An immunophilin is a component of the insect ecdysone receptor (EcR) complex

Cytoplasmic FKBPs are involved in molting and metamorphosis through regulating the nuclear localization of EcR

Molting and metamorphosis are important physiological processes in insects that are tightly controlled by ecdysone receptor (EcR) through the 20-hydroxyecdysone (20E) signaling pathway. EcR is a steroid nuclear receptor (SR). Several FK506-binding proteins (FKBPs) have been identified from the mammal SR complex, and are thought to be involved in the subcellular trafficking of SR. However, their roles in insects are poorly understood. To explore whether FKBPs are involved in insect molting or metamorphosis, we injected an FKBP inhibitor (FK506) into a lepidopteran insect, Spodoptera litura, and found that molting was inhibited in 61.11% of the larvae, and that the time for larvae to pupate was significantly extended. A total of 10 FKBP genes were identified from the genome of S. litura and were clustered into 2 distinct groups, according to their subcellular localization, with FKBP13 and FKBP14 belonging to the endoplasmic reticulum (ER) group and with the other members belonging to the cytoplasmic (Cy) group. All the CyFKBPs were significantly upregulated in the prepupal or pupal stages, with the opposite being observed for the ER group members. FK506 completely blocked the transfer of EcR to the nucleus under 20E induction, and significantly downregulated the transcriptional expression of many 20E signaling genes. A similar phenomenon was observed after RNA interference of 2 CyFKBPs (FKBP45 and FKBP12b), but not for FKBP13. Taken together, our data indicate that the cytoplasmic FKBPs, especially FKBP45 and FKBP12b, mediate the nuclear localization of EcR, thereby regulating the 20E signaling and ultimately affecting molting and metamorphosis in insects.

Molecular determinants of Drosophila immunophilin FKBP39 nuclear localization

FK506-binding proteins (FKBPs) belong to a distinct class of immunophilins that interact with immunosuppressants. They use their peptidyl-prolyl isomerase (PPIase) activity to catalyze the cis-trans conversion of prolyl bonds in proteins during protein-folding events. FKBPs also act as a unique group of chaperones. The Drosophila melanogaster peptidyl-prolyl cis-trans isomerase FK506-binding protein of 39 kDa (FKBP39) is thought to act as a transcriptional modulator of gene expression in 20-hydroxyecdysone and juvenile hormone signal transduction. The aim of this study was to analyze the molecular determinants responsible for the subcellular distribution of an FKBP39-yellow fluorescent protein (YFP) fusion construct (YFP-FKBP39). We found that YFP-FKBP39 was predominantly nucleolar. To identify the nuclear localization signal (NLS), a series of YFP-tagged FKBP39 deletion mutants were prepared and examined in vivo. The identified NLS signal is located in a basic domain. Detailed mutagenesis studies revealed that residues K188 and K191 are crucial for the nuclear targeting of FKBP39 and its nucleoplasmin-like (NPL) domain contains the sequence that controls the nucleolar-specific translocation of the protein. These results show that FKBP39 possesses a specific NLS in close proximity to a putative helix-turn-helix (HTH) motif and FKBP39 may bind DNA in vivo and in vitro.

Protein Discovery: Combined Transcriptomic and Proteomic Analyses of Venom from the Endoparasitoid Cotesia chilonis (Hymenoptera: Braconidae)

Many species of endoparasitoid wasps provide biological control services in agroecosystems. Although there is a great deal of information on the ecology and physiology of host/parasitoid interactions, relatively little is known about the protein composition of venom and how specific venom proteins influence physiological systems within host insects. This is a crucial gap in our knowledge because venom proteins act in modulating host physiology in ways that favor parasitoid development. Here, we identified 37 possible venom proteins from the polydnavirus-carrying endoparasitoid Cotesia chilonis by combining transcriptomic and proteomic analyses. The most abundant proteins were hydrolases, such as proteases, peptidases, esterases, glycosyl hydrolase, and endonucleases. Some components are classical parasitoid venom proteins with known functions, including extracellular superoxide dismutase 3, serine protease inhibitor and calreticulin. The venom contains novel proteins, not recorded from any other parasitoid species, including tolloid-like proteins, chitooligosaccharidolytic β-N-acetylglucosaminidase, FK506-binding protein 14, corticotropin-releasing factor-binding protein and vascular endothelial growth factor receptor 2. These new data generate hypotheses and provide a platform for functional analysis of venom components.

