Monitoring protein-protein interactions in intact eukaryotic cells by beta-galactosidase complementation

Subcellular localization of interacting proteins by bimolecular fluorescence complementation in planta

Bimolecular fluorescence complementation (BiFC) represents one of the most advanced and powerful tools for studying and visualizing protein-protein interactions in living cells. In this method, putative interacting protein partners are fused to complementary non-fluorescent fragments of an autofluorescent protein, such as the yellow spectral variant of the green fluorescent protein. Interaction of the test proteins may result in reconstruction of fluorescence if the two portions of yellow spectral variant of the green fluorescent protein are brought together in such a way that they can fold properly. BiFC provides an assay for detection of protein-protein interactions, and for the subcellular localization of the interacting protein partners. To facilitate the application of BiFC to plant research, we designed a series of vectors for easy construction of N-terminal and C-terminal fusions of the target protein to the yellow spectral variant of the green fluorescent protein fragments. These vectors carry constitutive expression cassettes with an expanded multi-cloning site. In addition, these vectors facilitate the assembly of BiFC expression cassettes into Agrobacterium multi-gene expression binary plasmids for co-expression of interacting partners and additional autofluorescent proteins that may serve as internal transformation controls and markers of subcellular compartments. We demonstrate the utility of these vectors for the analysis of specific protein-protein interactions in various cellular compartments, including the nucleus, plasmodesmata, and chloroplasts of different plant species and cell types.

Visualization of molecular interactions by fluorescence complementation

The visualization of protein complexes in living cells enables the examination of protein interactions in their normal environment and the determination of their subcellular localization. The bimolecular fluorescence complementation assay has been used to visualize interactions among multiple proteins in many cell types and organisms. Modified forms of this assay have been used to visualize the competition between alternative interaction partners and the covalent modification of proteins by ubiquitin-family peptides.

Systems for the detection and analysis of protein–protein interactions

The analysis of protein-protein interactions is important for developing a better understanding of the functional annotations of proteins that are involved in various biochemical reactions in vivo. The discovery that a protein with an unknown function binds to a protein with a known function could provide a significant clue to the cellular pathway concerning the unknown protein. Therefore, information on protein-protein interactions obtained by the comprehensive analysis of all gene products is available for the construction of interactive networks consisting of individual protein-protein interactions, which, in turn, permit elaborate biological phenomena to be understood. Systems for detecting protein-protein interactions in vitro and in vivo have been developed, and have been modified to compensate for limitations. Using these novel approaches, comprehensive and reliable information on protein-protein interactions can be determined. Systems that permit this to be achieved are described in this review.

A Homogeneous Cell-Based Assay to Measure Nuclear Translocation Using β-Galactosidase Enzyme Fragment Complementation

Positional complementation describes the use of homogeneous assays using beta- galactosidase (beta gal) enzyme fragment complementation to detect cellular protein translocation. This phenomenon occurs when the protein of interest, recombinantly expressed as a fusion protein with a modified alpha fragment of beta gal, translocates to a cellular compartment expressing an enzyme acceptor fragment of the enzyme. When these fragments interact, high-affinity complementation occurs, and a signal is generated that is then detected upon cell lysis. In the present paper the use of positional complementation is exemplified by measuring nuclear translocation of the glucocorticoid receptor in Chinese hamster ovary-K1 cells. The approach thus provides for homogeneous protocols, in an endpoint microtiter plate assay format, without the use of either imaging or reporter gene techniques. Consequently, these characteristics suggest that the technique is suitable for automated instrumentation protocols used in high throughput screening campaigns designed to identify activators or inhibitors of nuclear translocation.

Sequence-enabled reassembly of beta-lactamase (SEER-LAC): a sensitive method for the detection of double-stranded DNA

This work describes the development of a new methodology for the detection of specific double-stranded DNA sequences. We previously showed that two inactive fragments of green fluorescent protein, each coupled to engineered zinc finger DNA-binding proteins, were able to reassemble an active reporter complex in the presence of a predefined DNA sequence. This system, designated sequence-enabled reassembly (SEER), was demonstrated in vitro to produce a DNA-concentration-dependent signal. Here we endow the SEER system with catalytic capability using the reporter enzyme TEM-1 beta-lacatamase. This system could distinguish target DNA from nontarget DNA in less than 5 min, representing a more than 1000-fold improvement over our previous SEER design. A single base-pair substitution in the DNA binding sequence reduced the signal to nearly background levels. Substitution of a different custom zinc finger DNA-binding domain produced a signal only on the new cognate target. Signal intensity was not affected by genomic DNA when present in equal mass to the target DNA. These results present SEER as a rapid and sensitive method for the detection of double-stranded DNA sequences.

Conditional control of protein function

Deciphering the myriad ways in which proteins interact with each other to give rise to complex behaviors that define living systems is a significant challenge. Using perturbations of DNA, genetic analyses have provided many insights into the functions of proteins encoded by specific genes. However, it can be difficult to study essential genes using these approaches, and many biological processes occur on a fast timescale that precludes study using genetic methods. For these reasons and others, it is often desirable to target proteins directly rather than the genes that encode them. Over the past 20 years, several methods to regulate protein function have been developed. In this review, we discuss the genesis and use of these methods, with particular emphasis on the elements of specificity, speed, and reversibility.

Monitoring of conformational change in maltose binding protein using split green fluorescent protein

In this study, we describe a novel method for the detection of conformational changes in proteins, which is predicated on the reconstitution of split green fluorescent protein (GFP). We employed fluorescence complementation assays for the monitoring of the conformationally altered proteins. In particular, we used maltose binding protein (MBP) as a model protein, as MBP undergoes a characteristic hinge-twist movement upon substrate binding. The common feature of this approach is that GFP, as a reporter protein, splits into two non-fluorescent fragments, which are genetically fused to the N- and C-termini of MBP. Upon binding to maltose, the chromophores move closer together, resulting in the generation of fluorescence. This split GFP method also involves the reconstitution of GFP, which is determined via observations of the degree to which fluorescence intensity is restored. As a result, reconstituted GFP has been observed to generate fluorescence upon maltose binding in vitro, thereby allowing for the direct detection of changes in fluorescence intensity in response to maltose, in a concentration- and time-dependent fashion. Our findings showed that the fluorescence complementation assay can be used to monitor the conformational alterations of a target protein, and this ability may prove useful in a number of scientific and medical applications.

