Ligand-binding domain of estrogen receptors

Allosteric autoregulation of DNA binding via a DNA-mimicking protein domain: a biophysical study of ZNF410-DNA interaction using small angle X-ray scattering

ZNF410 is a highly-conserved transcription factor, remarkable in that it recognizes a 15-base pair DNA element but has just a single responsive target gene in mammalian erythroid cells. ZNF410 includes a tandem array of five zinc-fingers (ZFs), surrounded by uncharacterized N- and C-terminal regions. Unexpectedly, full-length ZNF410 has reduced DNA binding affinity, compared to that of the isolated DNA binding ZF array, both in vitro and in cells. AlphaFold predicts a partially-folded N-terminal subdomain that includes a 30-residue long helix, preceded by a hairpin loop rich in acidic (aspartate/glutamate) and serine/threonine residues. This hairpin loop is predicted by AlphaFold to lie against the DNA binding interface of the ZF array. In solution, ZNF410 is a monomer and binds to DNA with 1:1 stoichiometry. Surprisingly, the single best-fit model for the experimental small angle X-ray scattering profile, in the absence of DNA, is the original AlphaFold model with the N-terminal long-helix and the hairpin loop occupying the ZF DNA binding surface. For DNA binding, the hairpin loop presumably must be displaced. After combining biophysical, biochemical, bioinformatic and artificial intelligence-based AlphaFold analyses, we suggest that the hairpin loop mimics the structure and electrostatics of DNA, and provides an additional mechanism, supplementary to sequence specificity, of regulating ZNF410 DNA binding.

Estrogens and the regulation of glucose metabolism

The main estrogens: estradiol, estrone, and their acyl-esters have been studied essentially related to their classical estrogenic and pharmacologic functions. However, their main effect in the body is probably the sustained control of core energy metabolism. Estrogen nuclear and membrane receptors show an extraordinary flexibility in the modulation of metabolic responses, and largely explain gender and age differences in energy metabolism: part of these mechanisms is already sufficiently known to justify both. With regard to energy, the estrogen molecular species act essentially through four key functions: (1) Facilitation of insulin secretion and control of glucose availability; (2) Modulation of energy partition, favoring the use of lipid as the main energy substrate when more available than carbohydrates; (3) Functional protection through antioxidant mechanisms; and (4) Central effects (largely through neural modulation) on whole body energy management. Analyzing the different actions of estrone, estradiol and their acyl esters, a tentative classification based on structure/effects has been postulated. Either separately or as a group, estrogens provide a comprehensive explanation that not all their quite diverse actions are related solely to specific molecules. As a group, they constitute a powerful synergic action complex. In consequence, estrogens may be considered wardens of energy homeostasis.

Design and identification of a new farnesoid X receptor (FXR) partial agonist by computational structure-activity relationship analysis: Ligand-induced H8 helix fluctuation in the ligand-binding domain of FXR may lead to partial agonism

Farnesoid X receptor (FXR) controls gene-expression relevant to various diseases including nonalcoholic steatohepatitis and has become a drug target to regulate metabolic aberrations. However, some side effects of FXR agonists reported in clinical development such as an increase in blood cholesterol levels incentivize the development of partial agonists to minimize side effects. In this study, to identify a new partial agonist, we analyzed the computational structure-activity relationship (SAR) of FXR agonists previously developed in our laboratories using molecular dynamics simulations. SAR analysis showed that fluctuations in the H8 helix, by ligand binding, of the ligand-binding domain (LBD) of FXR may influence agonistic activity. Based on this observation, 6 was newly designed as a partial agonist and synthesized. As a result of biological evaluations, 6 showed weak agonistic activity (40.0% relative agonistic activity to the full-agonist GW4064) and a potent EC₅₀ value (55.5 nM). The successful identification of the new potent partial agonist 6 suggested that helix fluctuation in the LBD induced by ligands could be one way to develop partial agonists.