Cytoplasmic travels of the ecdysteroid receptor in target cells: pathways for both genomic and non-genomic actions

Signal transduction of the insect steroid hormones, ecdysteroids, is mediated by the ecdysteroid receptor, EcR. In various cells of the insect Rhodnius prolixus, EcR is present in both the nucleus and the cytoplasm, where it undergoes daily cycling in abundance and cellular location at particular developmental times of the last larval instar that are specific to different cell types. EcR favors a cytoplasmic location in the day and a nuclear location in the night. This study is the first to examine the potential mechanisms of intracellular transport of EcR and reveals close similarities with some of its mammalian counterparts. In double and triple labels using several antibodies, immunohistochemistry, and confocal laser scanning microscopy, we observed co-localization of EcR with the microtubules (MTs). Treatments with either the MT-stabilizing agent taxol or with colchicine, which depolymerizes MTs, resulted in considerable reduction in nuclear EcR with a concomitant increase in cytoplasmic EcR suggesting that MT disruption inhibits receptor accumulation in the nucleus. EcR also co-localizes with the chaperone Hsp90, the immunophilin FKBP52, and the light chain 1 of the motor protein dynein. All these factors also co-localize with MTs. We propose that in Rhodnius, EcR exerts its genomic effects by forming a complex with Hsp90 and FKBP52, which uses dynein on MTs as a mechanism for daily nucleocytoplasmic shuttling. The complex is transported intact to the nucleus and dissociates within it. We propose that EcR utilizes the cytoskeletal tracks for movement in a manner closely similar to that used by the glucocorticoid receptor. We also observed co-localization of EcR with mitochondria which suggests that EcR, like its mammalian counterparts, may be involved in the coordination of non-genomic responses of ecdysteroids in mitochondria.

Ecdysone receptor and ultraspiracle proteins are tyrosine phosphorylated during adult development of silkmoths

20-hydroxy ecdysone (20E) is essential to promote adult development in diapausing silkmoth pupae. Increases in protein tyrosine/serine-phosphorylations observed soon after 20E administration supported the initial hypothesis that activation of receptor tyrosine kinase-ras-MAPK pathway could be responsible for the growth promoting effects of 20E. This report pertains to the high levels of protein tyrosine phosphorylations (PTP) that occurred later during the growth to differentiation transition because of its novelty and relevance to 20E dependence of adult development. Further analyses demonstrated that both ecdysone receptor (EcR) and ultraspiracle (USP), the two dimerizing partners of the functional ecdysone receptor, are tyrosine phosphorylated coincidental with high PTP. Enhanced PTP during growth to differentiation transition and concomitant tyrosine phosphorylation of EcR and USP was shown to occur in another silkmoth species pointing to the necessity of similar protein tyrosine phosphorylation pathways for adult development. Properly timed increases in tissue protein tyrosine kinase (PTK) activity could explain the enhancement of PTP in the wing epidermis of both the silkmoths. Thymidine incorporation measurements showed that cessation of DNA synthesis preceded the increase in PTK activity thus emphasizing a role for PTP in aspects of tissue physiology related to differentiative events rather than cell proliferation. Phosphatase and tyrosine kinase inhibitors (Tyrphostins) had minimal effects on adult wing development in vivo. However, the escape of the adult from the pupal case was blocked by a tyrphostin indicating the importance of PTKs in eclosion.

Identification of 20-hydroxyecdysone late-response genes in the chitin biosynthesis pathway

Background

20-hydroxyecdysone (20E) and its receptor complex ecdysone receptor (EcR) and ultraspiracle (USP) play a crucial role in controlling development, metamorphosis, reproduction and diapause. The ligand-receptor complex 20E-EcR/USP directly activates a small set of early-response genes and a much larger set of late-response genes. However, ecdysone-responsive genes have not been previously characterized in the context of insect chitin biosynthesis.

