The structure of the ultraspiracle ligand-binding domain reveals a nuclear receptor locked in an inactive conformation

Nonsteroidal ecdysone agonists

Nonsteroidal ecdysone agonists are novel compounds that have become attractive candidates not only as pest control agents in agriculture but also as tools for research. Their narrow spectrum of activity makes them relatively safe as pesticides, and their mode of action as ligands for gene expression has found application in gene therapy and inducing transgenic gene expression in plants. These diacylhydrazines (DAHs) are potent nonsteroidal ecdysone agonists, and four of them, tebufenozide, methoxyfenozide, chromafenozide, and halofenozide, have been developed as insecticides. Although these compounds are very toxic to insects, they are safe for mammals and are environmentally benign. Their action on insects is also selective, the first three are effective against Lepidoptera but weakly active or inactive on Diptera and Coleoptera. On the other hand, halofenozide is effective on Coleoptera but mildly active on Lepidoptera. Previous reviews on ecdysone agonists have concentrated on the biological response of some DAHs and their effects on pests. In this review, the chemistry, biological effects and their modes of action at the molecular level will be covered. In addition, a few studies on other nonsteroidal ecdysone agonists, such as 3,5-di-tert-butyl-4-hydroxy-N-iso-butylbenzamide, acylaminoketones, and benzoyl-1,2,3,4-tetrahydroquinolines, will be briefly reviewed.

Juvenile hormones antagonize ecdysone actions through co-repressor recruitment to EcR/USP heterodimers

Insect development is controlled by the combined actions of ecdysteroid and juvenile hormones. Transcriptional control by ecdysteroid hormones is mediated via two nuclear receptor superfamily members, ecdysone receptor (EcR) and its heterodimeric partner, ultraspiracle (USP). Although the ecdysteroid hormone 20-hydroxyecdysone acts as an EcR ligand and activates transcription through EcR/USP heterodimers, the activity of juvenile hormones, such as Juvenile hormone III (JH III), and methoprenic acid (MA) via USP remains unclear. Here, we demonstrate that juvenile hormones act as USP ligands and exhibit suppressive effects on ecdysone-dependent EcR transactivation. JH III- and MA-bound USP markedly repressed ecdysone-dependent EcR transactivation through shifting of the USP ligand-binding domain alpha-helix 12 without affecting EcR/USP heterodimerization or DNA binding. Moreover, transcriptional repression by USP ligands was attenuated by a histone deacetylation inhibitor. Our results suggested that juvenile hormones serve as USP ligands that antagonize EcR-mediated ecdysone actions through the recruitment of histone deacetylase complexes.

Hormonal regulation and functional role of Drosophila E75A orphan nuclear receptor in the juvenile hormone signaling pathway

Ecdysone and juvenile hormone (JH) are important regulators of insect growth and development. While ecdysone initiates a transition from one developmental stage to another, JH determines the nature of the transition. How these two hormones interact at the molecular level is not known. Here we report the JH inducibility of the E75A nuclear receptor encoded by the E75 early ecdysone-inducible gene. In Drosophila S2 cells, E75A transcription is specifically activated by JH at concentrations well within the physiological range found in larvae and adults. The induction is rapid and does not require a concurrent protein synthesis, and thus represents a primary hormone response. Consistent with JH regulation, E75A mRNA levels are reduced in ovaries of apterous(4) mutant adults defective in JH secretion. Expression is rescued by topical methoprene application. We further provide evidence that ectopic E75A is sufficient to perform several functions in the JH signaling pathway. First, it can down-regulate its own transcription. Second, E75A can potentiate the JH inducibility of a secondary response gene, JhI-21. Finally, in the presence of JH, E75A can repress ecdysone activation of early genes including Broad-Complex. Based on these data, we propose a model for the role of E75A in the ecdysone-JH regulatory interplay.

