Studies on the structure and function of the chicken progesterone receptor

The avian vitamin D receptors: primary structures and their origins

cDNA clones encoding Japanese quail chorioallantoic membrane and chicken kidney 1,25-dihydroxyvitamin D3 receptors were isolated and the total 448-amino acid (aa) sequence was deduced. The sequences of the chicken and quail receptors are identical. The nucleotide and deduced amino acid sequences of the avian receptors are similar but not identical to the reported rat or human receptor sequences. There is a 78% similarity in the nucleotide sequences and 98.5% and 87.5% similarities in the amino acid sequences of the DNA-binding and ligand-binding domains, respectively. Two avian receptor proteins (58 and 60 kDa) arise from a single mRNA transcript by alternate initiation of translation. The avian 1,25-dihydroxyvitamin D3 receptors were produced using a bacterial expression system. Form A receptor was expressed from a cloned cDNA that contains the first translation signal (ATG) and corresponds with the 60-kDa avian receptor protein, and form B receptor was initiated from the third ATG on the same mRNA transcript to give rise to the 58-kDa protein. The cysteine-rich DNA-binding domain is almost conserved among human, rat, and avian receptors. The position of the nine cysteines was conserved in all three sequences. The avian receptor differs in the second zinc finger domain, where a methionine replaces a leucine, a serine replaces an asparagine, and a lysine replaces an arginine at aa 77, 83, and 87, respectively, of the avian sequence. The increased length of the avian receptor results from a 20-aa extension of the N-terminal region. RNA hybridization indicates there is a single mRNA species of approximately 2700 bp for both the chicken and quail receptors compared to 4400 bp for the rat transcript. Surprisingly, the translated avian sequence is larger (448 aa) than the 423-aa rat receptor protein. Therefore, our results confirm that despite the difference in molecular mass between different receptor proteins, there is a similarity in gene organization such that the DNA-binding and hormone-binding domains are positionally conserved from the C terminus.

Heterotetrameric structure of the human progesterone receptor

Nonactivated progesterone receptors in extracts of human T47D mammary carcinoma cells were investigated. Chemical cross-linking with dimethyl suberimidate resulted in complete stabilization of the A and B receptors with an average molecular mass of 340 kDa. For analyzing the subunit structure, we concentrated on the larger B receptor, which was separated from the A form by immunoaffinity chromatography. Progressive cross-linking of the photoaffinity-labeled receptor resulted in patterns of labeled bands in SDS gels, which are indicative of a heterotetrameric structure. It consists of one receptor polypeptide in association with two 90-kDa subunits and one polypeptide of approximately 60 kDa. The completely cross-linked B receptor has a molecular mass of approximately 390 kDa. To identify the subunits, the oligomeric B receptor was cross-linked with a cleavable bisimidate, highly purified by immunoaffinity chromatography, and analyzed by gel electrophoresis and immunoblotting. The receptor polypeptide has a mass of 116.5 kDa. The 90-kDa band was identified as the heat shock protein hsp90 and was roughly twice as intense as the receptor polypeptide. By use of specific antibodies, we identified the fourth receptor subunit as a 59-kDa protein (p59); we did not obtain any evidence for the heat shock protein hsp70 being a receptor component. We suggest an analogous heterotetrameric structure for the nonactivated A receptor.

Iodination of the progesterone receptor from hen oviduct spares the DNA-binding domain

The progesterone receptor from hen oviduct is isolated as a complex of two subunits, A and B. The A protein binds one molecule of progesterone and also binds to DNA with high affinity. The native A protein can be labeled with iodine with no loss of DNA-binding activity. Limited Staphylococcus aureus V8 protease digestion of the labeled preparation results in a number of DNA-binding and non-DNA-binding fragments of the receptor. The progesterone-binding domain contains iodine label. However, two low-molecular-weight DNA-binding fragments do not contain iodine label, indicating a lack of susceptible tyrosine residues near the DNA-binding site of the native receptor. The labeled receptor and its fragments will facilitate studies of the isolated DNA-binding and progesterone-binding domains of the hen A protein as well as of the activity of the native receptor in the presence and absence of hormone.