Principal Findings

Here, we show that injection-based RNA interference (RNAi) directed towards a common region of the two isoforms of SeEcR in a lepidopteron insect Spodoptera exigua was effective, with phenotypes including a high mortality prior to pupation and developmental defects. After gene specific RNAi, chitin contents in the cuticle of an abnormal larva significantly decreased. The expression levels of five genes in the chitin biosynthesis pathway, SeTre-1, SeG6PI, SeUAP, SeCHSA and SeCHSB, were significantly reduced, while there was no difference in the expression of SeTre-2 prior to 72 hr after injection of EcR dsRNA. Meanwhile, injection of 20E in vivo induced the expression of the five genes mentioned above. Moreover, the SeTre-1, SeG6PI, SeUAP and SeCHSB genes showed late responses to the hormone and the induction of SeTre-1, SeG6PI, SeUAP and SeCHSB genes by 20E were able to be inhibited by the protein synthesis inhibitor cycloheximide in vitro indicating these genes are 20E late-response genes.

Conclusions

We conclude that SeTre-1, SeG6PI, SeUAP and SeCHSB in the chitin biosynthesis pathway are 20E late-response genes and 20E and its specific receptors plays a key role in the regulation of chitin biosynthesis via inducing their expression.

Identification and characterization of a juvenile hormone response element and its binding proteins

Juvenile hormones (JH) regulate a wide variety of developmental and physiological processes in insects. Comparison of microarray data on JH-induced genes in the fruit fly, Drosophila melanogaster, L57 cells and in the honey bee, Apis mellifera, identified 16 genes that are induced in both species. Analysis of promoter regions of these 16 D. melanogaster genes identified DmJHRE1 (D. melanogaster JH response element 1). In L57 cells, the reporter gene regulated by DmJHRE1 was induced by JH III. Two proteins (FKBP39 and Chd64) that bind to DmJHRE1 were identified. FKBP39 and Chd64 double-stranded RNA inhibited JH III induction of a reporter gene regulated by DmJHRE1. FKBP39 and Chd64 proteins expressed in yeast bound to DmJHRE1. Two-hybrid and pull-down assays showed that these two proteins interact with each other as well as with ecdysone receptor, ultraspiracle, and methoprene-tolerant protein. Developmental expression profiles and JH induction of mRNA for FKBP39 and Chd64 proteins and their interaction with proteins known to be involved in both JH (methoprene-tolerant protein) and ecdysteroid action (ecdysone receptor and ultraspiracle) suggest that these proteins probably play important roles in cross-talk between JH and ecdysteroids.

Characterization of immunophilins in the silkmoth Bombyx mori

The FK506-binding proteins (FKBPs) perform an extensive variety of functions in numerous organisms from archaea to humans. The FKBPs are distinguished by their peptidyl-prolyl cis-trans isomerase (PPIase) activity and ability to bind the immunosuppressive drugs FK506 and rapamycin. Here, we report the isolation and characterization of FKBP45, a novel member of the FKBP family obtained from U1 small nuclear RNA (snRNA) binding assays using Bombyx mori nuclear extracts. The protein, an apparent orthologue of FKBP46 from the armyworm, Spodoptera frugiperda, was found to associate with U1 stem-loop I RNA in vitro. The FKBP45 cDNA was isolated and the genomic sequence was characterized, including the positions of exon/intron junctions and consensus splice sites. Using bioinformatics, transcription factor consensus binding sites were identified and subsequent Western blotting from developing eggs indicate that FKBP45 is differentially expressed during embryogenesis. A database was assembled using more than 1,800 available FKBP amino acid sequences and pairwise sequence alignments revealed several putative FKBP45 orthologues in various species. Analysis of these sequences revealed the position of an RNA binding domain within this new protein. In addition, FKBP45 possesses similar characteristics to several potential orthologues, including the presence of bipartite nuclear localization signals (NLSs) and phosphorylation sites.

Purification and characterization of recombinant ligand-binding domains from the ecdysone receptors of four pest insects

