Influence of chemical modifications of amino acid side chains on the binding or progesterone to uteroglobin

Differential inhibition of estrogen and antiestrogen binding to the estrogen receptor by diethylpyrocarbonate

Diethylpyrocarbonate differentially inhibited the specific binding, in lamb uterine cytosol, of estradiol (inhibition approximately 90% with 4 mM reagent) and 4-hydroxytamoxifen (inhibition approximately less than 50% with 4-16 mM reagent), a potent triphenylethylene antiestrogen. Saturation analysis experiments indicated that the effects of diethylpyrocarbonate were due to progressive but differing decreases in the concentration of binding sites for the two ligands, with no apparent change in the affinity constants. However, competitive binding and dissociation experiments evidenced that steroidal and nonsteroidal estrogens still bound, but with very low affinities, to diethylpyrocarbonate-modified receptor (greater than 1000-fold decrease in affinity) whereas the affinities of triphenylethylene antiestrogens were much less affected (less than 10-fold decrease). Both ligands prevented the inactivation of the estrogen receptor by diethylpyrocarbonate, estradiol being more efficient than 4-hydroxytamoxifen. These data indicate that the action of diethylpyrocarbonate results in the formation of two populations of estrogen receptor that are quantitatively nearly equivalent: the first does not bind estrogens or antiestrogens; the second does not bind estrogens significantly but still interacts with antiestrogens at a high affinity. The simplest interpretation is that these two populations arise from mutually exclusive modifications by diethylpyrocarbonate of at least two aminoacid residues located at or close to the ligand binding site; modification of one residue totally prevents the binding of estrogens and antiestrogens; the modification of the second impairs only the binding of estrogens. Considering that (i) hydroxylamine, which specifically reverses the diethylpyrocarbonate-induced modification of histidine and tyrosine residues, restored a large part (greater than 80%) of the estradiol- and 4-hydroxytamoxifen-binding capacity of diethylpyrocarbonate-inactivated cytosol, and that (ii) similar differential inhibition of estrogen and antiestrogen binding was observed following the action of tetranitromethane, it is likely that these residues are histidine(s) and/or tyrosine(s). These results evince a marked difference in the interaction of estrogens and triphenylethylene antiestrogens with the estrogen receptor, which could account for the altered activation of the receptor by triphenylethylene antiestrogens. Consequently, the screening of ligands with modified steroid receptors could be a useful method for distinguishing between potential hormone agonists and antagonists.

Isolation and structure of the gene for the progesterone-inducible protein uteroglobin

Uteroglobin is a small steroid-binding protein that is differentially regulated by steroid hormones in several tissues of the rabbit. In endometrium, the levels of uteroglobin mRNA increase after progesterone administration due to an enhanced rate of transcription of the uteroglobin gene. As a prerequisite for understanding the molecular mechanisms that modulate uteroglobin gene expression, we have isolated and characterized the uteroglobin gene. We first synthesized, cloned, and sequenced a uteroglobin cDNA that was used to screen a rabbit gene library and to show that the uteroglobin gene is not reiterated in the rabbit genome. We obtained three recombinant phages containing uteroglobin gene sequences and covering 35 kilobases of the rabbit genome. The uteroglobin gene is 3 kilobases long and is composed of three short exons separated by a long and a short intron. The complete coding sequence, the short intron, part of the large intron, and the flanking sequences have been subjected to sequence analysis. The salient features of the nucleotide sequence, including the absence of a canonical "T-A-T-A box," are discussed. A possible relationship is considered between the exon-intron structure of the gene and the known structure and function of uteroglobin.

Photoaffinity labeling of steroid binding proteins with unmodified ligands

Photoactivation of the alpha,beta-unsaturated ketones of natural and synthetic steroid molecules by light of lambda greater than or equal to 330 nm allows their covalent attachment to steroid-binding proteins. The general validity of this method is demonstrated with two steroid hormone receptors and the steroid-binding protein uteroglobin. Progesterone can be covalently attached to the partially purified progesterone receptor and to uteroglobin, and comigrates with the binding proteins upon electrophoresis in polyacrylamide gels containing sodium dodecyl sulfate. Similarly the synthetic glucocorticoid triamcinolone acetonide can be covalently bound to the partially purified glucocorticoid of rat liver. This method allows the identification of steroid hormone receptors after electrophoresis in polyacrylamide gels containing sodium dodecyl sulfate. Labeling with radioactive steroids is specific since it can be prevented by the addition of an excess of non-radioactive ligand. Digestion of the labeled binding proteins with trypsin or chymotrypsin yields a defined pattern of radioactive peptides, demonstrating that covalent attachment takes place at specific binding sites.