based cytometry, BRET, and fluorescence lifetime imaging microscopy (FLIM). Using all three methods, we were able to detect the interactions of the amyloid precursor protein with APBB1, APBB2, and APP itself. And we found an unreported interacting pair, APP-APH1A. In addition, we show that these four interacting pairs exhibit a strong FRET correlation with the acceptor/donor expression ratios. Overall the FRET based cytometry was the most sensitive and reliable approach to screen for new interacting proteins. Therefore, we applied FRET based cytometry to study competitive binding of two proteins, APBB1 and APBB2, with the same APP target.

Guide to red fluorescent proteins and biosensors for flow cytometry

Since the discovery of the first red fluorescent protein (RFP), named DsRed, 12 years ago, a wide pallet of red-shifted fluorescent proteins has been cloned and biotechnologically developed into monomeric fluorescent probes for optical microscopy. Several new types of monomeric RFPs that change the emission wavelength either with time, called fluorescent timers, or after a brief irradiation with violet light, known as photoactivatable proteins, have been also engineered. Moreover, RFPs with a large Stokes shift of fluorescence emission have been recently designed. Because of their distinctive excitation and fluorescence detection conditions developed specifically for microscopy, these fluorescent probes can be suboptimal for flow cytometry. Here, we have selected and summarized the advanced orange, red, and far-red fluorescent proteins with the properties specifically required for the flow cytometry applications. Their effective brightness was calculated for the laser sources available for the commercial flow cytometers and sorters. Compatibility of the fluorescent proteins of different colors in a multiparameter flow cytometry was determined. Novel FRET pairs, utilizing RFPs, RFP-based intracellular biosensors, and their application to a high-throughput screening, are also discussed.

Characterizing the interaction between the Rab6 GTPase and Mint3 via flow cytometry based FRET analysis

In extension to previously applied techniques like yeast two-hybrid and GST pull-down assays, we successfully established a FACS-based FRET analysis to investigate the interaction of the Mint3 adaptor protein and the small Rab GTPase Rab6A in living mammalian cells. A Mint3 mutant containing only the PTB domain (Mint3Delta6) is able to interact with the constitutively active form of Rab6A. Mint3Delta4, a mutant lacking part of the PTB domain was unable to interact with Rab6A in GST pull-down analysis and did not produce FRET signals, when co-expressed with active Rab6A. We demonstrate that this FACS-based FRET analysis is a suitable method for interaction studies between two proteins in living cells.

Molecular imaging of in vivo gene expression

Molecular imaging provides spatial and temporal information on cellular changes that occur during development and in disease. MRI and optical imaging of reporter genes allows for the visualization of promoter activity, protein-protein interactions, protein stability and the tracking of individual proteins and cells. Reporter genes can be genetically encoded in transgenic animals or detected through the administration of an exogenous contrast agent. Advances in molecular imaging of reporter genes have led to the development of imaging probes that detect changes in endogenous cellular changes. The ability to use contrast agents coupled with functional information on cellular events will allow for sensitive assessment of individual patient therapies, leading to an accurately tailored treatment regimen.

A flow cytometry-based FRET assay to identify and analyse protein-protein interactions in living cells

Background

Försters resonance energy transfer (FRET) microscopy is widely used for the analysis of protein interactions in intact cells. However, FRET microscopy is technically challenging and does not allow assessing interactions in large cell numbers. To overcome these limitations we developed a flow cytometry-based FRET assay and analysed interactions of human and simian immunodeficiency virus (HIV and SIV) Nef and Vpu proteins with cellular factors, as well as HIV Rev multimer-formation.

Results

Amongst others, we characterize the interaction of Vpu with CD317 (also termed Bst-2 or tetherin), a host restriction factor that inhibits HIV release from infected cells and demonstrate that the direct binding of both is mediated by the Vpu membrane-spanning region. Furthermore, we adapted our assay to allow the identification of novel protein interaction partners in a high-throughput format.