Cloned EcR and USP cDNAs encoding the ecdysone receptors of four insect pests (Lucilia cuprina, Myzus persicae, Bemisia tabaci, Helicoverpa armigera) were manipulated to allow the co-expression of their ligand binding domains (LBDs) in insect cells using a baculovirus vector. Recombinant DE/F segment pairs (and additionally, for H. armigera, an E/F segment pair) from the EcR and USP proteins associated spontaneously with high affinity to form heterodimers that avidly bound an ecdysteroid ligand. This shows that neither ligand nor D-regions are essential for the formation of tightly associated and functional LBD heterodimers. Expression levels ranged up to 16.6mg of functional apo-LBD (i.e., unliganded LBD) heterodimer per liter of recombinant insect cell culture. Each recombinant heterodimer was affinity-purified via an oligo-histidine tag at the N-terminus of the EcR subunit, and could be purified further by ion exchange and/or gel filtration chromatography. The apo-LBD heterodimers appeared to be more easily inactivated than their ligand-containing counterparts: after purification, populations of the former were <40% active, whereas for the latter >70% could be obtained as the ligand-LBD heterodimer complex. Interestingly, we found that the amount of ligand bound by recombinant LBD heterodimer preparations could be enhanced by the non-denaturing detergent CHAPS (3-[(3-cholamidopropyl)dimethyl-ammonio]-1-propanesulfonate). Purity, integrity, size and charge data are reported for the recombinant proteins under native and denaturing conditions. Certain intra- and intermolecular disulfide bonds were observed to form in the absence of reducing agents, and thiol-specific alkylation was shown to suppress this phenomenon but to introduce microheterogeneity.

A DNA-binding protein, tfp1, involved in juvenile hormone-regulated gene expression in Locusta migratoria

A partially palindromic 15-nt. sequence upstream from a juvenile hormone-regulated gene (jhp21) was previously identified in the African migratory locust, Locusta migratoria. This sequence was proposed as a juvenile hormone (JH) response element (JHRE), and a protein that bound to it, as a transcription factor (TF). A yeast strain was constructed containing four tandem copies of the JHRE and after transfection with a cDNA library made to fat bodies from vitellogenic females, yeast one-hybrid experiments yielded sequences for four putative binding proteins. One of these sequences, corresponding to a transcript that was present in fat body irrespective of JH stimulation, encodes a 35kDa protein. This was designated tfp1 and appears to have a leucine zipper motif and a lipid-binding motif. Recombinant tfp1 bound to JHRE in electrophoretic mobility shift experiments and addition of tfp1 antibody in the binding reaction resulted in the disappearance or shift of TF. We suggest that JH induces the association of pre-existing proteins, including tfp1, to form an active complex, which binds to the JHRE upstream from jhp21 and regulates its transcription.

PKC-mediated USP phosphorylation is required for 20E-induced gene expression in the salivary glands of Drosophila melanogaster

Ecdysone receptor (EcR) and its heterodimer, ultraspiracle protein (USP), are ligand-dependent transcriptional factors that mediate the action of molting hormone 20-hydroxyecdysone. The activities of transcriptional factors are subjected to regulation not only by transcriptional/translational mechanisms, but also by posttranslational mechanisms such as phosphorylation. Protein kinase consensus recognition sequence analysis of Drosophila EcR and USP reveals multiple phosphorylation sites for protein kinase C (PKC) and casein kinase II (CKII) on EcR and USP sequence. By using specific protein kinase inhibitors, we have shown that PKC, not CKII, is responsible for USP phosphorylation. Inhibition of PKC activity by protein kinase inhibitors blocked USP phosphorylation, resulting in inhibition of 20E-induced gene expression at both transcriptional and translational levels. The composite data suggest that PKC-mediated USP phosphorylation is required for 20E-induced gene expression in the salivary glands of Drosophila melanogaster.

Woc gene mutation causes 20E-dependent alpha-tubulin detyrosination in Drosophila melanogaster

The mutation without children(rgl) (woc(rgl)) is a newly described ecdysone-deficient Drosophila mutant. The woc(rgl) mutant larvae show developmental arrestment at the late larval stage and fail to form a puparium due to the failure of the ring gland to secret normal levels of ecdysone. Although a 6.8-kb woc gene transcript encoding a 187-kDa potential transcription factor has been cloned and lack of a specific cholesterol 7,8-dehydrogenease that mediates the first step in the ecdysteroidogenic pathway is likely the cause for ecdysteroid deficiency, the cellular events controlled by the woc gene remain unclear. In the present study, we investigated the effect of the woc gene mutation on the expression and tyrosination of alpha-tubulin in the woc(rgl) mutant. Our results demonstrated that the mutation in the woc gene caused 20E-dependent alpha-tubulin detyrosination, but had no significant effect on the expression of total alpha- and beta-tubulin in the homozygous woc(rgl) mutant larvae. Immunocytochemical study revealed that 20E-induced alpha-tubulin detyrosination led to the diminishing of tyrosinated alpha-tubulin signals from microtubules, resulting in the disruption of microtubule structure. The composite data suggest that the woc gene may regulate 20E-dependent alpha-tubulin detyrosination and that microtubules may be involved in sterol transport and sterol utilization in insect.

