Plasticity of tetramer formation by retinoid X receptors. An alternative paradigm for DNA recognition

Retinoid X Receptors Intersect the Molecular Clockwork in the Regulation of Liver Metabolism

Liver metabolic pathways are driven by the biological clock, and appropriate timing of 24-h patterns of metabolic gene expression as well as anabolic/catabolic processes with wake-related activity/feeding and sleep-related resting/fasting cycles preserves hepatic healthiness. The interplay among the liver metabolic pathways and the molecular clockwork is geared by the nuclear receptors, and ligand-dependent transcription factors that gauge the cellular nutritional status and redox balance, bind hormones and metabolites, and modulate the transcription of thousands target genes through their DNA-binding domain. Several nuclear receptors in the liver oscillate with circadian rhythmicity, and among these, the retinoid X receptors play a key role in metabolism regulation, intersecting with the cogs of the molecular clockwork.

Aberrant corepressor interactions implicated in PML-RAR(alpha) and PLZF-RAR(alpha) leukemogenesis reflect an altered recruitment and release of specific NCoR and SMRT splice variants

Human acute promyelocytic leukemia is causally linked to chromosomal translocations that generate chimeric retinoic acid receptor-α proteins (x-RARα fusions). Wild-type RARα is a transcription factor that binds to the SMRT/NCoR family of corepressors in the absence of hormone but releases from corepressor and binds coactivators in response to retinoic acid. In contrast, the x-RARα fusions are impaired for corepressor release and operate in acute promyelocytic leukemia as dominant-negative inhibitors of wild-type RARα. We report that the two most common x-RARα fusions, PML-RARα and PLZF-RARα, have gained the ability to recognize specific splice variants of SMRT and NCoR that are poorly recognized by RARα. These differences in corepressor specificity between the normal and oncogenic receptors are further magnified in the presence of a retinoid X receptor heteromeric partner. The ability of retinoids to fully release corepressor from PML-RARα differs for the different splice variants, a phenomenon relevant to the requirement for supraphysiological levels of this hormone in differentiation therapy of leukemic cells. We propose that this shift in the specificity of the x-RARα fusions to a novel repertoire of corepressors contributes to the dominant-negative and oncogenic properties of these oncoproteins and helps explain previously paradoxical aspects of their behavior.

Crystal Structure of a Novel Tetrameric Complex of Agonist-bound Ligand-binding Domain of Biomphalaria glabrata Retinoid X Receptor

Nuclear receptors form an important class of transcription regulators in metazoans. To learn more about the evolution of these proteins, we have initiated structural studies on nuclear receptor ligand-binding domains from various animals. Here we present the crystal structure of the ligand-binding domain (LBD) of the retinoid X receptor (RXR) from the mollusc Biomphalaria glabrata. The structure reveals a novel tetrameric association in which each monomer is complexed to the human RXR ligand 9-cis retinoic acid and to a human co-activator-derived peptide. The ligand and the co-activator peptide are bound in essentially the same manner as observed in previously reported human RXR LBD structures, suggesting that the mechanisms of RXR-mediated transcription regulation are very similar in mollusc and human. The structure shows further that binding of ligand and co-activator peptide does not necessarily lead to the typical holo-conformation in which helix 12 (H12) folds back and packs against the LBD. Within a canonical dimer, only one monomer is in this closed agonist conformation. The other monomer is in an open conformation with H12 protruding from the LBD core, occupying the H12 interaction groove of another open monomer in an adjacent dimer in a domain swapping fashion, thus resulting in a tetrameric association. Additional tetramer interfaces are formed between H11 of the closed LBD and H6 of the open LBD. This novel holo-tetramer configuration may have a biological role in activating genes whose promoters are poorly recognised by dimers but much more efficiently by the corresponding tetramers.

Transcriptional activities of retinoic acid receptors

Vitamin A derivatives plays a crucial role in embryonic development, as demonstrated by the teratogenic effect of either an excess or a deficiency in vitamin A. Retinoid effects extend however beyond embryonic development, and tissue homeostasis, lipid metabolism, cellular differentiation and proliferation are in part controlled through the retinoid signaling pathway. Retinoids are also therapeutically effective in the treatment of skin diseases (acne, psoriasis and photoaging) and of some cancers. Most of these effects are the consequences of retinoic acid receptors activation, which triggers transcriptional events leading either to transcriptional activation or repression of retinoid-controlled genes. Synthetic molecules are able to mimic part of the biological effects of the natural retinoic acid receptors, all-trans retinoic acid. Therefore, retinoic acid receptors are considered as highly valuable therapeutic targets and limiting unwanted secondary effects due to retinoid treatment requires a molecular knowledge of retinoic acid receptors biology. In this review, we will examine experimental evidence which provide a molecular basis for the pleiotropic effects of retinoids, and emphasize the crucial roles of coregulators of retinoic acid receptors, providing a conceptual framework to identify novel therapeutic targets.