Generation of Inducible Transgenic Lines of Arabidopsis Transcription Factors Regulated by MicroRNAs

Transcription factors play key regulatory roles in all the life processes across kingdoms. In plants, the genome of a typical model species such as Arabidopsis thaliana encodes over 1500 transcription factors that regulate the expression dynamics of all the genes in time and space. Therefore, studying their function by analyzing the loss and gain-of-function lines is of prime importance in basic plant biology and its agricultural application. However, the current approach of knocking out genes often causes embryonic lethal phenotype, while inactivating one or two members of a redundant gene family yields little phenotypic changes, thereby making the functional analysis a technically challenging task. In such cases, inducible knock-down or overexpression of transcription factors appears to be a more effective approach. Restricting the transcription factors in the cytoplasm by fusing them with animal glucocorticoid/estrogen receptors (GR/ER) and then re-localizing them to the nucleus by external application of animal hormone analogues has been a useful method of gene function analysis in the model plants. In this chapter, we describe the recent advancements in the GR and ER expression systems and their use in analyzing the function of transcription factors in Arabidopsis.

E6-Associated Protein Dependent Estrogen Receptor Regulation of Protein Kinase A Regulatory Subunit R2A Expression in Neuroblastoma

E6ap is a known transcriptional coregulator for estrogen receptor alpha (Er, Erα) in the presence of estrogen. Protein kinase A (PKA) contains two regulatory subunits derived from four genes. Recent evidence demonstrates that PKA regulates E6ap activity. Data generated in our lab indicated estrogen dependent regulation of Pkar2a levels. Our project sets to investigate a possible feedback mechanism constituting of Erα and E6ap transcriptional regulation of Pkar2a expression. Western blot evaluated protein regulation correlations with E2 in mouse neuroblastoma lines. Bioinformatics detected estrogen response element (ERE) sequences. quantitative polymerase chain reaction (qPCR) validated the western blot results. ERE oligonucleotides were synthesized. Reporter gene transcriptional activity was evaluated via Luciferase assay output. Electromobility shift assay (EMSA) assessed direct binding between Erα relevant sequences. Chromatin immunoprecipitation (ChIP) and Re-ChIP were conducted in quantifying protein complex recruitment levels. Pkar2a protein expression directly correlated with E2, and four putative ERE sequences were identified. Pkar2a mRNA expression reverted to baseline with either E2 or E6ap absent. In the presence of E2, ERE-1 and ERE-4 possessed Luciferase reporter gene transcriptional capabilities. ERE-1 portrayed band shifts, representing direct binding to Erα with E2 supplementation. With E2, ERE-1 significantly enhanced Erα and E6ap recruitment levels to the Pkar2a promoter. Pkar2a is directly regulated by Erα and E6ap in the presence of estrogen stimulus. This work indicates a feedback mechanism in the interplay between PKA and E6ap, which may prove crucial for the role of both proteins in cancers and neurogenetic diseases like Angelman syndrome.

Binding mechanism of the farnesoid X receptor marine antagonist suvanine reveals a strategy to forestall drug modulation on nuclear receptors. Design, synthesis, and biological evaluation of novel ligands

Here, we report suvanine, a marine sponge sesterterpene, as an antagonist of the mammalian bile acid sensor farnesoid-X-receptor (FXR). Using suvanine as a template, we shed light on the molecular bases of FXR antagonism, identifying the essential conformational changes responsible for the transition from the agonist to the antagonist form. Molecular characterization of the nuclear corepressor NCoR and coactivator Src-1 revealed that receptor conformational changes are associated with a specific dynamic of recruitment of these cofactors to the promoter of OSTα, a FXR regulated gene. Using suvanine as a novel hit, a library of semisynthetic derivatives has been designed and prepared, leading to pharmacological profiles ranging from agonism to antagonism toward FXR. Deep pharmacological evaluation demonstrated that derivative 19 represents a new chemotype of FXR modulator, whereas alcohol 6, with a simplified molecular scaffold, exhibits excellent antagonistic activity.