Ligand binding is without effect on complex formation of the ligand binding domain of the ecdysone receptor (EcR)

The ligand-binding domain (LBD) encompassing the C-terminal parts of the D- and the complete E-domains of the ecdysteroid receptor (EcR) fused to Gal4(AD) is present in two high molecular weight complexes (600 and 150 kDa) in yeast extracts according to size exclusion chromatography (Superdex 200 HR 10/30). Hormone binding is mainly associated with 150-kDa complexes. Complex formation is not influenced by hormone, but the ligand stabilizes the complexes at elevated salt concentrations. Mutational analysis of Gal4(AD)-EcR(LBD) revealed that formation of 600-kDa, but not 150-kDa, complexes depends on dimerization mediated by the EcR(LBD). Deletion of helix 12 is without effect. Mutation of K497 in helix 4, known to be essential for comodulator binding, abolishes 600-KDa complexes, but does not interfere with the formation of 150-kDa complexes. In contrast, the DE-domains of USP fused to Gal4(DBD) elute as monomer after elimination of the dimerization capacity of the ligand-binding domains by mutation of P463 in helix 10. The data presented here reveal that the complex formation of ligand-binding domains EcR and USP ligand is different.

Edysteroid receptor (EcR) shows marked differences in temporal patterns between tissues during larval-adult development in Rhodnius prolixus: correlations with haemolymph ecdysteroid titres

The presence of ecdysteroid receptor (EcR) in various tissues was studied throughout larval-adult development of the blood-sucking bug, Rhodnius prolixus, using an antibody to EcR that recognizes all isoforms. On Western blots, the antibody recognizes three peptides of approximate molecular masses of 70, 68 and 64 kDa, from epidermis and fat body of developing larvae, which contain high levels of haemolymph ecdysteroids. These peptides are absent from both unfed larvae and adults, which are devoid of ecdysteroids. In vitro treatment of epidermis and fat body from unfed larvae with 20E induces the appearance of all three EcR immunoreactive peptides. The stage-specific appearance and 20E inducibility of the peptides implies that they represent the native EcR(s) of Rhodnius. Confocal fluorescence analysis using this antibody revealed a great diversity of temporal profiles of EcR in various tissues during development. Developmental profiles of EcR were examined in abdominal epidermis, fat body, spermatocytes, brain (including the medial neurosecretory cells), prothoracic glands (PGs), rectal epithelium and Malpighian tubules. EcR fluorescence was confined to the nuclei in close association with chromatin. EcR was absent from tissues of unfed larvae or adults, supporting the results from Western blots. Different tissues develop EcR at different developmental times and in the presence of radically different concentrations of haemolymph ecdysteroids, retain EcR for different lengths of time and lose EcR at different concentrations of ecdysteroids. These results suggest that each tissue possesses a distinctive response mechanism to ecdysteroids. An exception to this, are the PGs, which exhibited no EcR fluorescence at any time during development.

Transcription of a lepidopteran cytochrome P450 promoter is modulated by multiple elements in its 5' UTR and repressed by 20-hydroxyecdysone

The biochemical response to the phytochemical xanthotoxin encountered in the diet of black swallowtail larvae is the induction of P450s capable of detoxifying this and other toxic furanocoumarins. As the xenobiotic response element to xanthotoxin (XRE-xan) is necessary but not sufficient for transcription of the CYP6B1v3 gene in Sf9 cells, sequences upstream of it, such as a putative EcRE, and downstream of it, such as a putative C/EBP binding site and Inr, have been tested for their roles in regulation. Mutation of the putative EcRE has indicated that it affects basal transcription of this promoter but not repression by 20-hydroxyecdysone. Mutation of the more proximal promoter sequence, including the C/EBP and Inr, have indicated that many core promoter elements between the TATA box and translation start site modulate basal and xanthotoxin-inducible expression of this composite promoter.

Effects of ecdysone agonists on the expression of EcR, USP and other specific proteins in the ovaries of the codling moth (Cydia pomonella L.)