Enhanced catalytic efficiency of aminoglycoside phosphotransferase (3')-IIa achieved through protein fragmentation and reassembly

Many monomeric proteins can be split into two fragments, yet the two fragments can associate to make an active heterodimer. However, for most locations in a protein such a conversion is not feasible, presumably due to inefficient assembly or improper folding of the fragments. For some locations, this can be overcome by fusion of the fragments to dimerization domains that facilitate correct assembly. A variety of heterodimers of aminoglycoside phosphotransferase (3')-IIa (Neo) were created in which the Neo fragments required fusion to a pair of leucine zippers for activity in vivo. However, the ability of these heterodimers to confer kanamycin resistance to Escherichia coli cells was impaired compared to wild-type Neo, primarily due to poor production of soluble protein. The mutations R177S and V198E restored the kanamycin resistance to wild-type levels while maintaining the dependence on leucine zippers for activity. These mutations restored high levels of kanamycin resistance not through an improvement in the production of soluble protein but rather by conferring a large improvement in k(cat)/K(m), surpassing that of Neo. Furthermore, whereas R177S and V198E served to improve k(cat)/K(m) 60-fold in the context of the heterodimer, the same mutations in the context of wild-type Neo had a ninefold negative effect on k(cat)/K(m). This demonstrates the possibility that enzymes with improved catalytic properties can be created through a process involving fragmentation and fusion to domains that facilitate assembly of the fragments.

Novel fusion protein approach for efficient high-throughput screening of small molecule-mediating protein-protein interactions in cells and living animals

Networks of protein interactions execute many different intracellular pathways. Small molecules either synthesized within the cell or obtained from the external environment mediate many of these protein-protein interactions. The study of these small molecule-mediated protein-protein interactions is important in understanding abnormal signal transduction pathways in a variety of disorders, as well as in optimizing the process of drug development and validation. In this study, we evaluated the rapamycin-mediated interaction of the human proteins FK506-binding protein (FKBP12) rapamycin-binding domain (FRB) and FKBP12 by constructing a fusion of these proteins with a split-Renilla luciferase or a split enhanced green fluorescent protein (split-EGFP) such that complementation of the reporter fragments occurs in the presence of rapamycin. Different linker peptides in the fusion protein were evaluated for the efficient maintenance of complemented reporter activity. This system was studied in both cell culture and xenografts in living animals. We found that peptide linkers with two or four EAAAR repeat showed higher protein-protein interaction-mediated signal with lower background signal compared with having no linker or linkers with amino acid sequences GGGGSGGGGS, ACGSLSCGSF, and ACGSLSCGSFACGSLSCGSF. A 9 +/- 2-fold increase in signal intensity both in cell culture and in living mice was seen compared with a system that expresses both reporter fragments and the interacting proteins separately. In this fusion system, rapamycin induced heterodimerization of the FRB and FKBP12 moieties occurred rapidly even at very lower concentrations (0.00001 nmol/L) of rapamycin. For a similar fusion system employing split-EGFP, flow cytometry analysis showed significant level of rapamycin-induced complementation.

Engineering novel binding proteins from nonimmunoglobulin domains

Not all adaptive immune systems use the immunoglobulin fold as the basis for specific recognition molecules: sea lampreys, for example, have evolved an adaptive immune system that is based on leucine-rich repeat proteins. Additionally, many other proteins, not necessarily involved in adaptive immunity, mediate specific high-affinity interactions. Such alternatives to immunoglobulins represent attractive starting points for the design of novel binding molecules for research and clinical applications. Indeed, through progress and increased experience in library design and selection technologies, gained not least from working with synthetic antibody libraries, researchers have now exploited many of these novel scaffolds as tailor-made affinity reagents. Significant progress has been made not only in the basic science of generating specific binding molecules, but also in applications of the selected binders in laboratory procedures, proteomics, diagnostics and therapy. Challenges ahead include identifying applications where these novel proteins can not only be an alternative, but can enable approaches so far deemed technically impossible, and delineate those therapeutic applications commensurate with the molecular properties of the respective proteins.

Noninvasive imaging of protein-protein interactions from live cells and living subjects using bioluminescence resonance energy transfer

This study demonstrates a significant advancement of imaging of a distance-dependent physical process, known as the bioluminescent resonance energy transfer (BRET2) signal in living subjects, by using a cooled charge-coupled device (CCD) camera. A CCD camera-based spectral imaging strategy enables simultaneous visualization and quantitation of BRET signal from live cells and cells implanted in living mice. We used the BRET2 system, which utilizes Renilla luciferase (hRluc) protein and its substrate DeepBlueC (DBC) as an energy donor and a mutant green fluorescent protein (GFP2) as the acceptor. To accomplish this objective in this proof-of-principle study, the donor and acceptor proteins were fused to FKBP12 and FRB, respectively, which are known to interact only in the presence of the small molecule mediator rapamycin. Mammalian cells expressing these fusion constructs were imaged using a cooled-CCD camera either directly from culture dishes or by implanting them into mice. By comparing the emission photon yields in the presence and absence of rapamycin, the specific BRET signal was determined. The CCD imaging approach of BRET signal is particularly appealing due to its capacity to seamlessly bridge the gap between in vitro and in vivo studies. This work validates BRET as a powerful tool for interrogating and observing protein-protein interactions directly at limited depths in living mice.

High-affinity fragment complementation of a fibronectin type III domain and its application to stability enhancement

The tenth fibronectin type III (FN3) domain of human fibronectin (FNfn10), a prototype of the ubiquitous FN3 domain, is a small, monomeric beta-sandwich protein. In this study, we have bisected FNfn10 in each loop to generate a total of six fragment pairs. We found that fragment pairs bisected at multiple loops of FNfn10 show complementation in vivo as tested with a yeast two-hybrid system. The dissociation constant of these fragment pairs determined in vitro were as low as 3 nM, resulting in one of the tightest fragment complementation systems reported so far. Furthermore, we show that the affinity of fragment complementation is correlated with the stability of the uncut parent protein. Exploring this correlation, we screened a yeast two-hybrid library of one fragment and identified mutations that suppress the effect of a destabilizing mutation in the other fragment. One of the identified mutations significantly increased the stability of the uncut wild-type protein, proving that fragment complementation can be used as a novel strategy for the selection of proteins with enhanced stability.