Nuclease resistance and the enrichment of native nuclear acceptor sites for the avian oviduct progesterone receptor

High-affinity nucleoprotein acceptor sites for the avian oviduct progesterone receptor (PR) have been enriched by a combination of nuclease digestion and centrifugation. These enriched binding elements exhibited markedly enhanced PR binding on a per mass DNA basis compared to chromatin (20- to 25-fold) or dehistonized chromatin (4- to 5-fold). Electrophoretic analysis of the nuclease-resistant DNA showed that there is a set of DNA fragments of 100-150 base pairs that are protected from digestion. Excessive digestion resulted in smaller DNA fragments and a loss of PR binding activity. The PR binding was saturable using a crude receptor preparation and displayed a competition with the same receptor preparation that was labeled with nonradioactive progesterone. The enhanced binding was also demonstrable using highly purified receptor preparations that exhibit two classes of binding sites both of which are of high affinity and saturable as assessed by Scatchard analyses. These two high-affinity classes of binding sites are shown to be competed by unlabeled purified PR. The nuclease resistance of these nucleoprotein acceptor sites from chromatin is a property similar to the nuclear matrix binding sites suggesting a relationship between these two classes of nuclear acceptor sites.

Tightly bound nuclear progesterone receptor is not phosphorylated in primary chick oviduct cultures

Oviduct cells from estradiol-treated chicks were grown in primary culture. After 3-5 days of culture in medium containing estradiol, 90% of the cellular progesterone binding sites were detected in the cytosol. After exposure to [3H]progesterone at 37 degrees C, 80% of the progesterone binding sites were found in nuclear fractions. Progesterone receptor phosphorylation was assessed after incubating the cells with [32P]orthophosphate. Receptor components were immunoprecipitated with a specific polyclonal antibody (IgG-G3) and analyzed by NaDodSO4/PAGE and autoradiography. In the cytosol, constant amounts of 32P-labeled 110-kDa subunit (the B subunit, one of the progesterone-binding components of the receptor) and of the non-steroid-binding heat shock protein hsp90 were found, whether cells had been exposed to progesterone or not. No 32P-labeled 79-kDa subunit (the A subunit, another progesterone-binding subunit) was detected. Various procedures were used to solubilize nuclear progesterone receptor (0.5 M KCl, micrococcal nuclease, NaDodSO4), and in no case was 32P-labeled B subunit detected in the extracts. However, nonradioactive B subunit was detected by immunoblot in a nuclear KCl extract of progesterone-treated cells. These results suggest that the fraction of the B subunit that becomes strongly attached to nuclear structures is not phosphorylated upon exposure of cells to progesterone.

Receptors for glucocorticosteroid and progesterone recognize distinct features of a DNA regulatory element

The chicken lysozyme gene can be induced in oviduct cells by four classes of steroid hormones, including glucocorticosteroids and progestins. The glucocorticosteroid receptor of rat liver and the progesterone receptor of rabbit uterus both bind, although with different relative affinities, to two sites in the promoter region of the chicken lysozyme gene located, respectively, between 50 and 80 and between 160 and 200 base pairs upstream of the transcription start point. Now we show that the purified progesterone binding unit of the chicken oviduct progesterone receptor (Mr 110,000, or so-called B subunit) generates a DNase I protection pattern ("footprint") in the promoter-distal site that is longer than the footprint generated by the glucocorticosteroid receptor. Methylation protection studies within the promoter-distal binding site identify four contact points for the chicken progesterone receptor and three contact points for the glucocorticosteroid receptor, of which only one is shared by both receptors. Computer graphics models allow one to envisage a different interaction of each receptor with the B form of the DNA double helix.

The common 90-kd protein component of non-transformed '8S' steroid receptors is a heat-shock protein

Non-transformed steroid receptors have an approximately 8S sedimentation coefficient that corresponds to an oligomeric structure of 250-300 kd which includes a non-hormone binding 90-kd protein. A monoclonal antibody BF4 raised against the purified, molybdate-stabilized, 8S progesterone receptor (8S-PR) from chick oviduct, recognizes 8S forms of all steroid hormone receptors. BF4 was found specific for a 90-kd protein present in great abundance in all chicken tissues, including that present in 8S-forms of steroid receptors. Here, using immunological and biochemical techniques, we demonstrate that this ubiquitous BF4-positive 90-kd protein is in fact the chicken 90 kd heat-shock protein (hsp 90): it increased in heat-shocked chick embryo fibroblasts, and displayed identical migration in two-dimensional gel electrophoresis and the same V8 peptide map as the already described hsp 90. We discuss the possibility that the interaction between hsp 90 and steroid hormone-binding subunits may play a role in keeping the receptor in an inactive form.