Novel mode of deoxyribonucleic acid recognition by thyroid hormone receptors: thyroid hormone receptor beta-isoforms can bind as trimers to natural response elements comprised of reiterated half-sites

Thyroid hormone receptors (TRs) regulate gene expression by binding to specific DNA sequences, denoted thyroid hormone response elements (TREs). The accepted paradigm for TRs proposes that they bind as homo- or heterodimers to TREs comprised of two AGGTCA half-site sequences. In the prototypic TRE, these half-sites are arranged as direct repeats separated by a four-base spacer. This dimeric model of TR binding, derived from analysis of artificial DNA sequences, fails to explain why many natural TREs contain more than two half-sites. Therefore, we investigated the ability of different TR isoforms to bind to TREs possessing three or more half-sites. We report that the TRbeta isoforms (TRbeta0, TRbeta1, TRbeta2), but not TRalpha1, can bind to reiterated DNA elements, such as the rat GH-TRE, as complexes trimeric or greater in size. The TRbeta0 isoform, in particular, formed homo- and heterotrimers (with the retinoid X receptor) with high efficiency and cooperativity, and TRbeta0 preferentially used reporters containing these reiterated elements to drive gene expression in vivo. Our data demonstrate that TRbeta isoforms can form multimeric receptor complexes on appropriately reiterated DNA response elements, providing a functional distinction between the TR isoforms and an explanation for TREs possessing three or more half-sites.

Probing protein oligomerization in living cells with fluorescence fluctuation spectroscopy

Fluorescence fluctuation spectroscopy provides information about protein interactions in the intercellular environment from naturally occurring equilibrium fluctuations. We determine the molecular brightness of fluorescent proteins from the fluctuations by analyzing the photon counting histogram (PCH) or its moments and demonstrate the use of molecular brightness in probing the oligomerization state of proteins. We report fluorescence fluctuation measurements of enhanced GFP (EGFP) in cells up to concentrations of 10 microM by using an improved PCH theory. The molecular brightness of EGFP is constant in the concentration range studied. The brightness of a tandem EGFP construct, which carries two fluorophores, increases by a factor of two compared with EGFP alone, demonstrating the sensitivity of molecular brightness as a probe for protein complex formation. Oligomerization of nuclear receptors plays a crucial role in the regulation of gene expression. We probe the oligomerization state of the testicular receptor 4 and the ligand-binding domains of retinoid X receptor and retinoic acid receptor by observing molecular brightness changes as a function of protein concentration. The large concentration range accessible by experiment allows us to perform titration experiments on EGFP fusion proteins. An increase in the molecular brightness with protein concentration indicates the formation of homocomplexes. We observe the formation of homodimers of retinoid X receptor ligand binding domain upon addition of ligand. Resolving protein interactions in a cell is an important step in understanding cellular function on a molecular level. Brightness analysis promises to develop into an important tool for determining protein complex formation in cells.

Retinoid receptors and their coregulators

Retinoids regulate gene transcription by binding to the nuclear receptors, the retinoic acid (RA) receptors (RARs), and the retinoid X receptors (RXRs). RARs and RXRs are ligand-activated transcription factors for the regulation of RA-responsive genes. The actions of RARs and RXRs on gene transcription require a highly coordinated interaction with a large number of coactivators and corepressors. This review focuses on our current understanding of these coregulators known to act in concert with RARs and RXRs. The mechanisms of action of these coregulators are beginning to be uncovered and include the modification of chromatin and the recruitment of basal transcription factors. Challenges remain to understand the specificity of action of RARs and RXRs and the formation of specific transcription complexes consisting of the receptors, coregulators, and other unknown factors.

Low resolution structures of the retinoid X receptor DNA-binding and ligand-binding domains revealed by synchrotron X-ray solution scattering