Highly flexible ligand binding pocket of ecdysone receptor: a single amino acid change leads to discrimination between two groups of nonsteroidal ecdysone agonists

The insect steroid hormone 20-hydroxyecdysone works through a ligand-activated nuclear receptor, the ecdysone receptor (EcR), which plays critical roles in insect development and reproduction. The EcR has been exploited to develop insecticides to control pests and gene switches for gene regulation. Recently reported crystal structures of the EcR protein show different but partially overlapping binding cavities for ecdysteroid (ECD) and diacylhydrazine (DAH) ligands, providing an explanation for the differential activity of DAH ligands in insects. 1-Aroyl-4-(arylamino)-1,2,3,4-tetrahydroquinoline (THQ) ligands were recently discovered as ecdysone agonists. Mutagenesis of the EcR (from Choristoneura fumiferana, CfEcR) ligand binding domain followed by screening in a reporter assay led to the identification of CfEcR mutants, which responded well to THQ ligands but poorly to both ECD and DAH ligands. These mutants were further improved by introducing a second mutation, A110P, which was previously reported to cause ECD insensitivity. Testing of these V128F/A110P and V128Y/A110P mutants in a C57BL/6 mouse model coactivator interaction assay and in insect cells showed that this mutant EcR is activated by THQ ligands but not by ECD or DAH ligands. The CfEcR and its V128F/A110P mutant were used to demonstrate simultaneous regulation of two reporter genes using THQ and DAH ligands.

Polyunsaturated fatty acids including docosahexaenoic and arachidonic acid bind to the retinoid X receptor alpha ligand-binding domain

Nuclear receptors (NRs) constitute a large and highly conserved family of ligand-activated transcription factors that regulate diverse biological processes such as development, metabolism, and reproduction. As such, NRs have become important drug targets, and the identification of novel NR ligands is a subject of much interest. The retinoid X receptor (RXR) belongs to a subfamily of NRs that bind vitamin A metabolites (i.e. retinoids), including 9-cis-retinoic acid (9-cis-RA). However, although 9-cis-RA has been described as the natural ligand for RXR, its endogenous occurrence has been difficult to confirm. Recently, evidence was provided for the existence of a different natural RXR ligand in mouse brain, the highly enriched polyunsaturated fatty acid (PUFA) docosahexaenoic acid (DHA) (Mata de Urquiza et al. (2000) Science 290, 2140-2144). However, the results suggested that supra-physiological levels of DHA were required for efficient RXR activation. Using a refined method for ligand addition to transfected cells, the current study shows that DHA is a more potent RXR ligand than previously observed, inducing robust RXR activation already at low micromolar concentrations. Furthermore, it is shown that other naturally occurring PUFAs can activate RXR with similar efficiency as DHA. In additional experiments, the binding of fatty acid ligands to RXRalpha is directly demonstrated by electrospray mass spectrometry of the noncovalent complex between the RXR ligand-binding domain (LBD) and its ligands. Data is presented that shows the noncovalent interaction between the RXR LBD and a number of PUFAs including DHA and arachidonic acid, corroborating the results in transfected cells. Taken together, these results show that RXR binds PUFAs in solution and that these compounds induce receptor activation, suggesting that RXR could function as a fatty acid receptor in vivo.

Nuclear receptors: the evolution of diversity

Nuclear receptors are an ancient family of transcription factors. Some receptors are regulated by small lipophilic ligands, whereas others are constitutive transcriptional activators or repressors. The evolution of this diversity is poorly understood, and it remains an open question as to whether or not the ancestral receptor was ligand-regulated. The recent cloning, from a snail, of an estrogen receptor that does not bind estrogen not only suggests that the steroid receptors are much more ancient than previous thought, but also points toward a mechanism through which nuclear receptors can lose the ability to be ligand regulated.

Structural adaptability in the ligand-binding pocket of the ecdysone hormone receptor

The ecdysteroid hormones coordinate the major stages of insect development, notably moulting and metamorphosis, by binding to the ecdysone receptor (EcR); a ligand-inducible nuclear transcription factor. To bind either ligand or DNA, EcR must form a heterodimer with ultraspiracle (USP), the homologue of retinoid-X receptor. Here we report the crystal structures of the ligand-binding domains of the moth Heliothis virescens EcR-USP heterodimer in complex with the ecdysteroid ponasterone A and with a non-steroidal, lepidopteran-specific agonist BYI06830 used in agrochemical pest control. The two structures of EcR-USP emphasize the universality of heterodimerization as a general mechanism common to both vertebrates and invertebrates. Comparison of the EcR structures in complex with steroidal and non-steroidal ligands reveals radically different and only partially overlapping ligand-binding pockets that could not be predicted by molecular modelling and docking studies. These findings offer new perspectives for the design of insect-specific, environmentally safe insecticides. The concept of a ligand-dependent binding pocket in EcR provides an insight into the moulding of nuclear receptors to their ligand, and has potential applications for human nuclear receptors.