Conclusion

The presented combination of FRET and FACS offers the precious possibility to discover and define protein interactions in living cells and is expected to contribute to the identification of novel therapeutic targets for treatment of human diseases.

Illuminating the life of GPCRs

The investigation of biological systems highly depends on the possibilities that allow scientists to visualize and quantify biomolecules and their related activities in real-time and non-invasively. G-protein coupled receptors represent a family of very dynamic and highly regulated transmembrane proteins that are involved in various important physiological processes. Since their localization is not confined to the cell surface they have been a very attractive "moving target" and the understanding of their intracellular pathways as well as the identified protein-protein-interactions has had implications for therapeutic interventions. Recent and ongoing advances in both the establishment of a variety of labeling methods and the improvement of measuring and analyzing instrumentation, have made fluorescence techniques to an indispensable tool for GPCR imaging. The illumination of their complex life cycle, which includes receptor biosynthesis, membrane targeting, ligand binding, signaling, internalization, recycling and degradation, will provide new insights into the relationship between spatial receptor distribution and function. This review covers the existing technologies to track GPCRs in living cells. Fluorescent ligands, antibodies, auto-fluorescent proteins as well as the evolving technologies for chemical labeling with peptide- and protein-tags are described and their major applications concerning the GPCR life cycle are presented.

Measuring the proximity of T-lymphocyte CXCR4 and TCR by fluorescence resonance energy transfer (FRET)

Multiprotein complexes play an important role in nearly all cell functions; therefore, the characterization of protein-protein interactions in living cells constitutes an important step in the analysis of cellular signaling pathways. Using fluorescence resonance energy transfer (FRET) as a "molecular ruler" is a powerful approach for identifying biologically relevant molecular interactions with high spatiotemporal resolution. Here, we describe two methods that use FRET to detect a physical interaction between the T-cell antigen receptor (TCR) and the CXCR4 chemokine receptor in living T lymphocytes. These FRET approaches use two different sets of chromophores. We discuss the design strategies, control experiments, and pitfalls involved in using these FRET approaches. Although there is no perfect pair of chromophores for FRET, the two FRET methods described here provide complementary and reliable insight into the molecular interactions between these receptor molecules.

Death receptors as targets for anti-cancer therapy

Human tumour cells are characterized by their ability to avoid the normal regulatory mechanisms of cell growth, division and death. The classical chemotherapy aims to kill tumour cells by causing DNA damage-induced apoptosis. However, as many tumour cells possess mutations in intracellular apoptosis-sensing molecules like p53, they are not capable of inducing apoptosis on their own and are therefore resistant to chemotherapy. With the discovery of the death receptors the opportunity arose to directly trigger apoptosis from the outside of tumour cells, thereby circumventing chemotherapeutic resistance. Death receptors belong to the tumour necrosis factor receptor superfamily, with tumour necrosis factor (TNF) receptor-1, CD95 and TNF-related apoptosis-inducing ligand-R1 and -R2 being the most prominent members. This review covers the current knowledge about these four death receptors, summarizes pre-clinical approaches engaging these death receptors in anti-cancer therapy and also gives an overview about their application in clinical trials conducted to date.

A quantitative protocol for dynamic measurements of protein interactions by Förster resonance energy transfer-sensitized fluorescence emission

Fluorescence detection of acceptor molecules sensitized by Förster resonance energy transfer (FRET) is a powerful method to study protein interactions in living cells. The method requires correction for donor spectral bleed-through and acceptor cross-excitation as well as the correct normalization of signals to account for varying fluorophore concentrations and imaging parameters. In this paper, we review different methods for FRET signal normalization and then present a rigorous model for sensitized emission measurements, which is both intuitive to understand and practical to apply. The method is validated by comparison with the acceptor photobleaching and donor lifetime-imaging techniques in live cell samples containing EYFP and ECFP tandem constructs exhibiting known amounts of FRET. By varying the stoichiometry of interaction in a controlled fashion, we show that information on the fractions of interacting donors and acceptors can be recovered. Furthermore, the method is tested by performing measurements on different microscopy platforms in both widefield and confocal imaging modes to show that signals recovered under different imaging conditions are in quantitative agreement. Finally, the method is applied in the study of dynamic interactions in the cyclin–cdk family of proteins in live cells. By normalizing the obtained signals for both acceptor and donor concentrations and using a FRET exhibiting control construct for calibration, stoichiometric changes in these interactions could be visualized in real time. The paper is written to be of practical use to researchers interested in performing sensitized emission measurements. The correct interpretation of the retrieved signals in a biological context is emphasized, and guidelines are given for the practical application of the developed algorithms.