Nuclear receptors are ligand-inducible transcription factors that share structurally related DNA-binding (DBD) and ligand-binding (LBD) domains. Biochemical and structural studies have revealed the modular nature of DBD and LBD. Nevertheless, the domains function in concert in vivo. While high-resolution crystal structures of nuclear receptor DBDs and LBDs are available, there are no x-ray structural studies of nuclear receptor proteins containing multiple domains. We report the solution structures of the human retinoid X receptor DBD-LBD (hRXRalphaDeltaAB) region. We obtained ab initio shapes of hRXRalphaDeltaAB dimer and tetramer to 3.3 and 1.7 nm resolutions, respectively, and established the position and orientation of the DBD and LBD by fitting atomic coordinates of hRXRalpha DBD and LBD. The dimer is U-shaped with DBDs spaced at approximately 2 nm in a head to head orientation forming an angle of about 10 degrees with respect to each other and with an extensive interface area provided by the LBD. The tetramer is a more elongated X-shaped molecule formed by two dimers in head to head arrangement in which the DBDs are extended from the structure and spaced at about 6 nm. The close proximity of DBDs in dimers may facilitate homodimer formation on DNA; however, for the homodimer to bind to a DNA element containing two directly repeated half-sites, one of the DBDs would need to rotate with respect to the other element. By contrast, the separation of DBDs in the tetramers may account for their decreased ability to recognize DNA.

Requirements for heterodimerization between the orphan nuclear receptor Nurr1 and retinoid X receptors

The nuclear receptor nurr1 is a transcription factor involved in the development and maintenance of neurons synthesizing the neurotransmitter dopamine. Although the lack of nurr1 expression has dramatic consequences for these cells either in terms of differentiation or survival, the mechanisms by which nurr1 controls gene transcription still remain unclear. In the intent to understand better the modalities of action of this nuclear receptor, we have undertaken a systematic analysis of the transcriptional effects and DNA binding properties of nurr1 as a monomer or when forming dimers with the different isotypes of the retinoic X receptor (RXR). Here, we show that nurr1 acts as a gene activator independently of RXR and through an AF2-independent mechanism. In addition, heterodimerization with RXR is isotype-specific, involves multiple domains in the C-terminal region of nurr1, and requires RXR binding to DNA. RXR(alpha)-nurr1 and RXRgamma-nurr1 heterodimers bind direct repeat response elements and display no specific requirements with respect to half-site spacing. However, the retinoid responsiveness of DNA-bound heterodimers requires the reiteration of at least three nurr1 binding sites, thereby limiting retinoid-induced nurr1 transcriptional activity to specific direct response elements.

Purification and characterization of a luxO promoter binding protein LuxT from Vibrio harveyi

Bioluminescence in the marine bacterium Vibrio harveyi is cell density dependent and is regulated by small molecules (autoinducers) excreted by the bacteria. The autoinducer signals are relayed to a central regulator, LuxO, which acts in its phosphorylated form as a repressor of the lux operon at the early stages of cell growth. We report in these studies the purification to homogeneity of a luxO DNA binding protein (LuxT) from V. harveyi after five major chromatography steps, including a highly effective DNA affinity chromatography step and reverse-phase HPLC. Regeneration of binding activity was accomplished after HPLC and SDS-PAGE by renaturation of LuxT from guanidine hydrochloride. It was also demonstrated that the functional LuxT was a dimer of 17 kDa that bound tightly (K(d) = 2 nM) to the luxO promoter. The sequences of three tryptic peptides obtained on digestion of the purified protein did not match any sequences in the Protein Data Bank, indicating that LuxT is a new V. harveyi lux regulatory protein.

Structural basis for autorepression of retinoid X receptor by tetramer formation and the AF-2 helix

The 9-cis-retinoic acid receptors (RXRalpha, RXRbeta, and RXRgamma) are nuclear receptors that play key roles in multiple hormone-signaling pathways. Biochemical data indicate that, in the absence of ligand, RXR can exist as an inactive tetramer and that its dissociation, induced by ligand, is important for receptor activation. In this article we report the inactivated tetramer structures of the RXRalpha ligand-binding domain (LBD), either in the absence of or in the presence of a nonactivating ligand. These structures reveal that the RXR LBD tetramer forms a compact, disc-shaped complex, consisting of two symmetric dimers that are packed along helices 3 and 11. In each monomer, the AF-2 helix protrudes away from the core domain and spans into the coactivator binding site in the adjacent monomer of the symmetric dimer. In this configuration, the AF-2 helix physically excludes the binding of coactivators and suggests an autorepression mechanism that is mediated by the AF-2 helix within the tetramer. The RXR-tetramer interface is assembled from amino acids that are conserved across several closely related receptors, including the HNF4s and COUP transcription factors, and may therefore provide a model for understanding structure and regulation of this subfamily of nuclear receptors.