A conserved surface on the ligand binding domain of nuclear receptors for allosteric control

Nuclear receptors (NRs) form a large superfamily of transcription factors that participate in virtually every key biological process. They control development, fertility, gametogenesis and are misregulated in many cancers. Their enormous functional plasticity as transcription factors relates in part to NR-mediated interactions with hundreds of coregulatory proteins upon ligand (e.g., hormone) binding to their ligand binding domains (LBD), or following covalent modification. Some coregulator association relates to the distinct residues that shape a coactivator binding pocket termed AF-2, a surface groove that primarily determines the preference and specificity of protein-protein interactions. However, the highly conserved AF-2 pocket in the NR superfamily appears to be insufficient to account for NR subtype specificity leading to fine transcriptional modulation in certain settings. Additional protein-protein interaction surfaces, most notably on their LBD, may contribute to modulating NR function. NR coregulators and chaperones, normally much larger than the NR itself, may also bind to such interfaces. In the case of the androgen receptor (AR) LBD surface, structural and functional data highlighted the presence of another site named BF-3, which lies at a distinct but topographically adjacent surface to AF-2. AR BF-3 is a hot spot for mutations involved in prostate cancer and androgen insensitivity syndromes, and some FDA-approved drugs bind at this site. Structural studies suggested an allosteric relationship between AF-2 and BF-3, as occupancy of the latter affected coactivator recruitment to AF-2. Physiological relevant partners of AR BF-3 have not been described as yet. The newly discovered site is highly conserved among the steroid receptors subclass, but is also present in other NRs. Several missense mutations in the BF-3 regions of these human NRs are implicated in pathology and affect their function in vitro. The fact that AR BF-3 pocket is a druggable site evidences its pharmacological potential. Compounds that may affect allosterically NR function by binding to BF-3 open promising avenues to develop type-specific NR modulators.

QSAR classification of estrogen receptor binders and pre-screening of potential pleiotropic EDCs

Endocrine disrupting chemicals (EDCs) are suspected of posing serious threats to human and wildlife health through a variety of mechanisms, these being mainly receptor-mediated modes of action. It is reported that some EDCs exhibit dual activities as estrogen receptor (ER) and androgen receptor (AR) binders. Indeed, such compounds can affect the normal endocrine system through a dual complex mechanism, so steps should be taken not only to identify them a priori from their chemical structure, but also to prioritize them for experimental tests in order to reduce and even forbid their usage. To date, very few EDCs with dual activities have been identified. The present research uses QSARs, to investigate what, so far, is the largest and most heterogeneous ER binder data set (combined METI and EDKB databases). New predictive classification models were derived using different modelling methods and a consensus approach, and these were used to virtually screen a large AR binder data set after strict validation. As a result, 46 AR antagonists were predicted from their chemical structure to also have potential ER binding activities, i.e. pleiotropic EDCs. In addition, 48 not yet recognized ER binders were in silico identified, which increases the number of potential EDCs that are substances of very high concern (SVHC) in REACH. Thus, the proposed screening models, based only on structure information, have the main aim to prioritize experimental tests for the highlighted compounds with potential estrogenic activities and also to design safer alternatives.

Nucleotide sequence, tissue expression patterns and phylogenetic analysis of estrogen receptor one mRNA in the Murray rainbowfish (Melanotaenia fluviatilis) (Atheriniformes, Actinopterygii)

Estrogens are steroidal hormones that control many physiological processes in both female and male vertebrates. Like other vertebrates, fish have two distinct estrogen receptors (Esr) subtypes, Esr1 and Esr2a that have been isolated in a number of species, as well as a third subtype, Esr2b. The mRNA encoding the Esr1 was isolated from the female liver of an Australian freshwater fish, the Murray rainbowfish, Melanotaenia fluviatilis. The rainbowfish esr1 cDNA was 2569 bp in length and with an open reading frame to encode a protein of 611 amino acids. Phylogenetic analysis and multiple amino acid sequence alignment indicated close relationship and high similarity with killifish (Fundulus heteroclitus) and gilthead sea bream (Sparus aurata). Expression of rainbowfish esr1 mRNA was abundant in the liver, gonads and intestine of adult female and male rainbowfish. This is the first isolation of the full-length nucleotide sequence of an estrogen receptor from rainbowfish. This sequence provides a valuable molecular tool that can be used in future studies investigating estrogen mechanisms, actions and tissue-specific expression in juvenile and adult rainbowfish.