Visualization of protein interactions in living cells using bimolecular fluorescence complementation (BiFC) analysis

Protein interactions integrate stimuli from different signaling pathways and developmental programs. Bimolecular fluorescence complementation (BiFC) analysis has been developed for visualization of protein interactions in living cells. This approach is based on complementation between two fragments of a fluorescent protein when they are brought together by an interaction between proteins fused to the fragments, and it enables visualization of the subcellular locations of protein interactions in the normal cellular environment. It can be used for the analysis of many protein interactions and does not require information about the structures of the interaction partners. A multicolor BiFC approach has been developed for simultaneous visualization of interactions with multiple alternative partners in the same cell, based on complementation between fragments of engineered fluorescent proteins that produce bimolecular fluorescent complexes with distinct spectral characteristics. This enables comparison of subcellular distributions of different protein complexes in the same cell and allows analysis of competition between mutually exclusive interaction partners.

Characterization of Isoprenaline- and Salmeterol-Stimulated Interactions between β2-Adrenoceptors and β-Arrestin 2 Using β-Galactosidase Complementation in C2C12 Cells

beta-Arrestin is an adaptor protein that has been shown to couple G protein-coupled receptors (GPCRs) to clathrin-coated pits and target them for subsequent internalization. More recently, beta-arrestin 2 has also been shown to be involved in the activation of mitogen-activated protein kinase cascades by G protein-coupled receptors independently of G protein activation. Direct interactions between proteins can be monitored using enzyme complementation between two inactive deletion mutants of beta-galactosidase (beta-gal; Deltaalpha and Deltaomega). In the present study, we have used fusion proteins of the human beta(2)-adrenoceptor (C-terminal beta-gal Deltaalpha) and beta-arrestin 2 (beta-gal Deltaomega) to study directly the pharmacology of this interaction in C2C12 cells expressing the beta(2)-adrenoceptor-beta-gal Deltaalpha fusion protein at low physiological levels (38.2 +/- 2.7 fmol . mg protein(-1)). Isoprenaline, noradrenaline, and adrenaline (-log EC(50) = 5.9, 5.5, and 5.7, respectively) stimulated an association between the beta(2)-adrenoceptor and beta-arrestin 2 at much higher concentrations than required for activation of cAMP accumulation (-log EC(50) = 7.6, 6.3, and 7.7, respectively). This was sensitive to inhibition by the beta(2)-adrenoceptor antagonists propranolol, timolol, and ICI 118551. Both salbutamol and terbutaline behaved as partial agonists of beta-gal complementation. Furthermore, the long-acting beta(2)-agonist salmeterol (-log K(D) for inhibition of [(3)H]CGP12177 binding = 8.7) behaved as an antagonist of isoprenaline-stimulated beta(2)-adrenoceptor-arrestin 2 interactions (-log K(D) = 8.0), whereas acting as a full agonist of cAMP accumulation in the same cells (-log EC(50) = 9.2). These data suggest that salmeterol can discriminate between receptor-G(S) protein and receptor-arrestin 2 complexes (in terms of efficacy and affinity) in a way that is favorable for its long duration of action.

Utilizing the split-ubiquitin membrane yeast two-hybrid system to identify protein-protein interactions of integral membrane proteins

Various modifications of the conventional yeast two-hybrid system have played an essential role in confirming or detecting protein-protein interactions among nuclear and cytoplasmic proteins. These approaches have permitted the identification of novel interaction partners, as well as provided hints as to their function. However, membrane proteins, such as receptor tyrosine kinases, G protein-coupled receptors, membrane-bound phosphatases, and transporters, which represent important classes of signaling molecules, are difficult to study using classical protein interaction assays because of their hydrophobic nature. Here, we describe a genetic system that allows the identification of integral membrane-interacting proteins. This so-called "split-ubiquitin membrane-based yeast two-hybrid assay" involves fusing the halves of ubiquitin to two interacting proteins, at least one of which is membrane bound. Upon interaction of these two proteins, the halves of ubiquitin are brought together, and the transcription factor that is fused to a membrane protein of interest is cleaved and released. The free transcription factor then enters the nucleus and activates transcription of reporter genes. We also describe how this technology is used to screen complementary DNA libraries to identify novel binding partners of a membrane protein of interest.

Application of the split-ubiquitin membrane yeast two-hybrid system to investigate membrane protein interactions

The characterization of protein-protein interactions provides the foundation for further studies concerning protein complex function and regulation. Since the advent of the yeast two-hybrid assay, many additional genetic systems based upon the principle of protein fragment complementation have been designed. One such system, the split-ubiquitin membrane yeast two-hybrid system (MbYTH), is able to analyze the interaction status between two integral membrane proteins. This ability of the MbYTH system augments genetic analysis of protein interactions by covering for the inherent limitation of the yeast two-hybrid system when studying membrane protein interactions. Herein, we provide a description of the MbYTH method and detailed protocols in order to monitor protein interactions and discover novel interacting partners using the MbYTH system.

Transforming a (beta/alpha)8--barrel enzyme into a split-protein sensor through directed evolution

Split-protein sensors have become an important tool for the analysis of protein-protein interactions in living cells. We present here a combinatorial method for the generation of new split-protein sensors and demonstrate its application toward the (beta/alpha)(8)-barrel enzyme N-(5'-phosphoribosyl)-anthranilate isomerase Trp1p from Saccharomyces cerevisiae. The generated split-Trp protein sensors allow for the detection of protein-protein interactions in the cytosol as well as the membrane by enabling trp1 cells to grow on medium lacking tryptophan. This powerful selection complements the repertoire of the currently used split-protein sensors and provides a new tool for high-throughput interaction screening.

Visualization of protein interactions in living plant cells using bimolecular fluorescence complementation

Dynamic networks of protein-protein interactions regulate numerous cellular processes and determine the ability to respond appropriately to environmental stimuli. However, the investigation of protein complex formation in living plant cells by methods such as fluorescence resonance energy transfer has remained experimentally difficult, time consuming and requires sophisticated technical equipment. Here, we report the implementation of a bimolecular fluorescence complementation (BiFC) technique for visualization of protein-protein interactions in plant cells. This approach relies on the formation of a fluorescent complex by two non-fluorescent fragments of the yellow fluorescent protein brought together by association of interacting proteins fused to these fragments (Hu et al., 2002). To enable BiFC analyses in plant cells, we generated different complementary sets of expression vectors, which enable protein interaction studies in transiently or stably transformed cells. These vectors were used to investigate and visualize homodimerization of the basic leucine zipper (bZIP) transcription factor bZIP63 and the zinc finger protein lesion simulating disease 1 (LSD1) from Arabidopsis as well as the dimer formation of the tobacco 14-3-3 protein T14-3c. The interaction analyses of these model proteins established the feasibility of BiFC analyses for efficient visualization of structurally distinct proteins in different cellular compartments. Our investigations revealed a remarkable signal fluorescence intensity of interacting protein complexes as well as a high reproducibility and technical simplicity of the method in different plant systems. Consequently, the BiFC approach should significantly facilitate the visualization of the subcellular sites of protein interactions under conditions that closely reflect the normal physiological environment.