Binding affinity of nonsteroidal ecdysone agonists against the ecdysone receptor complex determines the strength of their molting hormonal activity

N-tert-Butyl-N,N'-dibenzoylhydrazine and its analogs are nonsteroidal ecdysone agonists that exhibit insect molting hormonal and larvicidal activities. The interaction mode of those ecdysone agonists with the heterodimer of the ecdysone receptor and ultraspiracle has not been fully elucidated. We expressed the ecdysone receptor B1 and the ultraspiracle of the lepidopteran, Chilo suppressalis, using an in vitro transcription/translation system and confirmed, using gel-shift assays, that the proteins function as ecdysone receptors. We also analyzed their ligand-binding affinity. A potent ecdysteroid, ponasterone A, specifically bound to the ecdysone receptor with low affinity (KD = 55 nm), and the specific binding was dramatically increased (KD = 1.2 nm) in the presence of the ultraspiracle. For seven nonsteroidal ecdysone agonists and five ecdysteroids, the binding activity to the in vitro-translated ecdysone receptor-ultraspiracle complex was linearly correlated with the binding activity to the inherent receptor protein in the cell-free preparation of C. suppressalis integument. The binding to the ecdysone receptor-ultraspiracle complex for a series of compounds was highly correlated with their molting hormonal activity, indicating that the binding affinity of nonsteroidal ecdysone agonists to the ecdysone receptor-ultraspiracle complex primarily determines the strength of their molting hormonal activity.

Structural analysis of lipid complexes of GM2-activator protein

The GM2-activator protein (GM2-AP) is a small lysosomal lipid transfer protein essential for the hydrolytic conversion of ganglioside GM2 to GM3 by beta-hexosaminidase A. The crystal structure of human apo-GM2-AP is known to consist of a novel beta-cup fold with a spacious hydrophobic interior. Here, we present two new structures of GM2-AP with bound lipids, showing two different lipid-binding modes within the apolar pocket. The 1.9A structure with GM2 bound shows the position of the ceramide tail and significant conformational differences among the three molecular copies in the asymmetric unit. The tetrasaccharide head group is not visible and is presumed to be disordered. However, its general position could be established through modeling. The structure of a low-pH crystal, determined at 2.5A resolution, has a significantly enlarged hydrophobic channel that merges with the apolar pocket. Electron density inside the pocket and channel suggests the presence of a trapped phospholipid molecule. Structure alignments among the four crystallographically unique monomers provide information on the potential role for lipid binding of flexible chain segments at the rim of the cavity opening. Two discrete orientations of the S130-T133 loop define an open and a closed configuration of the hydrophobic channel that merges with the apolar pocket. We propose: (i) that the low-pH structure represents an active membrane-binding conformation; (ii) that the mobile S130-T133 loop serves as a gate for passage of ligand into the apolar pocket; and (iii) that this loop and the adjacent apolar V59-W63 loop form a surface patch with two exposed tryptophan residues that could interface with lipid bilayers.

Activation incarnate

Three new publications reveal that the nuclear receptors LRH-1, DHR38, and Nurr1 can assume active conformations in the absence of a ligand agonist. These reports help elucidate how transcriptional regulators are themselves regulated.

The Drosophila orphan nuclear receptor DHR38 mediates an atypical ecdysteroid signaling pathway

Ecdysteroid pulses trigger the major developmental transitions during the Drosophila life cycle. These hormonal responses are thought to be mediated by the ecdysteroid receptor (EcR) and its heterodimeric partner Ultraspiracle (USP). We provide evidence for a second ecdysteroid signaling pathway mediated by DHR38, the Drosophila ortholog of the mammalian NGFI-B subfamily of orphan nuclear receptors. DHR38 also heterodimerizes with USP, and this complex responds to a distinct class of ecdysteroids in a manner that is independent of EcR. This response is unusual in that it does not involve direct binding of ecdysteroids to either DHR38 or USP. X-ray crystallographic analysis of DHR38 reveals the absence of both a classic ligand binding pocket and coactivator binding site, features that seem to be common to all NGFI-B subfamily members. Taken together, these data reveal the existence of a separate structural class of nuclear receptors that is conserved from fly to humans.