Multiple protein domains mediate interaction between Bcl10 and MALT1

Bcl10 and MALT1 are essential mediators of NF-kappaB activation in response to the triggering of a diverse array of transmembrane receptors, including antigen receptors. Additionally, both proteins are translocation targets in MALT lymphoma. Thus, a detailed understanding of the interaction between these mediators is of considerable biological importance. Previous studies have indicated that a 13-amino acid region downstream of the Bcl10 caspase recruitment domain (CARD) is responsible for interacting with the immunoglobulin-like domains of MALT1. We now provide evidence that the death domain of MALT1 and the CARD of Bcl10 also contribute to Bcl10-MALT1 interactions. Although a direct interaction between the MALT1 death domain and Bcl10 cannot be detected via immunoprecipitation, FRET data strongly suggest that the death domain of MALT1 contributes significantly to the association between Bcl10 and MALT1 in T cells in vivo. Furthermore, analysis of point mutants of conserved residues of Bcl10 shows that the Bcl10 CARD is essential for interaction with the MALT1 N terminus. Mutations that disrupt proper folding of the Bcl10 CARD strongly impair Bcl10-MALT1 interactions. Molecular modeling and functional analyses of Bcl10 point mutants suggest that residues Asp(80) and Glu(84) of helix 5 of the Bcl10 CARD directly contact MALT1. Together, these data demonstrate that the association between Bcl10 and MALT1 involves a complex interaction between multiple protein domains. Moreover, the Bcl10-MALT1 interaction is the second reported example of interactions between a CARD and a non-CARD protein region, which suggests that many signaling cascades may utilize CARD interactions with non-CARD domains.

Bimolecular fluorescence complementation (BiFC) analysis of protein interactions in Caenorhabditis elegans

Protein interactions are essential components of signal transduction in cells. With the progress in genome-wide yeast two hybrid screens and proteomics analyses, many protein interaction networks have been generated. These analyses have identified hundreds and thousands of interactions in cells and organisms, creating a challenge for further validation under physiological conditions. The bimolecular fluorescence complementation (BiFC) assay is such an assay that meets this need. The BiFC assay is based on the principle of protein fragment complementation, in which two non-fluorescent fragments derived from a fluorescent protein are fused to a pair of interacting partners. When the two partners interact, the two non-fluorescent fragments are brought into proximity and an intact fluorescent protein is reconstituted. Hence, the reconstituted fluorescent signals reflect the interaction of two proteins under study. Over the past six years, the BiFC assay has been used for visualization of protein interactions in living cells and organisms, including our application of the BiFC assay to the transparent nematode Caenorhabditis elegans. We have demonstrated that BiFC analysis in C. elegans provides a direct means to identify and validate protein interactions in living worms and allows visualization of temporal and spatial interactions. Here, we provide a guideline for the implementation of BiFC analysis in living worms and discuss the factors that are critical for BiFC analysis.