Tebufenozide and methoxyfenozide have been previously shown to significantly reduce fecundity and cause vitellogenin accumulation in hemolymph of the codling moth Cydia pomonella L. In the present study, the effects of these ecdysone agonists, tebufenozide and methoxyfenozide, on the expression of ecdysone receptor (EcR), ultraspiracle protein (USP) and other proteins in the ovaries of C. pomonella L., were investigated at both the translational and/or transcriptional levels with an aim to elucidate the mechanisms by which the fecundity was reduced. Western and Northern blot analyses revealed that the expression of a 65 kDa (p65) EcR, and 60 and 64 kDa (p60 and p64) USP proteins were enhanced by tebufenozide and methoxyfenozide at both transcriptional and translational levels. Northern blot analysis indicated that the p65 EcR protein is encoded by EcRB1 transcript and that the p60 and p64 USP bands were the products of USP-1 transcript. Immunoprecipitation assays demonstrated that both the p60 and p64 USP coprecipitated with the p65 EcR and that p64 was a dominant USP to form complex with EcR. In addition, several other specific proteins were also identified and their expressions affected by the agonists. The data suggest that the ecdysone agonists regulate, via the EcR/USP complex, the expression of these specific proteins that might eventually lead to the inhibition of fecundity in the codling moth.

Effect of ecdysone agonists on vitellogenesis and the expression of EcR and USP in codling moth (Cydia pomonella)

The effects of tebufenozide and methoxyfenozide on vitellogenin (Vg) synthesis/release in the fat body, translocation in hemolymph, uptake by the ovary, and the expression of the ecdysone receptor (EcR) and its heterodimer partner, ultraspiracle protein (USP) in fat body, were investigated in Cydia pomonella. The results indicated that both ecdysone agonists significantly increased the Vg level in the adult hemolymph when the moths were exposed to agonist-treated surfaces. However, these agonists did not affect Vg release from the fat body nor Vg deposition in the first batch oocytes. Western blot analysis revealed that the expression of EcR and USP was significantly increased in tebufenozide- and methoxyfenozide-treated samples compared to the control, suggesting that ecdysone agonists regulated the Vg synthesis via the EcR and USP proteins complex.

Characterization of Arabidopsis thaliana AtFKBP42 that is membrane-bound and interacts with Hsp90

The twisted dwarf1 (twd1) mutant from Arabidopsis thaliana was identified in a screen for plant architecture mutants. The TWD1 gene encodes a 42 kDa FK506-binding protein (AtFKBP42) that possesses similarity to multidomain PPIases such as mammalian FKBP51 and FKBP52, which are known to be components of mammalian steroid hormone receptor complexes. We report here for the first time the stoichiometry and dissociation constant of a protein complex from Arabidopsis that consists of AtHsp90 and AtFKBP42. Recombinant AtFKBP42 prevents aggregation of citrate synthase in almost equimolar concentrations, and can be cross-linked to calmodulin. In comparison to one active and one inactive FKBP domain in FKBP52, AtFKBP42 lacks the PPIase active FKBP domain. While FKBP52 is found in the cytosol and translocates to the nucleus, AtFKBP42 was predicted to be membrane-localized, as shown by electron microscopy.

Expression of ecdysteroid receptor and ultraspiracle from Chironomus tentans (Insecta, Diptera) in E. coli and purification in a functional state

Full length clones of ecdysteroid receptor (EcR) and Ultraspiracle (USP) from Chironomus tentans were expressed as GST fusion proteins in E. coli and purified by affinity chromatography. The absence of detergents during the purification procedure is essential for retaining receptor function, especially ligand binding. Presence of USP is mandatory for ligand binding to EcR, but no other cofactors or posttranslational modifications seem to be important, since Scatchard plots revealed the same characteristics (two high affinity binding sites for Ponasterone A with K(D1)=0.24+/-0.1nM and K(D2)=3.9+/-1.3.nM) as found in 0.4 M NaCl extracts of Chironomus cells. Gel mobility shift assays showed binding of the heterodimer to PAL and DR5 even after removal of the GST-tag, whereas EcR binding to PAL1 is GST-dependent. USP binds preferentially to DR5. Addition of unprogrammed reticulocyte lysate improves ligand binding only slightly. Removal of GST has no effect on (3)H-ponasterone A binding, but alters DNA binding characteristics. Calculation of specific binding (5.3+3.0 nmol/mg GST EcR) revealed that 47+/-26% of purified receptor protein was able to bind ligand. The addition of purified EcR to cell extracts of hormone resistant subclones of the epithelial cell line from C. tentans, which have lost their ability to bind ligand, restores specific binding of (3)H-ponasterone A.