Binary classification models for endocrine disrupter effects mediated through the estrogen receptor

Endocrine disrupters (EDs) form an interesting field of application attracting great attention in the recent years. They represent a number of exogenous substances interfering with the function of the endocrine system, including the interfering with developmental processes. In particular EDs are mentioned as substances requiring a more detailed control and specific authorization within REACH, the new European legislation on chemicals, together with other groups of chemicals of particular concern. QSAR represents a challenging method to approach data gap which is foreseen by REACH. The aim of this study was to provide an insight into the use of QSAR models to address ED effects mediated through the estrogen receptor (ER). New predictive models were derived to assess estrogenicity for a very large and heterogeneous dataset of chemical compounds. QSAR binary classifiers were developed based on different data mining techniques such as classification trees, decision forest, fuzzy logic, neural networks and support vector machines. The focus was given to multiple endpoints to better characterize the effects of EDs evaluating both binding (RBA) and transcriptional activity (RA). A possible combination of the models was also explored. A very good accuracy was reached for both RA and RBA models (higher than 80%).

4-(4-Alkylpiperazin-1-yl)phenyl group: A novel class of basic side chains for selective estrogen receptor modulators

In the structure-activity relationships of spiro[indene-1,1'-indane] series, the 4-(4-alkylpiperazin-1-yl)phenyl group was found to be functionally equipotent to the 4-(1-piperidinoethoxy)phenyl group, the most widely used basic side chain in selective estrogen receptor modulators.

Ligand-Induced Regulation of ERα and ERβ is Indicative of Human Breast Cancer Cell Proliferation

Two estrogen receptors (ER), ERalpha and ERbeta, are expressed in breast cancer but their role in treatment response is unclear. The overall objective of this study was to determine if the presence of ERbeta protein in breast cancer cell lines is an indicator of a poor prognosis based on cell proliferation. In addition, we determined the effect of estradiol (E2) and selective estrogen receptor modulators (SERMs), such as tamoxifen and genistein, on ERalpha and ERbeta protein regulation, to help in the understanding of the mechanism behind their role in modulating cell proliferation. Using western blot and immunofluorescence analysis, the ER positive cell lines, MCF-7 and T47D, were found to contain both ERalpha and ERbeta, and thus were used as model systems. E2 and genistein, which increased cell proliferation in both cell lines, induced an up regulation of ERbeta in both cell lines. This suggests that an estrogenic response in breast cancer cells is indicated by an increase in ERbeta expression. Tamoxifen decreased cell proliferation in both cell lines, while up regulating ERalpha in both cell lines, suggesting that antiestrogenic response is indicated by an increase in ERalpha expression. Although a change in the ERalpha/ERbeta ratio may play a role in the effect seen in cell proliferation, this study indicates that ERbeta is a poor prognosticator of cell proliferation in breast cancer and that ERalpha is a positive prognosticator of responsiveness to antiestrogen treatment.