Plasmid copy-number control and better-than-random segregation genes of pSM19035 share a common regulator

Transcription initiation of the copy-number control and better-than-random segregation genes of the broad-host-range and low-copy-number plasmid pSM19035 are subjected to repression by the autoregulated pSM19035-encoded omega product in Bacillus subtilis cells. The promoters of the copS (Pcop1 and Pcop2), delta (Pdelta), and omega (Pomega) genes have been mapped. These promoters are embedded in a set of either seven copies of a 7-bp direct repeat or in a block consisting of two 7-bp direct repeats and one 7-bp inverted repeat; the blocks are present either two or three times. The cooperative binding of omega protein to the repeats on the Pcop1, Pcop2, Pdelta, and Pomega promoters represses transcription initiation by a mechanism that does not exclude sigma(A)RNAP from the promoters. These results indicate that omega protein regulates plasmid maintenance by controlling the copy number on the one hand and by regulating the amount of proteins required for better-than-random segregation on the other hand.

Role of estrogen receptor ligand and estrogen response element sequence on interaction with chicken ovalbumin upstream promoter transcription factor (COUP-TF)

Estrogen-responsive genes are regulated by altering the balance of estrogen receptor (ER) interaction with transcription activators and inhibitors. Here we examined the role of ER ligand on ER interaction with the Chicken Ovalbumin Upstream Promoter Transcription Factor (COUP-TF) orphan nuclear receptor. COUP-TF binding to half-site estrogen response elements (EREs) was increased by the addition of estradiol (E2) -liganded ER (E2-ER), but not by ER liganded with the antiestrogens 4-hydroxytamoxifen (4-OHT-ER) or tamoxifen aziridine (TAz-ER). ER did not bind to single half-sites. Conversely, COUP-TF enhanced the ERE binding of purified E2-ER, but did not affect TAz-ER-ERE binding. In contrast, only antiestrogens enhanced direct interaction between ER and COUP-TF as assessed by GST pull-down assays. Identical results were obtained using either purified bovine or recombinant human ERalpha. Co-immunoprecipitation assays showed that ER and COUP-TF interact in extracts from MCF-7 and ERalpha-transfected MDA-MB-231 cells. Here we document that ER ligand impacts COUP-TF-ER interaction. COUP-TF interaction is mediated by the DNA binding and ligand-binding domains of ER. We suggest that changes in ER conformation induced by DNA binding reduce ER-COUP-TF interaction. Transient transfection of human MCF-7 breast cancer cells with a COUP-TFI expression vector repressed E2-induced luciferase reporter gene expression from single or multiple tandem copies of a consensus ERE. COUP-TFI stimulated 4-OHT-induced luciferase activity from a minimal ERE. Alone, COUP-TFI increased transcription from ERE half-sites or a single ERE in a sequence-dependent manner. These data provide evidence that the ERE sequence and its immediate flanking regions influence whether COUP-TF enhances, inhibits, or has no effect on ER ligand-induced ERE reporter gene expression and that COUP-TFI activates gene transcription from ERE half-sites. We suggest that COUP-TFI plays a role in mitigating estrogen-responsive gene expression.

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.

Binding of the fur (ferric uptake regulator) repressor of Escherichia coli to arrays of the GATAAT sequence

The mode of DNA binding of the Fur (ferric uptake regulator) repressor which controls transcription of iron-responsive genes in Escherichia coli, has been re-examined. Using as a reference the known sites at the promoter of the aerobactin operon of Escherichia coli, we have compared in detail the patterns of interaction between the purified Fur protein and natural or synthetic DNA targets. DNase I and hydroxyl radical footprinting, as well as missing-T assays, consistently revealed that functional Fur sites are composed of a minimum of three repeats of the hexameric motif GATAAT rather than by a palindromic 19 bp target sequence. Extended binding sites, constructed by stepwise addition of one or two direct repeats of the same sequence, were occupied co-operatively by Fur with the same pattern of interactions as those observed with the core of three repeats. This indicated that functional sites with a range of affinities can be formed by the addition of discrete GATAAT extensions to a minimal recognition sequence. The fashion in which Fur binds its target, virtually unknown in prokaryotic transcriptional regulators, accounts for the observed helical wrapping of the protein around the DNA helix.

Transcriptional silencing is defined by isoform- and heterodimer-specific interactions between nuclear hormone receptors and corepressors

Nuclear hormone receptors are ligand-regulated transcription factors that play critical roles in metazoan homeostasis, development, and reproduction. Many nuclear hormone receptors exhibit bimodal transcriptional properties and can either repress or activate the expression of a given target gene. Repression appears to require a physical interaction between a receptor and a corepressor complex containing the SMRT/TRAC or N-CoR/RIP13 polypeptides. We wished to better elucidate the rules governing the association of receptors with corepressors. We report here that different receptors interact with different domains in the SMRT and N-CoR corepressors and that these divergent interactions may therefore contribute to distinct repression phenotypes. Intriguingly, different isoforms of a single nuclear hormone receptor class also differ markedly in their interactions with corepressors, indicative of their nonidentical actions in cellular regulation. Finally, we present evidence that combinatorial interactions between different receptors can, through the formation of heterodimeric receptors, result in novel receptor-corepressor interactions not observed for homomeric receptors.