MUC20 suppresses the hepatocyte growth factor-induced Grb2-Ras pathway by binding to a multifunctional docking site of met

A cDNA encoding a novel mucin protein, MUC20, was isolated as a gene that is up-regulated in the renal tissues of patients with immunoglobulin A nephropathy. We demonstrate here that the C terminus of MUC20 associates with the multifunctional docking site of Met without ligand activation, preventing Grb2 recruitment to Met and thus attenuating hepatocyte growth factor (HGF)-induced transient extracellular signal-regulated kinase-1 and -2 activation. Production of MUC20 reduced HGF-induced matrix metalloproteinase expression and proliferation, which require the Grb2-Ras pathway, whereas cell scattering, branching morphogenesis, and survival via the Gab1/phosphatidylinositol 3-kinase (PI3K) pathways was not affected. Thus, MUC20 reduces HGF-induced activation of the Grb2-Ras pathway but not the Gab1/PI3K pathways. We further demonstrate that the cytoplasmic domain of MUC20 has the ability to oligomerize and that the oligomerization augments its affinity for Met. Taken together, these results suggest that MUC20 is a novel regulator of the Met signaling cascade which has a role in suppression of the Grb2-Ras pathway.

A toll-like receptor-based two-hybrid assay for detecting protein--protein interactions on live eukaryotic cells

Protein--protein interactions underly diverse biological processes. Here, we describe a method for detecting protein interactions on the cell surface. This method is based on the mechanism of TLR2 activation whereby extracellular (EC) domain-mediated heterodimerization of TLR2 and TLR1 activates NF-kappaB and other signalling processes. Test proteins were expressed as the EC domains of TLR1 and TLR2 in fusion with the transmembrane/cytoplasmic (TM/Cyt) domains, i.e. tmTIR1 and tmTIR2. The feasibility of this TIR1/2-based method was examined by expression of IL-4 and the EC domains of the interleukin-4 receptor alpha (IL-4Ralpha) and the cytokine receptor common gamma chain (gammaC) as hybrid receptors with tmTIR1 and tmTIR2. Upon co-expression of IL4Ralpha-TIR1 and gammaC-TIR2 in 293T cells, NF-kappaB activation was found to be inducible by IL-4. Co-expression of IL4-TIR1 with IL4Ralpha-TIR2, but not gammaC-TIR2, led to constitutive NF-kappaB activation. This is consistent with IL-4 primarily binding to IL4Ralpha but not gammaC. Co-expression of the IL4Ralpha-TIR1/2, IL4-TIR1/2 or gammaC-TIR1/2 hybrid receptor pairs also constitutively activated NF-kappaB suggesting that IL-4, IL4Ralpha and gammaC form homodimers or homotypic interactions. This was confirmed by immunoprecipitation studies. In summary, we report a TIR1/2-based assay for detecting interactions between membrane proteins, receptors/ligands and secreted proteins on live eukaryotic cells.

Protein fragment complementation strategies for biochemical network mapping

The organization of biochemical networks that make up the living cell can be defined by studying the dynamics of protein-protein interactions. To this end, experimental strategies based on protein fragment complementation assays (PCAs) have been used to map biochemical networks and to identify novel components of these networks. Pharmacological perturbations of the interactions can be observed, and the resulting pharmacological profiles and subcellular locations of interactions allow each gene product to be 'placed' at its relevant point in a network. Network mapping by PCA could be used with, or instead of, traditional target-based drug discovery strategies to increase the quantity and quality of information about the actions of small molecules on living cells and the intricate networks that make up their chemical machinery.

Synergistic transcription activation by Maf and Sox and their subnuclear localization are disrupted by a mutation in Maf that causes cataract

Crystallin genes are selectively expressed during lens development. Maf and Sox family proteins synergistically enhanced gammaF-crystallin promoter activity in a lens cell line. Mutational analysis of the gammaF-crystallin promoter identified a composite regulatory element containing nonconsensus Maf and Sox recognition sequences. Mutations in these recognition sequences or changes in their spacing eliminated synergistic transcription activation. The transcriptional synergy was also affected by changes in the orientation of the Maf recognition sequence that had no detectable effect on binding affinity. The interaction between Maf and Sox proteins was visualized in living cells by bimolecular fluorescence complementation analysis. The N-terminal region of Maf mediated the interaction with Sox proteins in cells. Synergistic transcription activation required the N-terminal region of Maf as well as the ancillary DNA binding domain and the unique portion of the basic region that mediate specific recognition of the gammaF-crystallin promoter element. A mutation in the ancillary DNA binding domain of Maf (R288P) that has been shown to cause cataract eliminated the transcriptional activity of Maf but had no detectable effect on DNA binding in vitro. Whereas wild-type Maf was uniformly distributed in the nucleoplasm, R288P Maf was enriched in nuclear foci. Cajal bodies and gemini of coiled bodies were closely associated with the foci occupied by R288P Maf. Wild-type Maf formed complexes with Sox proteins in the nucleoplasm, whereas R288P Maf recruited Sox proteins as well as other interaction partners to the nuclear foci. The mislocalization of normal cellular proteins to these foci provides a potential explanation for the dominant disease phenotype of the R288P mutation in Maf.

Kinetics of regulated protein-protein interactions revealed with firefly luciferase complementation imaging in cells and living animals

Signaling pathways regulating proliferation, differentiation, and apoptosis are commonly mediated through protein-protein interactions as well as reversible phosphorylation of proteins. To facilitate the study of regulated protein-protein interactions in cells and living animals, we optimized firefly luciferase protein fragment complementation by screening incremental truncation libraries of N- and C-terminal fragments of luciferase. Fused to the rapamycin-binding domain (FRB) of the kinase mammalian target of rapamycin and FK506-binding protein 12 (FKBP), respectively, the optimized FRB-N-terminal luciferase fragment (NLuc)/C-terminal luciferase fragment (CLuc)-FKBP luciferase complementation imaging (LCI) pair reconstituted luciferase activity in cells upon single-site binding of rapamycin in an FK506-competitive manner. LCI was used in three independent applications. In mice bearing implants of cells expressing the FRB-NLuc/CLuc-FKBP LCI pair, dose- and time-dependent luciferase activity allowed target-specific pharmacodynamic analysis of rapamycin-induced protein-protein interactions in vivo. In cells expressing a Cdc25C-NLuc/CLuc-14-3-3epsilon LCI pair, drug-mediated disruption of cell cycle regulated protein-protein interactions was demonstrated with the protein kinase inhibitor UCN-01 in a phosphoserine-dependent manner. When applied to IFN-gamma-dependent activation of Janus kinase/signal transducer and activator of transcription 1 (STAT1), LCI revealed, in the absence of ligand-induced phosphorylation, STAT1 proteins existing in live cells as preformed dimers. Thus, optimized LCI provides a platform for near real-time detection and characterization of regulated and small molecule-induced protein-protein interactions in intact cells and living animals and should enable a wide range of novel applications in drug discovery, chemical genetics, and proteomics research.