Progesterone receptor in the chick oviduct: an immunohistochemical study with antibodies to distinct receptor components

We performed immunohistochemical studies of chicken oviduct after different fixation procedures, by using antibodies against the progesterone receptor: polyclonal antibodies IgG-G3 against the "8S" form (an oligomere containing progesterone-binding and nonprogesterone-binding units), polyclonal antibodies IgG-RB against the progesterone-binding B subunit, and monoclonal BF4 against the non-progesterone-binding 90,000-mol-wt protein component. Chickens were immature animals with or without estrogen priming, and with or without progesterone treatment. The antibodies were revealed by means of an immunoperoxidase technique that used the avidin-biotin-peroxidase complex, and controls were performed by presaturation of antibodies with the purified 8S-progesterone receptor, the B subunit, and 90,000-mol-wt protein. The progesterone receptor was detected not only in well-characterized target tissues, i.e., in glands and luminal epithelium, but also in stromal cells (some displayed the strongest reaction), in mesothelium, and in fibers of smooth muscles. Only in cell nuclei, whether or not the animals received an injection of progesterone was an antigen revealed corresponding to the B subunit (and/or to the A subunit, because there is immunoreactivity of IgG-RB with both hormone-binding subunits A and B). The 90,000-mol-wt protein was revealed in both cytoplasm and nuclei. These immunohistological data suggest that the concept of steroid action that necessarily involves the original formation of the hormone-receptor complexes in the cytoplasm before translocation to the nucleus, may have to be revised.

Molecular cloning of a cDNA for the chicken progesterone receptor B antigen

A cDNA for the chicken progesterone receptor B subunit antigen (Mr, 108,000) has been isolated from a cDNA library prepared from size-selected chicken oviduct poly(A)+RNA. A specific monoclonal antibody raised against hen progesterone receptor B subunit (alpha PR-B) was used to screen the library. Recombinant clones reacting with the antibody by virtue of antigen expression were used in hybrid-selected translation. A single clone, pPRB-1, hybridized specifically to a mRNA that yielded a Mr 108,000 protein when translated in vitro and which was immunoprecipitable by the alpha PR-B antibody. This cDNA represents a 470-base-pair portion of the PR-B nucleotide sequence. Additional clones have been subsequently isolated from the recombinant library using the insert from pPRB-1 as a specific probe. A mRNA size of approximately 3000 nucleotides was determined for the chicken progesterone receptor B subunit by formaldehyde/agarose gel electrophoresis and blot hybridization using pPRB-1 as a probe. Preliminary studies show that withdrawal of hormone from chickens treated chronically with estrogen leads to a dramatic decrease in the cellular RNA concentration of receptor B, indicating that target tissue levels of receptor B RNA are under hormonal control.

Endogenous avidin found in the magnum gland of the hen oviduct. Comparative cytochemical studies utilizing both biotinyl-peroxidase and immunoperoxidase methods

The location of endogenous avidin was studied cytochemically in the magnum tissue of the oviduct of laying hens. Two methods, based on an interaction of avidin-biotin with biotin hydrazide-peroxidase (B-HRP) as an affinity reagent, and on an immunoperoxidase technique, were tested by morphological analysis. The data obtained by both methods showed that in the magnum B-HRP is a strictly substitutive reagent for endogenous avidin. Avidin was clearly demonstrated in large amounts in the secretory granules of some epithelial cells and tubular gland cells, but was absent from mucous cells, the goblet cells, which had been believed to be the location of avidin production, and from ciliated cells. These granules had previously been demonstrated by both electron-microscopic cytochemical techniques. Especially in acinar cells, they were nonhomogeneous with a speckled core and a dense peripheral part. They ranged in size from 500 to 2200 nm in diameter in the gland and 180 to 720 nm in the epithelium. Columnar epithelial cells containing avidin granules had a strong resemblance to those of the protodifferentiated tubular gland cells in the magnum of chicks pretreated with daily estrogen or estrogen plus progesterone, and might have migrated towards the acinus as substitutional secretory cells. Therefore, the acinar cells of the magnum, considered to be composed of several secretory protein-producing systems, are dependent on estrogen and/or progesterone in the oviduct of the laying hen.