Isotype-restricted corepressor recruitment: a constitutively closed helix 12 conformation in retinoic acid receptors beta and gamma interferes with corepressor recruitment and prevents transcriptional repression

Retinoic acid receptors (RARs) are ligand-regulated transcription factors that play multiple roles in vertebrate development and differentiation. RARs as a class are capable of both repressing and activating target gene expression. Transcriptional repression is mediated through the recruitment of corepressor proteins such as SMRT. Notably, vertebrates encode three major forms of RARs, alpha, beta, and gamma, and these distinct RAR isotypes differ in the ability to recruit a corepressor. RAR alpha strongly interacts with SMRT and can repress target gene transcription, whereas RAR beta and -gamma interact with SMRT only weakly and fail to repress. We report here the use of a genetic suppressor approach, based on a yeast two-hybrid interaction assay using Saccharomyces cerevisiae, for the isolation of RAR beta mutants that have gained the RAR alpha-like corepressor phenotype, i.e., a strong interaction with SMRT and the ability to repress gene expression in vertebrate cells. Analysis of these gain-of-function mutants indicates that the different corepressor interaction properties of RAR alpha, -beta and -gamma are determined by a gating mechanism through which amino acid differences in the helix 3 region of these receptors influence the position of the receptor C-terminal helix 12 domain. As a consequence, the RAR beta and RAR gamma receptors appear to adopt a constitutively closed helix 12 conformation in the absence of hormone that may approximate the conformation of RAR alpha when bound to hormone agonist. This closed helix 12 conformation in RAR beta and RAR gamma blocks corepressor binding, prevents repression, and permits significant levels of target gene activation even in the absence of hormone. We refer to this phenomenon as a "gate-latch" model of corepressor regulation.

Transcription activation by the ecdysone receptor (EcR/USP): identification of activation functions

The ecdysone receptor is a heterodimer of the two nuclear receptors EcR and ultraspiracle (USP). We have identified the regions of Drosophila EcR and USP responsible for transcriptional activation of a semisynthetic Eip71CD promoter in Kc cells. The isoform-specific A/B domains of EcR-B1 and B2, but not those of EcR-A or USP, exhibit strong activation activity [activation function 1 (AF1)], both in isolation and in the context of the intact receptor. AF1 activity in isoform B1 derives from dispersed elements; the B2-specific AF1 consists of a 17-residue amphipathic helix. AF2 function was studied using a two-hybrid assay in Kc cells, based on the observation that potent hormone-dependent activation by the EcR/USP ligand-binding domain heterodimer requires the participation of both partners. Mutagenesis reveals that AF2 function depends on EcR helix 12, but not on the cognate USP region. EcR helix 12 mutants (F645A and W650A) exhibit a dominant negative phenotype. Thus, in the setting tested, the ecdysone receptor can activate transcription using the AF1 regions of EcR-B1 or -B2 and the AF2 region of EcR. USP acts as an allosteric effector for EcR, but does not contribute any intrinsic function.

Activation of orphan nuclear constitutive androstane receptor requires subnuclear targeting by peroxisome proliferator-activated receptor gamma coactivator-1 alpha. A possible link between xenobiotic response and nutritional state

In contrast to the classical nuclear receptors, the constitutive androstane receptor (CAR) is transcriptionally active in the absence of ligand. In the course of searching for the mediator of CAR activation, we found that ligand-independent activation of CAR was achieved in cooperation with the peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1 alpha). PGC-1 beta, a PGC-1 alpha homologue, also activated CAR to less of an extent than PGC-1 alpha. Coexpression of the ligand-binding domain of a heterodimerization partner, retinoid X receptor alpha, enhanced the PGC-1 alpha-mediated activation of CAR, although it had a weak effect on the basal activity of CAR in the absence of PGC-1 alpha. Both the N-terminal region, with the LXXLL motif, and the C-terminal region, with a serine/arginine-rich domain (RS domain), in PGC-1 alpha were required for full activation of CAR. Pull-down experiments using recombinant proteins revealed that CAR directly interacted with both the LXXLL motif and the RS domain. Furthermore, we demonstrated that the RS domain of PGC-1 alpha was required for CAR localization at nuclear speckles. These results indicate that PGC-1 alpha mediates the ligand-independent activation of CAR by means of subnuclear targeting through the RS domain of PGC-1 alpha.