Estrogen receptor alpha polymorphisms and renal cell carcinoma--a possible risk

Renal cell carcinoma is the most common form of kidney cancer. However, the genetic basis of renal cancer is not fully understood. Estrogens and their receptors (ERs) have been shown to play a role in various cancers and it is speculated that they can also affect the human kidney. One of the animal models utilized to study the effects of estrogens on renal cancer is the Syrian hamster. Exposing these hamsters to estrogens results in the development of kidney cancer and thus, the hormone-ER complex may be playing a role. The ER is expressed in reproductive as well as non-reproductive tissues and is implicated in the control of proliferation, differentiation, and development of many tissues. There are two types of ERs and they are the alpha and beta forms. Genetic polymorphisms of various factors have been shown to play a role in the alteration of their functions. The NH2-terminal region of the ERalpha protein influences its structure and function and thus, inherited variants of the ERalpha gene may alter tissue responsiveness to estrogens and possibly lead to renal carcinogenesis. Polymorphisms have been determined in the coding region of the human ERalpha gene and are located at the following codons: 10 T-->C, 85 G-->C, 87 G-->C, 243 C-->T, 325 C-->G, and 594 G-->A. There are also two polymorphisms that have been identified in intron 1 and give rise to a PvuII and XbaI restriction site. These polymorphisms of ERalpha have been shown to be associated with various cancers. Based on the evidence, it is hypothesized that polymorphisms of the ERalpha gene are associated with renal cell carcinoma.

Limited species differences in estrogen receptor alpha-medicated reporter gene transactivation by xenoestrogens

Estrogen receptors (ERs) play an important role in estrogen function. However, it is well known that there are species differences in amino acid sequences of the ligand binding domains. Here, we report on the analysis of species differences in ER-dependent transactivation with some chemicals using reporter gene assays. Full-length ER cDNAs from human, rat, chicken, alligator (Caiman), whiptail lizard, African clawed frog and rainbow trout were prepared from hepatic mRNA by the RT-PCR method and inserted into expression plasmids. Both expression and reporter plasmids were transiently transfected into HeLa cells, and then the estrogenic effects of chemicals were analyzed in terms of induction of luciferase activity. No species differences in transactivation were found among human, rat, chicken, alligator, whiptail lizard and African clawed frog ERs. However, thermo-dependent alteration in susceptibility to 17-beta-estradiol was observed with the rainbow trout ER because of thermo-dependence of estrogen binding.

The crystal structure of the quorum sensing protein TraR bound to its autoinducer and target DNA

The quorum sensing system allows bacteria to sense their cell density and initiate an altered pattern of gene expression after a sufficient quorum of cells has accumulated. In Agrobacterium tumefaciens, quorum sensing controls conjugal transfer of the tumour- inducing plasmid, responsible for plant crown gall disease. The core components of this system are the transcriptional regulator TraR and its inducing ligand N-(3-oxo-octanoyl)-L-homoserine lactone. This complex binds DNA and activates gene expression. We have determined the crystal structure of TraR in complex with its autoinducer and target DNA (PDB code 1h0m). The protein is dimeric, with each monomer composed of an N-terminal domain, which binds the ligand in an enclosed cavity far from the dimerization region, and a C-terminal domain, which binds DNA via a helix-turn-helix motif. The structure reveals an asymmetric homodimer, with one monomer longer than the other. The N-terminal domain resembles GAF/PAS domains, normally fused to catalytic signalling domains. In TraR, the gene fusion is between a GAF/PAS domain and a DNA-binding domain, resulting in a specific transcriptional regulator involved in quorum sensing.

Detecting endocrine-disrupting compounds by fast impedance measurements

The increasing concern worldwide over the adverse effects of endocrine disruptors on human health has created a need for screening systems to detect xenoestrogens, a diverse group of environmental chemicals that mimic estrogenic actions and are hypothesized to decrease male fertility. Here, we describe a novel, class-selective detector that uses fast impedance measurements to monitor the binding of estrogen and xenoestrogens to a native estrogen receptor. We embedded the receptor in synthetic lipid bilayers attached to gold electrodes. The lipid bilayers serve as electrical circuits constructed of resistors and capacitors. Estrogen binding to the receptor-modified electrode is immediately followed by conformational changes in the lipid layer, leading to alterations of the electrical circuit components that are detected by fast impedance measurements. The electrochemical system enabled characterization of changes in the bilayer structure and quantification of estrogen binding to the receptor. To assess the effectiveness of the method for detecting environmental estrogenic chemicals, we chose two classes of xenoestrogens: bisphenol A, a synthetic xenoestrogen, and genistein, a phytoestrogen. This system is highly sensitive and amenable to use in the field, providing an efficient and economic tool for measuring minuscule amounts of endocrine-disrupting chemicals in environmental or human samples.