Phosphorylation of thyroid hormone receptors by protein kinase A regulates DNA recognition by specific inhibition of receptor monomer binding

Thyroid hormone receptor (T3R) alpha-1 and its oncogenic derivative, the v-ERB A protein, are phosphorylated by cAMP-dependent protein kinase A. Although this phosphorylation appears to be necessary for the oncogenic properties of v-ERB A, the mechanism by which phosphorylation influences the functions of v-ERB A and of the normal T3R has not been established. The protein kinase A phosphorylation site in T3Ralpha-1 is within a domain that is known to contribute to the DNA recognition properties of these receptors. We therefore analyzed the effects of protein kinase A phosphorylation on DNA recognition by the normal T3Ralpha and by the v-ERB A oncoprotein. We report here that phosphorylation of these receptor derivatives does not significantly alter the overall affinity of receptor dimers for DNA. However, phosphorylation does notably alter DNA recognition by preventing, or greatly inhibiting, the ability of these receptors to bind to DNA as protein monomers. These studies suggest that the phosphorylation of T3Ralpha-1 and v-ERB A by protein kinase A may provide a means of altering promoter recognition through a post-translational modification.

Chicken ovalbumin upstream promoter-transcription factor interacts with estrogen receptor, binds to estrogen response elements and half-sites, and inhibits estrogen-induced gene expression

Chicken ovalbumin upstream promoter-transcription factor (COUP-TF) was identified as a low abundance protein in bovine uterus that co-purified with estrogen receptor (ER) in a ligand-independent manner and was separated from the ER by its lower retention on estrogen response element (ERE)-Sepharose. In gel mobility shift assays, COUP-TF bound as an apparent dimer to ERE and ERE half-sites. COUP-TF bound to an ERE half-site with high affinity, Kd = 1.24 nM. In contrast, ER did not bind a single ERE half-site. None of the class II nuclear receptors analyzed, i.e. retinoic acid receptor, retinoid X receptor, thyroid receptor, peroxisome proliferator-activated receptor, or vitamin D receptor, were constituents of the COUP-TF.DNA binding complex detected in gel mobility shift assays. Direct interaction of COUP-TF with ER was indicated by GST "pull-down" and co-immunoprecipitation assays. The nature of the ER ligand influenced COUP-TF-ERE half-site binding. When ER was liganded by the antiestrogen 4-hydroxytamoxifen (4-OHT), COUP-TF-half-site interaction decreased. Conversely, COUP-TF transcribed and translated in vitro enhanced the ERE binding of purified estradiol (E2)-liganded ER but not 4-OHT-liganded ER. Co-transfection of ER-expressing MCF-7 human breast cancer cells with an expression vector for COUP-TFI resulted in a dose-dependent inhibition of E2-induced expression of a luciferase reporter gene under the control of three tandem copies of EREc38. The ability of COUP-TF to bind specifically to EREs and half-sites, to interact with ER, and to inhibit E2-induced gene expression suggests COUP-TF regulates ER action by both direct DNA binding competition and through protein-protein interactions.

The tetramerization region of the retinoid X receptor is important for transcriptional activation by the receptor

The retinoid X receptor (RXR), a member of the superfamily of hormone nuclear receptors, is a ligand-inducible transcription factor that is activated by the vitamin A derivative 9-cis-retinoic acid. We previously showed that RXR self-associates into tetramers with a high affinity and that ligand binding induces rapid dissociation of receptor tetramers to smaller species. Here, the RXR region that is responsible for mediating tetramer formation is identified. It is shown that this interface, which we term the "tetramerization domain," critically contains two consecutive phenylalanine residues located at the C-terminal region of the receptor. Mutation of these residues is sufficient to disrupt RXR tetramers without affecting the overall fold of the protein or interfering with ligand binding, dimer formation, or DNA binding by the receptor. Nevertheless, the tetramer-impaired mutant was found to be transcriptionally defective. The newly characterized tetramerization domain and the previously identified main dimerization interface of RXR act autonomously to affect separate intersubunit interactions that, overall, lead to formation of tetramers. Protein-protein interactions mediated by the tetramerization domain, but not those that involve the dimerization interface, are disrupted following ligand binding by RXR. Overall, these data attest to the specificity of the interaction and implicate the tetramerization interface in playing a direct role in regulating transcriptional activation by RXR.

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.