Retinoic acid receptors beta and gamma do not repress, but instead activate target gene transcription in both the absence and presence of hormone ligand

Retinoic acid receptors (RARs) are important mediators of retinoid signaling in morphogenesis, development, and cell differentiation. Three major isotypes of RARs, denoted alpha, beta, and gamma, have been identified, each encoded by a distinct genetic locus. Although RARalpha, RARbeta, and RARgamma share many structural and functional features, these three isotypes are known to play unique, as well as overlapping, roles in physiology and development. We report here that the three RAR isotypes display different transcriptional properties in the absence of hormone ligand; under these conditions, RARalpha is a strong repressor of target gene expression, whereas both RARbeta and RARgamma fail to repress and instead are able to mediate substantial levels of hormone-independent transcriptional activation. These differing transcriptional properties appear to reflect the differing abilities of the three RAR isotypes to interact with the SMRT (silencing mediator of retinoic acid and thyroid hormone receptor) corepressor protein: RARalpha binds to SMRT strongly both in vitro and in vivo, whereas RARbeta and RARgamma interact only weakly with SMRT. The ability to repress or to activate transcription in the absence of hormone maps predominantly to isotype-specific differences in the sequence of helix 3 within the hormone binding domain of the RARs, and the transcriptional properties of one isotype can be exchanged with that of another by exchanging portions of helix 3. The different transcriptional properties of RARalpha, RARbeta, and RARgamma in the absence of hormone contribute to the distinctive biological functions of these proteins and provide a rationale for the strong conservation of the three distinct isotypes during the vertebrate evolutionary radiation.

Beyond the Human Genome

The nuclear receptor (NR) superfamily is a large group of related, pharmacologically important receptors, comprising the targets for over 10% of commonly prescribed drugs. Cross-genome analysis of NR sequence, structure, and biological function, provides an important source of information on the function of human NRs and thus plays a role in NR drug discovery. For example, research on the pregnane X receptor (PXR; NR1I2), constitutive androstane receptor (CAR; NR1I3), hepatocyte nuclear factor 4 (HNF4; NR2A1), and farnesoid X receptor (FXR) illustrate how the study of nonhuman orthologs has provided new insights into NR biology and has increased our understanding of human NRs and orphan NR function. Understanding differences between humans and pharmacological model species may provide useful tools for the development of new NR-binding drugs.

EcR isoforms in Drosophila: testing tissue-specific requirements by targeted blockade and rescue

The three Drosophila EcR isoforms differ only at their N termini; thus, they share the conserved ligand-binding domain transcriptional activation function (AF2) and only differ in the unconserved A/B region, which contains a second, isoform-specific, activation function (AF1). We have developed a dominant-negative mutant EcR (EcR-DN), expressed it in flies with the GAL4/UAS system, and used it to block ecdysone signaling in eight tissues or groups of tissues. Localized EcR-DN arrests ecdysone-dependent development in the target cells and often--because of a molting checkpoint--arrests development globally. Simultaneously expressing individual wild-type EcR isoforms in the same target tissues suppresses the EcR-DN phenotype and identifies the rescuing isoform as sufficient to support the development of the target. Every isoform, and even an N-terminal truncated EcR that lacks any AF1, supports development in the fat body, eye discs, salivary glands, EH-secreting neurosecretory cells and in the dpp expression domain, implying that AF1 is dispensable in these tissues. By contrast, only EcR-A is able to support development in the margins of the wing discs, and only EcR-B2 can do so in the larval epidermis and the border cells of the developing egg chamber. In light of our results, the simplest explanations for the widespread spatial and temporal variations in EcR isoform titers appear untenable.

RORalpha: an orphan nuclear receptor on a high-cholesterol diet

A high-resolution X-ray crystal structure of the retinoic acid receptor-related orphan receptor (RORalpha; NR1F1), which reveals a molecule of cholesterol within the ligand binding pocket, is a breakthrough in functional analysis of this orphan nuclear receptor.