Cloning and characterization of porcine ovarian estrogen receptor beta isoforms

The cDNA for the full-length porcine estrogen receptor beta (ER beta) and an alternatively spliced transcript with a deletion of exon 5 (ER beta delta 5) was cloned from pig ovary. RNase protection assays revealed that ER beta mRNA was expressed in the preovulatory follicles and early, midluteal, and regressing corpora lutea (CL) of eCG +/- hCG-primed gilts. ER beta and ER beta delta 5 transcripts were shown by semiquantitative reverse transcription polymerase chain reaction to be expressed at a ratio of approximately 2:1 in granulosa cells, small, medium, and large antral follicles, and midluteal phase corpora lutea of unprimed animals. Immunoreactive ER beta proteins corresponding to the size of in vitro translated ER beta and ER beta delta 5 were detected by immunoblot. Full-length ER beta was detected in granulosa, small, medium, and large antral follicles, and midluteal phase CL of unprimed animals. Putative ER beta delta 5 immunoreactive bands were abundant only in granulosa cell extracts. In COS-1 cells, transfected ER beta delta 5 had no effect on basal transcription of an estrogen-responsive reporter construct but did repress wild-type ER beta transactivation when cotransfected at 10-fold excess plasmid. No repression of ER alpha transactivation was observed. In primary granulosa cell cultures, transfected ER beta delta 5 plasmid did not inhibit basal reporter activation. ER beta delta 5 was shown by immunofluorescence to localize to the nucleus in transfected COS-1 cells. In vitro translated ER beta delta 5 proteins bound estrogen response elements in DNA in electrophoretic mobility shift assays, as indicated by supershift analysis. ER beta is abundant in porcine ovary, and a naturally occurring splice variant missing exon 5 may have biological function.

The conserved residues of the ligand-binding domains of steroid receptors are located in the core of the molecules

The relationship between conserved residues and biochemical functions of steroid receptors was investigated. Pairwise three-dimensional (3D) alignment of the ligand-binding domains of the human estrogen (1A52) and progesterone (1A28) receptors revealed two conserved domains; Asn313-Ser456 and Gln471-Lys531 (numbering reflects the sequence in the human estrogen receptor). Alignment of the protein sequences of 39 steroid receptors revealed 36 highly conserved residues (i.e., the residues commonly found in more than 80% of sequences aligned). They were distributed throughout the sequences but formed a contiguous 3D structure. Most of these highly conserved residues were buried in the ligand-binding domain, but several residues were exposed on the surface. The well-known functions commonly associated with the ligand-binding domain of steroid receptors are ligand binding, HSP90 binding, transcriptional activation and dimerization. The relationship between the residues and these functions were checked. To determine the residues involved in dimerization, the differences between the solvent accessibilities of the monomeric and dimeric forms were calculated. These results revealed 32 residues of 1A52 and 15 residues of 1A28 potentially involved in dimerization. Their distribution areas do not overlap greatly. Comparing these putative dimerization sites with highly conserved residues, many of the exposed conserved residues were observed on the side of the domain opposite are the dimerization sites. Some highly conserved residues are located in a steroid-binding site and in transcriptional activation domain. However, few of them were observed in the HSP90 binding site. These results indicate that the core structure made by most of the highly conserved residues among the ligand-binding domains of steroid receptors is important. These conserved residues may be essential for conformational change in the ligand-binding domain from its inactive to active form.

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.