Fluorescence lifetime imaging: association of cortical actin with a PIP3-rich membrane compartment

We have used fluorescence lifetime imaging (FLIM) to study actin and plasma membrane dynamics in B16-F1 melanoma cells. In the absence of a FRET acceptor, significant changes in the fluorescence lifetime of GFP were induced simply by linking the fluorophore to different functional probes, including beta-actin, the PH domains of PLCdelta and Akt, the Ras farnesylation signal, and the neuromodulin palmitoylation signal (MEM). In contrast, the lifetime of GFP-actin was constant despite the many different local environments of G- and F-actin within the cell. Treatment with cytochalasin D but not latrunculin A significantly shortened the lifetime of GFP-beta-actin in the absence of a FRET acceptor. Robust lifetime shifts were observed using either a GFP-RFP chimera or co-transfection of GFP-MEM with RFP-MEM. In contrast to previous reports we observed a photobleaching-dependent change in the lifetime of GFP which could complicate the interpretation of FRET experiments. Of the membrane probes tested only the fluorescence lifetime of GFP-Akt was influenced by the presence of mRFP-actin, suggesting that the cortical actin meshwork is associated with a PIP3-enriched compartment of the plasma membrane. These results will aid in the design of new FRET-based approaches to study cytoskeletal interactions at the molecular level.

T cell receptor engagement triggers its CD3epsilon and CD3zeta subunits to adopt a compact, locked conformation

How the T cell antigen receptor (TCR) discriminates between molecularly related peptide/Major Histocompatibility Complex (pMHC) ligands and converts this information into different possible signaling outcomes is still not understood. One current model proposes that strong pMHC ligands, but not weak ones, induce a conformational change in the TCR. Evidence supporting this comes from a pull-down assay that detects ligand-induced binding of the TCR to the N-terminal SH3 domain of the adapter protein Nck, and also from studies with a neoepitope-specific antibody. Both methods rely on the exposure of a polyproline sequence in the CD3ε subunit of the TCR, and neither indicates whether the conformational change is transmitted to other CD3 subunits. Using a protease-sensitivity assay, we now show that the cytoplasmic tails of CD3ε and CD3ζ subunits become fully protected from degradation upon TCR triggering. These results suggest that the TCR conformational change is transmitted to the tails of CD3ε and CD3ζ, and perhaps all CD3 subunits. Furthermore, the resistance to protease digestion suggests that CD3 cytoplasmic tails adopt a compact structure in the triggered TCR. These results are consistent with a model in which transduction of the conformational change induced upon TCR triggering promotes condensation and shielding of the CD3 cytoplasmic tails.

A cysteine-rich receptor-like kinase NCRK and a pathogen-induced protein kinase RBK1 are Rop GTPase interactors

In plants, Rop/Rac GTPases have emerged as central regulators of diverse signalling pathways in plant growth and pathogen defence. When active, they interact with a wide range of downstream effectors. Using yeast two-hybrid screening we have found three previously uncharacterized receptor-like protein kinases to be Rop GTPase-interacting molecules: a cysteine-rich receptor kinase, named NCRK, and two receptor-like cytosolic kinases from the Arabidopsis RLCK-VIb family, named RBK1 and RBK2. Uniquely for Rho-family small GTPases, plant Rop GTPases were found to interact directly with the protein kinase domains. Rop4 bound NCRK preferentially in the GTP-bound conformation as determined by flow cytometric fluorescence resonance energy transfer measurements in insect cells. The kinase RBK1 did not phosphorylate Rop4 in vitro, suggesting that the protein kinases are targets for Rop signalling. Bimolecular fluorescence complementation assays demonstrated that Rop4 interacted in vivo with NCRK and RBK1 at the plant plasma membrane. In Arabidopsis protoplasts, NCRK was hyperphosphorylated and partially co-localized with the small GTPase RabF2a in endosomes. Gene expression analysis indicated that the single-copy NCRK gene was relatively upregulated in vasculature, especially in developing tracheary elements. The seven Arabidopsis RLCK-VIb genes are ubiquitously expressed in plant development, and highly so in pollen, as in case of RBK2. We show that the developmental context of RBK1 gene expression is predominantly associated with vasculature and is also locally upregulated in leaves exposed to Phytophthora infestans and Botrytis cinerea pathogens. Our data indicate the existence of cross-talk between Rop GTPases and specific receptor-like kinases through direct molecular interaction.