Control and biochemical nature of the ecdysteroidogenic pathway

Molting is elicited by a critical titer of ecdysteroids that includes the principal molting hormone, 20-hydroxyecdysone (20E), and ecdysone (E), which is the precursor of 20E but also has morphogenetic roles of its own. The prothoracic glands are the predominate source of ecdysteroids, and the rate of synthesis of these polyhydroxylated sterols is critical for molting and metamorphosis. This review concerns three aspects of ecdysteroidogenesis: (a) how the brain neuropeptide prothoracicotropic hormone (PTTH) initiates a transductory cascade in cells of the prothoracic gland, which results in an increased rate of ecdysteroid biosynthesis (upregulation); (b) how the concentrations of 20E in the hemolymph feed back on the prothoracic gland to decrease rates of ecdysteroidogenesis (downregulation); and (c) how the prothoracic gland cells convert cholesterol to the precursor of E and then 20E, a series of reactions only now being understood because of the use of a combination of classical biochemistry and molecular genetics.

Receptor accessory folding helper enzymes: the functional role of peptidyl prolyl cis/trans isomerases

Receptor accessory peptidyl prolyl cis/trans isomerases (PPIases) of the FKBP and cyclophilin types form receptor heterocomplexes with different stabilities. PPIases have been found to associate with other receptor heterocomplex constituents via either proline-directed active sites or additional domains of the enzymes. The single-domain PPIases FKBP12 and FKBP12.6 are shown to interact with receptor protein kinases and calcium channels at their active sites. In contrast, heterooligomeric nuclear receptors contain multi-domain PPIases like FKBP51, FKBP52 or cyclophilin 40 that directly interact with the chaperone hsp90 via the tetratricopeptide repeat modules of the folding helper enzymes. PPIases play a critical role in the functional arrangement of components within receptor heterocomplexes.

Phosphorylation of the insect immunophilin FKBP46 by the Spodoptera frugiperda homolog of casein kinase II

Immunophilins are a family of conserved proteins found in both prokaryotes and eukaryotes, that exhibit peptidylprolyl isomerase (PPIase) activity. Members of this family bind to immunosuppressive drugs and on this basis are divided into two classes: FKBPs bind to FK506 and rapamycin, while cyclophilins bind to cyclosporin A. In this paper, we report on insect immunophilin FKBP46 and its associated kinase. The insect FKBP46 belongs to the high-molecular-weight immunophilins and shares many characteristic features with its mammalian counterparts, but its functional role remains unclear. Here, we show that FKBP46 is phosphorylated by a protein kinase present in the nucleus of both insect Spodoptera frugiperda (Sf9) and human Jurkat cells. This protein kinase is immunoreactive with polyclonal antiserum raised against Drosophila melanogaster casein kinase II (CKII). We have cloned, overexpressed and characterized a new member of the CKII family derived from Spodoptera frugiperda cells. Recombinant Sf9 CKII alpha subunit shares 75% identity to human, chicken and Drosophila melanogaster homologs, whereas the Sf9 CKII beta subunit is 77% identical to rat, chicken and human. Moreover, we demonstrate that the insect immunophilin FKBP46 can be phosphorylated by human and Sf9 casein kinase II. Finally, we show that FKBP46 interacts with DNA, and this interaction is not prevented by phosphorylation.

FK506 and the role of the immunophilin FKBP-52 in nerve regeneration

In summary, FKBP-12 does not mediate the neurite outgrowth-promoting properties of neuroimmunophilin ligands (e.g., FK506). Instead, the neurotrophic properties of neuroimmunophilin ligands (FK506) and steroid hormones are mediated by disruption of steroid-receptor complexes. It remains unclear which component mediates neurite outgrowth, although the most likely candidates are FKBP-52, hsp-90, and p23 [42]. Regardless of the underlying mechanism involved, the FKBP-52 antibody data reveal that it should be possible to design, based on the structure of FK506, non-FKBP-12-binding (nonimmunosuppressant) compounds selective for FKBP-52 and test these new libraries for their ability to augment nerve regeneration. It may also be possible to exploit the structure of geldanamycin to develop a new class of hsp-90-binding compounds for use in nerve regeneration.