Activation of transcription through the ligand-binding pocket of the orphan nuclear receptor ultraspiracle

The invertebrate nuclear receptor, ultraspiracle (USP), an ortholog of the vertebrate RXR, is typically modelled as an orphan receptor that functions without a ligand-binding activity. The identification of a ligand that can transcriptionally activate USP would provide heuristic leads to the structure of potentially high affinity activating compounds, with which to detect unknown regulatory pathways in which this nuclear receptor participates. We show here that the application of the sesquiterpenoid methyl epoxyfarnesoate (juvenile hormone III) to Sf9 cells induces transcription from a transfected heterologous core promoter, through a 5'-placed DR12 enhancer to which the receptor ultraspiracle (USP) binds. Isolated, recombinant USP from Drosophila melanogaster specifically binds methyl epoxyfarnesoate, whereupon the receptor homodimerizes and changes tertiary conformation, including the movement of the ligand-binding domain alpha-helix 12. Ligand-binding pocket point mutants of USP that do not bind methyl epoxyfarnesoate act as dominant negative suppressors of methyl epoxyfarnesoate-activation of the reporter promoter, and addition of wild-type USP rescues this activation. These data establish a paradigm in which the USP ligand-binding pocket can productively bind ligand with a functional outcome of enhanced promoter activity, the first such demonstration for an invertebrate orphan nuclear receptor. USP thus establishes the precedent that invertebrate orphan receptors are viable targets for development of agonists and antagonists with which to discern and manipulate transcriptional pathways dependent on USP or other orphan receptors. The demonstration here of these functional capacities of USP in a transcriptional activation pathway has significant implications for current paradigms of USP action that do not include for USP a ligand-binding activity.

A single point mutation in ecdysone receptor leads to increased ligand specificity: implications for gene switch applications

The ecdysone receptor (EcR), a member of the nuclear receptor superfamily, plays an important role in regulating development and reproduction in insects. The EcR binds to ecdysteroids and regulates transcription of genes that contain ecdysone response elements. The EcR has been used to develop inducible gene switches for efficient regulation of foreign genes in applications such as gene therapy, protein production, and functional genomics. An EcR [Choristoneura fumiferana EcR (CfEcR)] homology model was constructed, and 17 amino acid residues were identified as critical for 20-hydroxyecdysone binding. Mutation of these amino acids followed by analysis of these mutants in transactivation (in insect and mammalian cells and in vivo in mice) and ligand-binding assays identified one particular mutant (A110P) that failed to respond to steroids, but its response to the diacylhydrazine nonsteroidal ligands RG-102240 (GS(TM)E) and RG-102317 was unaffected. This steroid-insensitive EcR mutant has potential gene switch applications in insects and plants that have endogenous ecdysteroids. In addition, this mutant would be also useful for developing orthogonal EcR-ligand pairs for simultaneous regulation of multiple genes in the same cell.

Hepatocyte nuclear factor 4 is a transcription factor that constitutively binds fatty acids

The 2.7 A X-ray crystal structure of the HNF4gamma ligand binding domain (LBD) revealed the presence of a fatty acid within the pocket, with the AF2 helix in a conformation characteristic of a transcriptionally active nuclear receptor. GC/MS and NMR analysis of chloroform/methanol extracts from purified HNF4alpha and HNF4gamma LBDs identified mixtures of saturated and cis-monounsaturated C14-18 fatty acids. The purified HNF4 LBDs interacted with nuclear receptor coactivators, and both HNF4 subtypes show high constitutive activity in transient transfection assays, which was reduced by mutations designed to interfere with fatty acid binding. The endogenous fatty acids did not readily exchange with radiolabeled palmitic acid, and all attempts to displace them without denaturing the protein failed. Our results suggest that the HNF4s may be transcription factors that are constitutively bound to fatty acids.