Historical developments in the research of interferon receptors

Interferons (IFNs) were discovered 50 years ago independently by Isaacs and Lindemann and by Nagata and Kojima. When it was later realized that IFNs are active at very low concentrations, research began to determine how their powerful effects were generated from such a small initial signal. It has since been established that interferons, as well as all other cytokines, employ cell surface receptors to translate their presence in the serum to a potent cellular response to a viral infection. These receptor complexes are composed of multiple distinct glycosylated transmembrane polypeptides, a number of protein tyrosine kinases, and interact transiently with a large variety of other proteins including transcription factors, phosphatases, signaling repressors, and adaptor proteins coupling the receptor to alternative signaling pathways. Three major receptor complexes exist that are exclusive to each of three major classes of interferon. Even though the effects of each major class of interferon vary physiologically, each receptor complex interacts with its ligand in similar ways and activates similar signaling cascades. In this mini-review, we take a historical perspective at the major events in the characterization of interferon receptors, discussing interesting results that still need to be explained.

Fluorescent protein FRET: the good, the bad and the ugly

Dynamic protein interactions play a significant part in many cellular processes. A technique that shows considerable promise in elucidating such interactions is Förster resonance energy transfer (FRET). When combined with multiple, colored fluorescent proteins, FRET permits high spatial resolution assays of protein-protein interactions in living cells. Because FRET signals are usually small, however, their measurement requires careful interpretation and several control experiments. Nevertheless, the use of FRET in cell biological experiments has exploded over the past few years. Here we describe the physical basis of FRET and the fluorescent proteins appropriate for these experiments. We also review the approaches that can be used to measure FRET, with particular emphasis on the potential artifacts associated with each approach.

Prestin-prestin and prestin-GLUT5 interactions in HEK293T cells

The remarkable hearing sensitivity and frequency selectivity in mammals is attributed to cochlear amplifier in the outer hair cells (OHCs). Prestin, a membrane protein in the lateral wall of OHC plasma membrane, is required for OHC electromotility and cochlear amplifier. In addition, GLUT5, a fructose transporter, is reported to be abundant in the plasma membrane of the OHC lateral wall and has been originally proposed as the OHC motor protein. Here we provide evidence of interactions between prestin/prestin and prestin/GLUT5 in transiently transfected HEK293T cells. We used a combination of techniques: (1) membrane colocalization by confocal microscopy, (2) fluorescence resonance energy transfer (FRET) by fluorescence activated cell sorting (FACS), (3) FRET by acceptor photobleaching, (4) FRET by fluorescence lifetime imaging (FRET-FLIM), and (5) coimmunoprecipitation. Our results suggest that homomeric and heteromeric prestin interactions occur in native OHCs to facilitate its electromotile function and that GLUT5 interacts with prestin for its elusive function.

Three is better than one: pre-ligand receptor assembly in the regulation of TNF receptor signaling

The tumor necrosis factor (TNF) family of cytokines and their receptors regulates many areas of metazoan biology. Specifically, this cytokine-receptor family plays crucial roles in regulating myriad aspects of immune development and functions. Disruption of ligand-receptor interaction or downstream signal transduction components in the TNF family often leads to pathological conditions. Historically, members of the TNF receptor family (TNFRs) were thought to exist as monomeric receptor chains prior to stimulation. Binding of the trimeric ligand then induces the trimerization of the receptors and activation of downstream signaling. However, recent evidence indicates that many TNFRs exist as pre-assembled oligomers on the cell surface. Pre-ligand assembly of TNFR oligomers is mediated by the pre-ligand assembly domain (PLAD), which resides within the membrane distal cysteine-rich domain of the receptors. Growing evidence indicates that PLAD-mediated receptor association regulates cellular responses to TNF-like cytokines, especially in cells of the immune system. Thus, targeting pre-ligand assembly may offer new possibilities for therapeutic intervention in different pathological conditions involving TNF-like cytokines.