Enzyme fragment complementation: a flexible high throughput screening assay technology

High-affinity complementation of a small fragment of beta-galactosidase to an inactive deletion mutant of the enzyme forms a stable heteromeric enzyme complex capable of hydrolyzing substrates to produce either chemiluminescent or fluorescent signals. This review describes a series of screening assays in which the small beta-galactosidase fragment, Enzyme Donor or ProLabel, is either chemically conjugated or recombinantly fused to small molecules or proteins, respectively. Chemical conjugation forms the basis of several HitHunter HTS assays in which competitive displacement of the ProLabel conjugate from either a binding protein (receptor or antibody) is induced by the analyte in question. In this manner, a calibration curve is generated, to measure cellular analytes including 3',5'-cyclic AMP. Changes in this second messenger, occurring due to G protein-coupled receptor (GPCR) activation, can thus be easily measured in a homogeneous assay. Similar assays have been developed for tyrosine kinases, serine threonine kinases, nuclear hormone receptors, and proteases. A second form of assay technology involves measurement of cellular protein expression, in which the protein is fused to ProLabel. Analysis can be undertaken in crude cell lysates, or with intact cells, using beta-galactosidase complementation in a microtiter plate. This homogeneous technology is highly sensitive and has been developed to measure protein expression changes occurring in response to pathway activation by targets such as GPCRs, tyrosine kinase receptors, and proteases. In summary, the DiscoveRx technology using beta-galactosidase complementation provides a robust and flexible assay technology for use in cell-free and cell-based HTS.

The yeast split-ubiquitin membrane protein two-hybrid screen identifies BAP31 as a regulator of the turnover of endoplasmic reticulum-associated protein tyrosine phosphatase-like B

In the past decade, traditional yeast two-hybrid techniques have identified a plethora of interactions among soluble proteins operating within diverse cellular pathways. The discovery of associations between membrane proteins by genetic approaches, on the other hand, is less well established due to technical limitations. Recently, a split-ubiquitin system was developed to overcome this barrier, but so far, this system has been limited to the analysis of known membrane protein interactions. Here, we constructed unique split-ubiquitin-linked cDNA libraries and provide details for implementing this system to screen for binding partners of a bait protein, in this case BAP31. BAP31 is a resident integral protein of the endoplasmic reticulum, where it operates as a chaperone or cargo receptor and regulator of apoptosis. Here we describe a novel human member of the protein tyrosine phosphatase-like B (PTPLB) family, an integral protein of the endoplasmic reticulum membrane with four membrane-spanning alpha helices, as a BAP31-interacting protein. PTPLB turns over rapidly through degradation by the proteasome system. Comparisons of mouse cells with a deletion of Bap31 or reconstituted with human BAP31 indicate that BAP31 is required to maintain PTPLB, consistent with a chaperone or quality control function for BAP31 in the endoplasmic reticulum membrane.

Molecular imaging of drug-modulated protein-protein interactions in living subjects

Networks of protein interactions mediate cellular responses to environmental stimuli and direct the execution of many different cellular functional pathways. Small molecules synthesized within cells or recruited from the external environment mediate many protein interactions. The study of small molecule-mediated interactions of proteins is important to understand abnormal signal transduction pathways in cancer and in drug development and validation. In this study, we used split synthetic renilla luciferase (hRLUC) protein fragment-assisted complementation to evaluate heterodimerization of the human proteins FRB and FKBP12 mediated by the small molecule rapamycin. The concentration of rapamycin required for efficient dimerization and that of its competitive binder ascomycin required for dimerization inhibition were studied in cell lines. The system was dually modulated in cell culture at the transcription level, by controlling nuclear factor kappaB promoter/enhancer elements using tumor necrosis factor alpha, and at the interaction level, by controlling the concentration of the dimerizer rapamycin. The rapamycin-mediated dimerization of FRB and FKBP12 also was studied in living mice by locating, quantifying, and timing the hRLUC complementation-based bioluminescence imaging signal using a cooled charged coupled device camera. This split reporter system can be used to efficiently screen small molecule drugs that modulate protein-protein interactions and also to assess drugs in living animals. Both are essential steps in the preclinical evaluation of candidate pharmaceutical agents targeting protein-protein interactions, including signaling pathways in cancer cells.

Alpha-complementation assay for HIV envelope glycoprotein-mediated fusion

The fusion reaction mediated by viral envelope glycoproteins proceeds through an ordered series of conformational changes in the envelope glycoprotein. Fusion inhibitors have been developed that target glycoprotein subunits, arresting the reaction at different points in the process. We report the development of a novel method for detecting viral glycoprotein-mediated fusion that is based on the principle of alpha-complementation of beta-galactosidase. The method is simple, accurate, has a high signal-to-noise ratio, is suited for high-throughput screening, and does not require new transcription or protein synthesis. Cells expressing a viral envelope glycoprotein and the N-terminal alpha fragment of beta-galactosidase were mixed with cells expressing the C-terminal beta-galactosidase fragment, CD4, CCR5, or CXCR4. Fusion was detected after 30 min and continued to increase to very high levels for more than 5 h. The assay was used to examine the temperature dependence of fusion and the effect of coreceptor and glycoprotein density on inhibitor activity.