Folding and stability of the ligand-binding domain of the glucocorticoid receptor

A complex pathway involving many molecular chaperones has been proposed for the folding, assembly, and maintenance of a high-affinity ligand-binding form of steroid receptors in vivo, including the glucocorticoid receptor. To better understand this intricate folding and assembly process, we studied the folding of the ligand-binding domain of the glucocorticoid receptor in vitro. We found that this domain can be refolded into a compact, highly structured state in vitro in the absence of chaperones. However, the presence of zwitterionic detergent is required to maintain the domain in a soluble form. In this state, the protein is dimeric and has considerable helical structure as shown by far-UV circular dichroism. Further investigation of the properties of this in vitro refolded state show that it is stable and resistant to denaturation by heat or low concentrations of chemical denaturants. A detailed analysis of the unfolding equilibria using three different structural probes demonstrated that this state unfolds via a highly populated dimeric intermediate state. Together, these data clearly show that the ligand-binding domain of the glucocorticoid receptor does not require chaperones for folding per se. However, this in vitro refolded state binds the ligand dexamethasone only weakly (K(d) = 45 microM) compared to the in vivo assembled receptor (K(d) = 3.4 nM). We suggest that the role of Hsp90 and associated chaperones is to bind to, and stabilize, a specific conformational state of the receptor which binds ligand with high affinity.

Ecdysone-controlled expression of transgenes

Inducible expression systems show great potential for use in human gene therapy and systems based on insect ecdysone receptors are particularly promising candidates. This article describes such systems and reviews actual and potential uses of ecdysone-controlled transgenes in vitro and in vivo. The ligand specificity of ecdysone receptor-based systems is considered, along with the safety and efficacy of the ecdysteroid and non-steroidal compounds used to activate them.

Molecular recognition of agonist ligands by RXRs

The nuclear receptor RXR is an obligate partner in many signal transduction pathways. We report the high-resolution structures of two complexes of the human RXRalpha ligand-binding domain specifically bound to two different and chemically unrelated agonist compounds: docosa hexaenoic acid, a natural derivative of eicosanoic acid, present in mammalian cells and recently identified as a potential endogenous RXR ligand in the mouse brain, and the synthetic ligand BMS 649. In both structures the RXR-ligand-binding domain forms homodimers and exhibits the active conformation previously observed with 9-cis-RA. Analysis of the differences in ligand-protein contacts (predominantly van der Waals forces) and binding cavity geometries and volumes for the several agonist-bound RXR structures clarifies the structural features important for ligand recognition. The L-shaped ligand-binding pocket adapts to the diverse ligands, especially at the level of residue N306, which might thus constitute a new target for drug-design. Despite its highest affinity 9-cis-RA displays the lowest number of ligand-protein contacts. These structural results support the idea that docosa hexaenoic acid and related fatty acids could be natural agonists of RXRs and question the real nature of the endogenous ligand(s) in mammalian cells.

Endocrine insights into the evolution of metamorphosis in insects

This review explores the roles of ecdysone and juvenile hormone (JH) in the evolution of complete metamorphosis and how metamorphosis, in turn, has impacted endocrine signaling. JH is a key player in the evolution of metamorphosis because it can act on embryos from more basal insect groups to suppress morphogenesis and cause premature differentiation, functions needed for transforming the transitional pronymphal stage of hemimetabolous insects into a functional larval stage. In the ancestral condition, imaginal-related growth is then delayed until JH finally disappears during the last larval instar. In the more derived groups of the Holometabola, selective tissues have escaped this JH suppression to form early-growing imaginal discs. We discuss how complete metamorphosis may have influenced the molecular aspects of both ecdysone and JH signaling.

Different ligands-different receptor conformations: modeling of the hER alpha LBD in complex with agonists and antagonists

The aim of this study is to compare crystal structures of nuclear receptor ligand binding domains in complex with different agonists and partial agonists to achieve a better understanding of the three-dimensional structures and their ligand-induced conformational changes. This led to the identification of structurally conserved "rigid" regions and more flexible parts of the proteins. The analysis was found to be of great value in fitting selected non-steroidal compounds into the human estrogen receptor alpha (hER alpha) ligand binding pocket. The experimentally determined binding affinities for a number of 2-aryl indoles and 2-aryl indenones are in good agreement with the subsequently modeled binding interactions. To date, no crystal structure is published for a complex with a pure antagonist. We therefore used the available structural information on complexes with partial agonists and the crystal structure of a mutant protein in complex with estradiol displaying a similar conformation to predict binding interactions for antagonists. The results are discussed in detail.