Photophysics of Clomeleon by FLIM: discriminating excited state reactions along neuronal development

In this work, fluorescence lifetime imaging microscopy in the time domain was used to study the fluorescence dynamics of ECFP and of the ratiometric chloride sensor Clomeleon along neuronal development. The multiexponential analysis of fluorophores combined with the study of the contributions of the individual lifetimes (decay-associated spectra) was used to discriminate the presence of energy transfer from other excited state reactions. A characteristic change of sign of the pre-exponential factors of lifetimes from positive to negative near the acceptor emission maxima was observed in presence of energy transfer. By fluorescence lifetime imaging microscopy, we could show that the individual conformations of CFP display differential quenching properties depending on their microenvironment. Suitability of Clomeleon as an optical indicator to obtain a direct readout of the intracellular chloride concentrations in living cells was verified by steady-state and time-resolved spectroscopy. The simultaneous study of the photophysical properties of Clomeleon, the calcium indicator Cameleon, and ECFP with neuronal development provided a kinetic model for the mechanism when competitive quenching effects as well as energy transfer occur in the same molecule. Simultaneous analysis of donor and acceptor kinetics was necessary to discriminate Försters resonance energy transfer along neuronal development due to the different cellular effects involved.

Functional characterization of calcium sensing receptor polymorphisms and absence of association with indices of calcium homeostasis and bone mineral density

OBJECTIVES

Associations between calcium-sensing receptor (CaSR) polymorphisms and serum calcium, PTH and bone mineral density (BMD) have been reported by six studies. However, three other studies have failed to detect such associations. We therefore further investigated three CaSR coding region polymorphisms (Ala986Ser, Arg990Gly and Gln1011Glu) for associations with indices of calcium homeostasis and BMD and for alterations in receptor function.

PATIENTS AND DESIGN

One hundred and ten adult, Caucasian, female, dizygotic twin pairs were investigated for associations between the three CaSR polymorphisms and serum calcium, albumin, PTH, 25-hydroxyvitamin D(3) (25OHD(3)), 1,25-dihydroxyvitamin D(3)[1,25(OH)(2)D(3)], urinary calcium excretion and BMD. Each polymorphic CaSR was also transfected into HEK293 cells and functionally evaluated.

RESULTS

There was a lack of association between each of these three CaSR polymorphisms and serum calcium corrected for albumin, PTH, 25OHD(3), 1,25(OH)(2)D(3), urinary calcium excretion or BMD at the hip, forearm and lumbar spine. These findings were supported by a lack of functional differences in the dose-response curves of the CaSR variants, with the EC(50) values (mean +/- SEM) of the wild-type (Ala986/Arg990/Gln1011), Ser986, Gly990 and Glu1011 CaSR variants being 2.74 +/- 0.29 mm, 3.09 +/- 0.34 mm (P > 0.4), 2.99 +/- 0.23 mm (P > 0.4) and 2.96 +/- 0.30 mm (P > 0.5), respectively.

CONCLUSIONS

Our study, which was sufficiently powered to detect effects that would explain up to 5%, but not less than 1%, of the variance has revealed that the three CaSR polymorphisms of the coding region have no major influence on indices of calcium homeostasis in this female population, and that they do not alter receptor function.

Effects of excited-state absorption on two-photon absorbing materials

Many chromophores with a large two-photon absorptive cross section are hybrid materials where the two-photon absorption (TPA) is coupled to an excited-state absorption (ESA). We develop a numerical technique to investigate hybrid two-photon processes in nonlinear absorbers. Our numerical method compares well with published results. In addition to customary calculation of the transmission curve, we demonstrate the importance of the ESA following the TPA, which may cause significant temporal and radial distortion. We also show that improvements in the transmission can result in significant radial and temporal pulse distortion, which may actually reduce the material effect.