Heterologous expression of soluble, active proteins in Escherichia coli: the human estrogen receptor hormone-binding domain as paradigm

The human estrogen receptor ligand-binding domain (hER-E/F), including the distal F domain, has been expressed to high levels in a soluble, active form in Escherichia coli in order to facilitate biophysical studies. The ability of a series of vectors incorporating strong transcriptional and translational signals to provide an efficient expression system for hER-E/F was investigated. High-level expression was obtained from all of the vectors used in the study. Although the majority of hER-E/F protein was produced in insoluble form under standard bacterial culture conditions, hER-E/F could be produced in soluble, biologically active form by altering the sequence of the expressed protein and by varying the host culture conditions. Several parameters, including the presence of a His tag, growth temperature, and addition of ethanol and 17beta-estradiol to the growth medium were shown to have a positive effect on production of soluble hER-E/F. An optimized expression system capable of producing from 25 to 35 mg of biologically active hER-E/F in 1 liter of cell culture was designed, and a simple, rapid purification protocol for hER-E/F produced in this system was developed. Characterization of purified hER-E/F by Edman degradation and mass spectrometry verified the identity of the protein. The K(D) for 17beta-estradiol binding to purified hER-E/F was determined to be 0.6 +/- 0.1 nM. The parameters controlling soluble, heterologous protein production observed in this study may be generally applicable to the expression of other heterologous proteins in E. coli.

Nuclear tyrosine phosphorylation: the beginning of a map

Tyrosine phosphorylation is usually associated with cytoplasmic events. Yet, over the years, many reports have accumulated on tyrosine phosphorylation of individual molecules in the nucleus, and several tyrosine kinases and phosphatases have been found to be at least partially nuclear. The question arises as to whether nuclear tyrosine phosphorylation represents a collection of loose ends of events originating in the cytoplasm or if there may be intranuclear signaling circuits relying on tyrosine phosphorylation to regulate specific processes. The recent discovery of a mechanism causing nuclear tyrosine phosphorylation has prompted us to review the cumulative evidence for nuclear tyrosine phosphorylation pathways and their possible role. While we found that no complex nuclear function has yet been shown to rely upon intranuclear tyrosine phosphorylation in an unambiguous fashion, we found a very high number of compelling observations on individual molecules that suggest underlying networks linking individual events. A systematic proteomics approach to nuclear tyrosine phosphorylation should help chart possible interaction pathways.

Technical advance: An estrogen receptor-based transactivator XVE mediates highly inducible gene expression in transgenic plants

We have developed an estrogen receptor-based chemical-inducible system for use in transgenic plants. A chimeric transcription activator, XVE, was assembled by fusion of the DNA-binding domain of the bacterial repressor LexA (X), the acidic transactivating domain of VP16 (V) and the regulatory region of the human estrogen receptor (E; ER). The transactivating activity of the chimeric XVE factor, whose expression was controlled by the strong constitutive promoter G10-90, was strictly regulated by estrogens. In transgenic Arabidopsis and tobacco plants, estradiol-activated XVE can stimulate expression of a GFP reporter gene controlled by the target promoter, which consists of eight copies of the LexA operator fused upstream of the -46 35S minimal promoter. Upon induction by estradiol, GFP expression levels can be eightfold higher than that transcribed from a 35S promoter, whereas the uninduced controls have no detectable GFP transcripts, as monitored by Northern blot analysis. Neither toxic nor adverse physiological effects of the XVE system have been observed in transgenic Arabidopsis plants under all the conditions tested. The XVE system thus appears to be a reliable and efficient chemical-inducible system for regulating transgene expression in plants.

Chemical-inducible systems for regulated expression of plant genes

Chemical regulation of transgene expression presents a powerful tool for basic research in plant biology and biotechnological applications. Various chemical-inducible systems based on de-repression, activation and inactivation of the target gene have been described. The utility of inducible promoters has been successfully demonstrated by the development of a marker-free transformation system and large-scale gene profiling. In addition, field applications appear to be promising through the use of registered agrochemicals (e.g. RH5992) as inducers.