Attenuation of HIV-1 Replication in Primary Human Cells with a Designed Zinc Finger Transcription Factor

Small molecule inhibitors of human immunodeficiency virus, type 1 (HIV-1) have been extremely successful but are associated with a myriad of undesirable effects and require lifelong daily dosing. In this study we explore an alternative approach, that of inducing intracellular immunity using designed, zinc finger-based transcription factors. Three transcriptional repression proteins were engineered to bind sites in the HIV-1 promoter that were expected to be both accessible in chromatin structure and highly conserved in sequence structure among the various HIV-1 subgroups. Transient transfection assays identified one factor, KRAB-HLTR3, as being able to achieve 100-fold repression of an HIV-1 promoter. Specificity of repression was demonstrated by the lack of repression of other promoters. This factor was further shown to repress the replication of several HIV-1 viral strains 10- to 100-fold in T-cell lines and primary human peripheral blood mononuclear cells. Repression was observed for at least 18 days with no significant cytotoxicity. Stable T-cell lines expressing the factor also do not show obvious signs of cytotoxicity. These characteristics present KRAB-HLTR3 as an attractive candidate for development in an intracellular immunization strategy for anti-HIV-1 therapy.

Creation of an Allosteric Enzyme by Domain Insertion

Two allosteric enzymes have been created by the covalent linkage of non-interacting, monomeric proteins with the prerequisite effector-binding and catalytic functionalities, respectively. This was achieved through a combinatorial process called random domain insertion. The fragment of the TEM-1 beta-lactamase gene coding for the mature protein lacking its signal sequence was randomly inserted into the Escherichia coli maltose-binding protein (MBP) gene to create a domain insertion library. This library's diversity derived both from the site of insertion and from a distribution of tandem duplications or deletions of a portion of the MBP gene at the insertion site. From a library of approximately 2 x 10(4) in-frame fusions, approximately 800 library members conferred a phenotype to E.coli cells that was consistent with the presence of bifunctional fusions that could hydrolyze ampicillin and transport maltose in E.coli. Partial screening of this bifunctional sublibrary resulted in the identification of two enzymes in which the presence of maltose modulated the rate of nitrocefin hydrolysis. For one of these enzymes, the presence of maltose increased k(cat) by 70% and k(cat)/K(m) by 80% and resulted in kinetic parameters that were almost identical to TEM-1 beta-lactamase. Such an increase in activity was only observed with maltooligosaccharides whose binding to MBP is known to induce a conformational change. Modulation of the rate of nitrocefin hydrolysis could be detected at maltose concentrations less than 1 microM. Intrinsic protein fluorescence studies were consistent with a conformational change being responsible for the modulation of activity.

Mutant Frizzled 4 associated with vitreoretinopathy traps wild-type Frizzled in the endoplasmic reticulum by oligomerization

nt signalling pathways regulate cell proliferation, cell fate and morphogenetic movements. Here, we demonstrate that the Frizzled (Fz) family of Wnt receptors, similarly to G-protein-coupled receptors (GPCRs), form specific homo- and hetero-oligomers. Two lines of evidence suggest that oligomerization occurs in the endoplasmic reticulum: first, a mutant allele of Fz4, encoding a truncated protein that is retained in the endoplasmic reticulum, is linked to the autosomal-dominant retinal degenerative disease, familial exudative vitreoretinopathy (FEVR). We show that this mutant form of Fz4 oligomerizes with wild-type Fz4, retains it in the endoplasmic reticulum and inhibits its signalling. Second, a derivative of Fz1 targeted to the endoplasmic reticulum traps wild-type Fz1 in the endoplasmic reticulum and blocks its signalling. These data support the hypothesis that oligomerization of mutant and wild-type Fz proteins occurs in the endoplasmic reticulum and may explain the genetic dominance of this FEVR allele.

A homogeneous enzyme fragment complementation cyclic AMP screen for GPCR agonists

In the new high-throughput screening (HTS) campaign, receptor functional assays, 3',5'-cyclic adenosine monophosphate (cAMP), intracellular [Ca(2)+](i), phosphatidylinositol turnover, and reporter-based assays are being used as primary screens as they are now developed as homogeneous and automation-friendly assays. FlashPlate assay and scintillation proximity assay using radiolabeled cAMP have been used for measuring cAMP. A nonradioactive homogeneous HTS assay using HitHunter trade mark enzyme fragment complementation (EFC) technology was evaluated for measuring cAMP in adherent and suspension cells overexpressing a Galpha(s)-coupled receptor. In the EFC-cAMP assay, the beta-galactosidase (beta-gal) donor fragment-cAMP (ED-cAMP) conjugate complements with the beta-gal enzyme acceptor (EA) fragment to form an active beta-gal enzyme. Binding of ED-cAMP conjugate to the anti-cAMP antibody prevents its complementation with the EA fragment to form an active enzyme. Cyclic AMP in the samples compete with ED-cAMP to bind to the anti-cAMP antibody, thus increasing the free ED-cAMP that can complement with the EA fragment to form an active enzyme that is assayed with a luminescent substrate. Thus, this assay results in a positive signal unlike other technologies, wherein the signal is completed by cAMP in the sample. Glucagon-like peptide (GLP)-1 binds to GLP-1 receptor (with a Kd of 0.2 nM) signals through Galpha(s) to activate adenylate cyclase, which results in an increase of intracellular cAMP (EC(50) of 0.3 nM). GLP-1 stimulation of cAMP levels measured by the EFC method was similar in both adherent and suspension cell formats (EC(50)~0.3 nM) at different cell numbers. The assay was further validated with forskolin, exendin, and several active GLP-1 peptide analogues. The stimulation of cAMP by GLP-1 and forskolin was effectively inhibited by the adenylate cyclase inhibitors MDL-12330A and SQ-22536, confirming that the increased cAMP is through the AC pathway. The assay tolerates dimethyl sulfoxide (DMSO) up to 10%, and tartrazine does not interfere with the assay with the adherent cells up to 1 mM and affects minimally up to 10 microM in suspension cells. The assay is very robust, with a Z' value of 0.7 to 0.8. The assay was validated with several plates of low molecular weight nonpeptide compounds and peptide agonists with different potencies. The suspension cell protocol is a robust homogeneous assay that involves fewer steps than the adherent cell protocol and is suitable for HTS. The cAMP assay using EFC technology is advantageous in that it has a greater dynamic range of detection; is nonradioactive, very sensitive, robust; has minimal interference from DMSO and colored compounds; and is amenable for automation. An added advantage of this assay is that the cAMP is measured as a positive signal, thereby reducing the incidence of false positives.