Homotypic FADD interactions through a conserved RXDLL motif are required for death receptor-induced apoptosis

Death receptors in the TNF receptor superfamily signal for apoptosis via the ordered recruitment of FADD and caspase-8 to a death-inducing signaling complex (DISC). However, the nature of the protein-protein interactions in the signaling complex is not well defined. Here we show that FADD self-associates through a conserved RXDLL motif in the death effector domain (DED). Despite exhibiting similar binding to both Fas and caspase-8 and preserved overall secondary structure, FADD RDXLL motif mutants cannot reconstitute FasL- or TRAIL-induced apoptosis and fail to recruit caspase-8 into the DISC of reconstituted FADD-deficient cells. Abolishing self-association can transform FADD into a dominant-negative mutant that interferes with Fas-induced apoptosis and formation of microscopically visible receptor oligomers. These findings suggest that lateral interactions among adapter molecules are required for death receptor apoptosis signaling and implicate self-association into oligomeric assemblies as a key function of death receptor adapter proteins in initiating apoptosis.

Isolation of FRET-positive cells using single 408-nm laser flow cytometry

BACKGROUND

Flow cytometry may be used to isolate large amounts of living, fluorescently labeled cells. Certain fluorescent labels, like enhanced cyan fluorescent protein (ECFP) and enhanced yellow fluorescent protein (EYFP), allow the assessment of direct protein-protein interaction in situ, by fluorescence resonance energy transfer (FRET). However, current flow cytometric methods either require elaborate technical adaptations or, using a single laser protocol, are hampered by background signal. We optimized a single 408-nm laser protocol to detect FRET between ECFP/EYFP-tagged proteins.

METHODS

Cell lines stably expressing ECFP and/or EYFP or an EYFP-ECFP fusion protein were used to design the settings for the flow cytometer to detect FRET-positive cells using a single 408-nm laser. Using these settings, interactions between the subunits of the transcription factor NF-Y were studied.

RESULTS

Flow cytometric analysis of the cells expressing an EYFP-ECFP fusion protein yielded a discrete FRET-positive population. Using the same settings, in cells expressing NF-YB-CFP and NF-YC-YFP fusion proteins, FRET could also be detected. These cells were sorted and FRET was confirmed by confocal microscopy.

CONCLUSION

FRET-positive cells, expressing ECFP- and EYFP-tagged proteins, can be detected using single 408-nm laser excitation, with low background signal. This allows high-throughput analysis and isolation of viable FRET-positive and -negative cells for subsequent biological experiments.

Preligand assembly domain-mediated ligand-independent association between TRAIL receptor 4 (TR4) and TR2 regulates TRAIL-induced apoptosis

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a cytokine with potential therapeutic value against cancers because of its selective cytotoxicity to many transformed, but not normal, cells. The "decoy receptors" TRAIL-R3 (TR3) and TRAIL-R4 (TR4) were believed to negatively regulate TRAIL-induced cytotoxicity by competing for ligand binding with TRAIL-R1 (TR1) and TRAIL-R2 (TR2). Here, we show that inhibition of TRAIL-induced apoptosis by TR4 critically depends on its association with TR2 via the NH(2)-terminal preligand assembly domain overlapping the first partial cysteine-rich domain of both receptors. By contrast, ligand binding by TR4 is dispensable for its apoptosis inhibitory function, thereby excluding the possibility that TR4 was a "decoy" to inhibit apoptosis by binding up TRAIL. In primary CD8(+) T cells, which express only TR2 and TR4 and are resistant to TRAIL-induced apoptosis, stimulation with phorbol myristate acetate abrogated the ligand-independent interaction between TR2 and TR4 and enhanced their sensitivity to TRAIL-induced apoptosis. Hence, whereas most TNF receptors normally form only homotrimeric complexes, the preligand assembly domains in TR2 and TR4 permit mixed complex formation as a means to regulate apoptosis induction. We propose that TR4 is a "regulatory" rather than "decoy" receptor that inhibits apoptosis signaling by TRAIL through this previously uncharacterized ligand-independent mechanism.