DNA-binding properties of the ecdysteroid receptor-complex (EcR/USP) of the epithelial cell line from Chironomus tentans

DNA-binding features of EcR and USP were investigated using a 0.4 M NaCl extract of the epithelial cell line of Chironomus tentans by means of electrophoretic mobility shift assays (EMSAs). It is shown that the DNA-binding is enhanced by hormone administration and that in the hormone dependent shift, both EcR and USP, are present. Furthermore, we demonstrate that under these conditions, EcR/USP form a unique complex on inverted repeat elements (PAL1 and hsp27-EcRE), while on direct repeat elements (DR1-5), a second complex with higher mobility is formed. In this second complex, neither EcR nor USP are present. Thus, an additional difference between PAL1 and DR-elements is the competition of other factors for DR-elements, modulating its function as an EcRE. A competition EMSA, using PAL1 as radiolabeled probe, reveals the following order of binding strength: PAL1>DR4/5>DR1>DR2/3/hsp27. Surprisingly, using DR1 as radiolabeled probe, shows a different order of binding strength: DR1>DR2>DR3/4/5/PAL1>hsp27. This indicates that the complexes formed on PAL1 are not identical to the ones formed on DR1 and that both are not easily convertible. Furthermore, the affinity of the EcR/USP complex may be altered under various conditions or by interaction with cofactors. Upon hormone administration, DNA binding of the receptor complex is enhanced, but the difference to hormone-free binding reactions decreases in course of time, indicating an additional hormone independent activation. Arch.

Alterations in ultraspiracle (USP) content and phosphorylation state accompany feedback regulation of ecdysone synthesis in the insect prothoracic gland

Insect molting and metamorphosis are elicited by a class of ecdysteroids, mainly 20-hydroxyecdysone (20E), the precursor of which is synthesized in the prothoracic gland. 20E acts via the ecdysone receptor (EcR) and its heterodimer partner ultraspiracle (USP). Analysis of the prothoracic gland of Manduca sexta revealed that the developmental expression and phosphorylation of a specific USP form, p47, is positively correlated with ecdysteroidogenesis and that 20E, but not ecdysone, is responsible for initiating the translational expression and phosphorylation of p47. The latter forms a functional complex with EcR and the ligand-complex interaction results in the down regulation of ecdysteroidogenesis and the inhibition of prothoracicotropic hormone (PTTH)-stimulated ecdysteroidogenesis. The composite data suggest that USP plays a key role in modulating PTTH-stimulated ecdysteroid biosynthesis through the selective expression and phosphorylation of the p47 USP isoform.

Control of ecdysteroidogenesis: activation and inhibition of prothoracic gland activity

The ecdysteroid hormones, mainly 20-hydroxyecdysone (20E), play a pivotal role in insect development by controlling gene expression involved in molting and metamorphosis. In the model insect Manduca sexta the production of ecdysteroids by the prothoracic gland is acutely controlled by a brain neurohormone, prothoracicotropic hormone (PTTH). PTTH initiates a cascade of events that progresses from the influx of Ca2+ and cAMP generation through phosphorylation of the ribosomal protein S6 and S6-dependent protein synthesis, and concludes with an increase in the synthesis and export of ecdysteroids from the gland. Recent studies indicate that S6 phosphorylation probably controls the steroidogenic effect of PTTH by gating the translation of selected mRNAs whose protein products are required for increased ecdysteroid synthesis. Inhibition of S6 phosphorylation prevents an increase in PTTH-stimulated protein synthesis and subsequent ecdysteroid synthesis. Two of the proteins whose translations are specifically stimulated by PTTH have been identified, one being a beta tubulin and the other a heat shock protein 70 family member. Current data suggest that these two proteins could be involved in supporting microtubule-dependent protein synthesis and ecdysone receptor assembly and/or function. Recent data also indicate that the 20E produced by the prothoracic gland feeds back upon the gland by increasing expression and phosphorylation of a specific USP isoform that is a constituent of the functional ecdysone receptor. Changes in the concentration and composition of the ecdysone receptor complex of the prothoracic gland could modulate the gland's potential for ecdysteroid synthesis (e.g. feedback inhibition) by controlling the levels of enzymes or other proteins in the ecdysteroid biosynthetic pathway.