Cell-based high-throughput screening assay system for monitoring G protein-coupled receptor activation using beta-galactosidase enzyme complementation technology

A novel cell-based functional assay to directly monitor G protein-coupled receptor (GPCR) activation in a high-throughput format, based on a common GPCR regulation mechanism, the interaction between beta-arrestin and ligand-activated GPCR, is described. A protein-protein interaction technology, the InteraX trade mark system, uses a pair of inactive beta-galactosidase (beta-gal) deletion mutants as fusion partners to the protein targets of interest. To monitor GPCR activation, stable cell lines expressing both GPCR- and beta-arrestin-beta-gal fusion proteins are generated. Following ligand stimulation, beta-arrestin binds to the activated GPCR, and this interaction drives functional complementation of the beta-gal mutant fragments. GPCR activation is measured directly by quantitating restored beta-gal activity. The authors have validated this assay system with two functionally divergent GPCRs: the beta2-adrenergic amine receptor and the CXCR2 chemokine-binding receptor. Both receptors are activated or blocked with known agonists and antagonists in a dose-dependent manner. The beta2-adrenergic receptor cell line was screened with the LOPAC trade mark compound library to identify both agonists and antagonists, validating this system for high-throughput screening performance in a 96-well microplate format. Hit specificity was confirmed by quantitating the level of cAMP. This assay system has also been performed in a high-density (384-well) microplate format. This system provides a specific, sensitive, and robust methodology for studying and screening GPCR-mediated signaling pathways.

An optimized homogeneous noncompetitive immunoassay based on the antigen-driven enzymatic complementation

We describe an optimized noncompetitive and homogeneous immunoassay based on the antigen-dependent reassociation of antibody variable domains and beta-galactosidase (beta-gal) complementation (open sandwich enzymatic complementation immunoassay, OS-ECIA). The reassociation of two fusion proteins, an antibody heavy chain variable region fragment tethered to an N-terminal deletion mutant of beta-gal, V(H)Deltaalpha, and the light chain variable region fragment tethered to a C-terminal deletion mutant of beta-gal, V(L)Deltaomega, was monitored by the enzymatic complementation between the two. With the use of anti-hen egg lysozyme (HEL) antibody HyHEL10, an antigen-dependent enhancement in the enzymatic activity was clearly observed. To optimize the assay, the lengths of the linkers connecting the two domains of each fusion protein were varied, and the optimal pair V(H)(G(4)S)(2)Deltaalpha/V(L)(G(4)S)Deltaomega showed much improved antigen-responsive beta-gal activity. After various optimizations, almost 1000-fold improvement in sensitivity compared with that of our corresponding homogeneous open sandwich (OS) assays based on the energy transfer was observed, possibly due to lower V(H)/V(L) concentration and background heterodimer association.

Heteromeric MAPPIT: a novel strategy to study modification-dependent protein-protein interactions in mammalian cells

We recently reported a two-hybrid trap for detecting protein-protein interactions in intact mammalian cells (MAPPIT). The bait protein was fused to a STAT recruitment-deficient, homodimeric cytokine receptor and the prey protein to functional STAT recruitment sites. In such a configuration, STAT-dependent responses can be used to monitor a given bait-prey interaction. Using this system, we were able to demonstrate both modification-independent and tyrosine phosphorylation- dependent interactions. Protein modification in this approach is, however, strictly dependent on the receptor-associated JAK tyrosine kinases. We have now extended this concept by using extracellular domains of the heteromeric granulocyte/macrophage colony-stimulating factor receptor (GM-CSFR). Herein, the bait was fused to the (beta)c chain and its modifying enzyme to the GM-CSFRalpha chain (or vice versa). We demonstrate several serine phosphorylation-dependent interactions in the TGFbeta/Smad pathway using the catalytic domains of the ALK4 or ALK6 serine/threonine kinase receptors. In all cases tested, STAT-dependent signaling was completely abolished when mutant baits were used wherein critical serine residues were replaced by alanines. This approach operates both in transient and stable expression systems and may not be limited to serine phosphorylation but has the potential for studying various different types of protein modification-dependent interactions in intact cells.

Novel level of signalling control in the JAK/STAT pathway revealed by in situ visualisation of protein-protein interaction during Drosophila development

It is commonly accepted that activation of most signalling pathways is induced by ligand receptor dimerisation. This belief has been challenged for some vertebrate cytokine receptors of the JAK/STAT pathway. Here we study whether DOME, the Drosophila receptor of the JAK/STAT pathway, can dimerise and if the dimerisation is ligand-dependent. To analyse DOME homo-dimerisation, we have applied a beta-gal complementation technique that allows the detection of protein interactions in situ. This technique has been used previously in cell culture but this is the first time that it has been applied to whole embryos. We show that this technique, which we rename betalue-betalau technique, can be used to detect DOME homo-dimerisation in Drosophila developing embryos. Despite DOME being ubiquitously expressed, dimerisation is developmentally regulated. We investigate the state of DOME dimerisation in the presence or absence of ligand and show that DOME dimerisation is not ligand-induced, indicating that ligand independent cytokine receptor dimerisation is a conserved feature across phyla. We have further analysed the functional significance of ligand-independent receptor dimerisation by comparing the effects of ectopic ligand expression in cells in which the receptor is, or is not, dimerised. We show that ligand expression can only activate STAT downstream targets or affect embryo development in cells in which the receptor is dimerised. These results suggest a model in which ligand-independent dimerisation of the JAK/STAT receptor confers cells with competence to activate the pathway prior to ligand reception. Thus, competence to induce the JAK/STAT signalling pathway in Drosophila can be regulated by controlling receptor dimerisation prior to ligand binding. These results reveal a novel level of JAK/STAT signalling regulation that could also apply to vertebrates.

Simultaneous visualization of multiple protein interactions in living cells using multicolor fluorescence complementation analysis

The specificity of biological regulatory mechanisms relies on selective interactions between different proteins in different cell types and in response to different extracellular signals. We describe a bimolecular fluorescence complementation (BiFC) approach for the simultaneous visualization of multiple protein interactions in the same cell. This approach is based on complementation between fragments of fluorescent proteins with different spectral characteristics. We have identified 12 bimolecular fluorescent complexes that correspond to 7 different spectral classes. Bimolecular complex formation between fragments of different fluorescent proteins did not differentially affect the dimerization efficiency of the bZIP domains of Fos and Jun or the subcellular sites of interactions between these domains. Multicolor BiFC enables visualization of interactions between different proteins in the same cell and comparison of the efficiencies of complex formation with alternative interaction partners.

