Oestrogens are involved in the growth, development and homeostasis of a number of tissues. The physiological effects of these steroids are mediated by a ligand-inducible nuclear transcription factor, the oestrogen receptor (ER). Hormone binding to the ligand-binding domain (LBD) of the ER initiates a series of molecular events culminating in the activation or repression of target genes. Transcriptional regulation arises from the direct interaction of the ER with components of the cellular transcription machinery. Here we report the crystal structures of the LBD of ER in complex with the endogenous oestrogen, 17beta-oestradiol, and the selective antagonist raloxifene, at resolutions of 3.1 and 2.6 A, respectively. The structures provide a molecular basis for the distinctive pharmacophore of the ER and its catholic binding properties. Agonist and antagonist bind at the same site within the core of the LBD but demonstrate different binding modes. In addition, each class of ligand induces a distinct conformation in the transactivation domain of the LBD, providing structural evidence of the mechanism of antagonism.

Here we describe the three-dimensional crystal structures of human glucocorticoid receptor ligand-binding domain (GR-LBD) in complex with the antagonist RU-486 at 2.3 Å resolution and with the agonist dexamethasone ligand together with a coactivator peptide at 2.8 Å. The RU-486 structure was solved in several different crystal forms, two with helix 12 intact (GR1 and GR3) and one with a protease-digested C terminus (GR2). In GR1, part of helix 12 is in a position that covers the co-activator pocket, whereas in the GR3, domain swapping is seen between the crystallographically identical subunits in the GR dimer. An arm consisting of the end of helix 11 and beyond stretches out from one molecule, and helix 12 binds to the other LBD, partly blocking the coactivator pocket of that molecule. This type of GR-LBD dimer has not been described before but might be an artifact from crystallization. Furthermore, the subunits of the GR3 dimers are covalently connected via a disulfide bond between the Cys-736 residues in the two molecules. All three RU-486 GR-LBD structures show that GR has a very flexible region between the end of helix 11 and the end of helix 12.The glucocorticoid receptor (GR) 1 is a foundational member of the nuclear receptor family. A large part of the basic knowledge of the mechanism of nuclear receptor function and action has been obtained by analysis of glucocorticoid receptor function. The three-domain structure of the nuclear receptor was originally described based on results from proteolytic cleavage of GR (1, 2). The first specific binding of a nuclear receptor to a defined DNA sequence, the glucocorticoid response element, was shown using purified GR (3, 4), and GR was the first steroid receptor to be cloned (partial clone) (5). The first threedimensional structure of a nuclear receptor was obtained when the structure of the GR DNA-binding domain was solved (6, 7). However, until recently, the structure of the GR ligand-binding domain (LBD) has proven elusive. The structures of numerous other nuclear receptor LBDs have been described, including those of most of the homologues of progesterone and androgen receptors (8, 9). Very recently, the first GR-LBD structure was described in complex with the agonist dexamethasone and a coactivator peptide (10).The four steroid receptors, GR, the progesterone (PR), androgen (AR), and mineralocorticoid receptors, are very closely related. They all bind to response elements with the same degenerate consensus sequence (11), and there is considerable overlap in ligand specificity and action (12)(13)(14). Progesterone is a glucocorticoid antagonist, and many synthetic progestins are also androgens. Glucocorticoids, and particularly the endogenous hormone cortisol, bind with similar affinities to both GR and the mineralocorticoid receptor, although aldosterone is a poor GR agonist. Thus, detailed structural and functional data will be needed to understand the specific function of these four steroid receptors. Despite the problems purifying and crystallizing GR-LBD...

The tartrate-resistant acid phosphatase (TRAP) of rat osteoclasts has been shown to exhibit high (85-94%) identity at the amino acid sequence level with the purple acid phosphatase (PAP) from bovine spleen and with pig uteroferrin. These iron-containing purple enzymes contain a binuclear iron centre, with a tyrosinate-to-Fe(III) charge-transfer transition responsible for the purple colour. In the present study, production of rat osteoclast TRAP could be achieved at a level of 4.3 mg/litre of medium using a baculovirus expression system. The enzyme was purified to apparent homogeneity using a combination of cation-exchange, hydrophobic-interaction, lectin-affinity and gel-permeation chromatography steps. The protein as isolated had a purple colour, a specific activity of 428 units/mg of protein and consisted of the single-chain form of molecular mass 34 kDa, with only trace amounts of proteolytically derived subunits. The recombinant enzyme had the ability to dephosphorylate bone matrix phosphoproteins, as previously shown for bone TRAP. Light absorption spectroscopy of the isolated purple enzyme showed a lambda max at 544 nm, which upon reduction with ascorbic acid changed to 515 nm, concomitant with the transition to a pink colour. EPR spectroscopic analysis of the reduced enzyme at 3.6 K revealed a typical mu-hydr(oxo)-bridged mixed-valent Fe(II)Fe(III) signal with g-values at 1.96, 1.74 and 1.60, proving that recombinant rat TRAP belongs to the family of PAPs. To validate the use of recombinant PAP in substituting for the rat bone counterpart in functional studies, various comparative studies were carried out. The enzyme isolated from bone exhibited a lower K(m) for p-nitrophenyl phosphate and was slightly more sensitive to PAP inhibitors such as molybdate, tungstate, arsenate and phosphate. In contrast with the recombinant enzyme, TRAP from bone was isolated predominantly as the proteolytically cleaved, two-subunit, form. Both the recombinant enzyme and rat bone TRAP were shown to be substituted with N-linked oligosaccharides. A slightly higher apparent molecular mass of the monomeric form and N-terminal chain of bone TRAP compared with the recombinant enzyme could not be accounted for by differential N-glycosylation. Despite differences in specific post-translational modifications, the recombinant PAP should be useful in future studies on the properties and regulation of the mammalian PAP enzyme.

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