Detection of integrin alpha IIbbeta 3 clustering in living cells

In platelets, bidirectional signaling across integrin alpha(IIb)beta(3) regulates fibrinogen binding, cytoskeletal reorganization, cell aggregation, and spreading. Because these responses may be influenced by the clustering of alpha(IIb)beta(3) heterodimers into larger oligomers, we established two independent methods to detect integrin clustering and evaluate factors that regulate this process. In the first, weakly complementing beta-galactosidase mutants were fused to the C terminus of individual alpha(IIb) subunits, and the chimeras were stably expressed with beta(3) in Chinese hamster ovary cells. Clustering of alpha(IIb)beta(3) should bring the mutants into proximity and reconstitute beta-galactosidase activity. In the second method, alpha(IIb) was fused to either a green fluorescent protein (GFP) or Renilla luciferase and transiently expressed with beta(3). Here, integrin clustering should stimulate bioluminescence resonance energy transfer between a cell-permeable luciferase substrate and GFP. These methods successfully detected integrin clustering induced by anti-alpha(IIb)beta(3) antibodies. Significantly, they also detected clustering upon soluble fibrinogen binding to alpha(IIb)beta(3). In contrast, no clustering was observed following direct activation of alpha(IIb)beta(3) by MnCl(2) or an anti-alpha(IIb)beta(3)-activating antibody Fab in the absence of fibrinogen. Intracellular events also influenced alpha(IIb)beta(3) clustering. For example, a cell-permeable, bivalent FK506-binding protein (FKBP) ligand stimulated clustering when added to cells expressing an alpha(IIb)(FKBP)(2) chimera complexed with beta(3). Furthermore, alpha(IIb)beta(3) clustering occurred in the presence of latrunculin A or cytochalasin D, inhibitors of actin polymerization. These effects were enhanced by fibrinogen, suggesting that actin-regulated clustering modulates alpha(IIb)beta(3) interaction with ligands. These studies in living cells establish that alpha(IIb)beta(3) clustering is modulated by fibrinogen and actin dynamics. More broadly, they should facilitate investigations of the mechanisms and consequences of integrin clustering.

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A sense of closeness: protein detection by proximity ligation

Highly specific and sensitive procedures will be required to evaluate proteomes. Proximity ligation is a recently introduced mechanism for protein analysis. In this technique, the convergence of sets of protein-binding reagents on individual target molecules juxtaposes attached nucleic acid sequences. Through a ligation reaction a DNA reporter sequence is created, which can be amplified. The procedure thus encodes detected proteins as specific nucleic acid sequences in what may be viewed as a reverse translation reaction.

Visualizing protein-protein interactions in living animals

A variety of techniques have been developed to analyze protein-protein interactions in vitro and in cultured cells. However, these methods do not determine how protein interactions affect and are regulated by physiologic and pathophysiologic conditions in living animals. This article describes methodology for detecting and quantifying protein interactions in living mice, using an inducible two-hybrid system developed for positron emission tomography (PET) imaging. We discuss the methods to establish stably transfected cells with components of the imaging system, create tumor xenografts, synthesize PET radiopharmaceuticals used to visualize the imaging reporter, perform microPET imaging, and analyze data from imaging studies. Development and application of technologies for molecular imaging of protein-protein interactions in vivo should enable researchers to investigate intrinsic binding specificities of proteins during normal development and disease progression as well as aid drug development through direct interrogation of molecular targets within intact animals.

Creating new fluorescent probes for cell biology

Fluorescent probes are one of the cornerstones of real-time imaging of live cells and a powerful tool for cell biologists. They provide high sensitivity and great versatility while minimally perturbing the cell under investigation. Genetically-encoded reporter constructs that are derived from fluorescent proteins are leading a revolution in the real-time visualization and tracking of various cellular events. Recent advances include the continued development of 'passive' markers for the measurement of biomolecule expression and localization in live cells, and 'active' indicators for monitoring more complex cellular processes such as small-molecule-messenger dynamics, enzyme activation and protein-protein interactions.

Noninvasive imaging of protein-protein interactions in living subjects by using reporter protein complementation and reconstitution strategies

In this study we have developed bioluminescence-imaging strategies to noninvasively and quantitatively image protein-protein interactions in living mice by using a cooled charge-coupled device camera and split reporter technology. We validate both complementation and intein-mediated reconstitution of split firefly luciferase proteins driven by the interaction of two strongly interacting proteins, MyoD and Id. We use transient transfection of cells and image MyoD-Id interaction after induction of gene expression in cell culture and in cells implanted into living mice. Techniques to study protein-protein interactions in living subjects will allow the study of cellular networks, including signal transduction pathways, as well as development and optimization of pharmaceuticals for modulating protein-protein interactions.

Analysis of membrane protein interactions using yeast-based technologies

Proteins associated with membranes total approximately a third of all proteins in a typical eukaryotic cell. However, the analysis of interactions between membrane proteins is difficult because of the hydrophobic nature of these proteins, and conventional biochemical and genetic assays are often of limited use. We summarize here recent yeast-based interaction technologies that can be applied to membrane proteins.

LMP-1's transmembrane domains encode multiple functions required for LMP-1's efficient signaling

The latent membrane protein-1 (LMP-1) of Epstein-Barr virus (EBV) contributes to the proliferation of infected B lymphocytes by signaling through its binding to cellular signaling molecules. It apparently mimics members of the tumor necrosis factor receptor family, in particular, CD40, by binding a similar set of cellular molecules as does CD40. LMP-1 differs dramatically in its structure from CD40. LMP-1 has six membrane-spanning domains as opposed to CD40's one. LMP-1 also differs from CD40 in its apparent independence of a ligand for its signaling. We have examined the role of LMP-1's membrane-spanning domains in its signaling. Their substitution with six membrane-spanning domains from the LMP-2A protein of EBV yields a derivative which neither coimmunoprecipitates with LMP-1 nor signals to increase the activity of NF-kappaB as does wild-type LMP-1. These observations indicate that LMP-1 has specific sequences in its membrane-spanning domains required for these activities. LMP-1's first and sixth membrane-spanning domains have multiple leucine residues potentially similar to leucine-heptad motifs that can mediate protein-protein interactions in membranes (Gurezka et al., J. Biol. Chem. 274:9265-9270, 1999). Substitution of seven leucines in LMP-1's sixth membrane-spanning domain has no effect on its function, whereas similar substitutions in its first membrane-spanning domain yielded a derivative which aggregates as does wild-type LMP-1 but has only 3% of wild-type's ability to signal through NF-kappaB. Importantly, this derivative complements a mutant of LMP-1 with wild-type membrane-spanning domains but no carboxy-terminal signaling domain. These findings together indicate that the membrane-spanning domains of LMP-1 contribute multiple functions to its signaling.