The nuclear receptors belong to a superfamily of proteins, many of which are ligand-regulated, that bind to specific DNA sequences and control specific gene transcription. Recent data show that, in addition to contacting the basal transcription machinery directly, nuclear receptors inhibit or enhance transcription by recruiting an array of coactivator or corepressor proteins to the transcription complex. In this review we define the properties of these putative coregulatory factors; we describe the basal and coregulatory factors that are currently known to interact with nuclear receptors; we suggest various mechanisms by which coactivators and corepressors act; we discuss issues that are raised by the presence of multiple, perhaps competing, coregulatory factors; and we speculate how these additional regulatory layers may explain the heterogeneity of hormone responses that are observed in normal and malignant tissues.

Small lipophilic molecules such as steroidal hormones, retinoids, and free fatty acids control many of the reproductive, developmental, and metabolic processes in eukaryotes. The mediators of these effects are nuclear receptor proteins, ligand-activated transcription factors capable of regulating the expression of complex gene networks. This review addresses the structure and structural properties of nuclear receptors, focusing on the well-studied ligand-binding and DNA-binding domains as well as our still-emerging understanding of the largely unstructured N-terminal regions. To emphasize the allosteric interdependence among these subunits, a more detailed inspection of the structural properties of the human progesterone receptor is presented. Finally, this work is placed in the context of developing a quantitative and mechanistic understanding of nuclear receptor function.

Two proteins are encoded by the mammalian retrotransposon long interspersed nuclear element 1 (LINE-1 or L1); both are essential for retrotransposition. The function of the protein encoded by the 5-most ORF, ORF1p, is incompletely understood, although the ORF1p from mouse L1 is known to bind single-stranded nucleic acids and function as a nucleic acid chaperone. ORF1p selfassociates by means of a long coiled-coil domain in the N-terminal region of the protein, and the basic, C-terminal region (C-1͞3 domain) contains the nucleic acid binding activity. The full-length and C-1͞3 domains of ORF1p were purified to near homogeneity then analyzed by gel filtration chromatography and analytical ultracentrifugation. Both proteins were structurally homogeneous and asymmetric in solution, with the full-length version forming a stable trimer and the C-1͞3 domain remaining a monomer. Examination of the full-length protein by atomic force microscopy revealed an asymmetric dumbbell shape, congruent with the chromatography and ultracentrifugation results. These structural features are compatible with the nucleic acid binding and chaperone activities of L1 ORF1p and offer further insight into the functions of this unique protein during LINE-1 retrotransposition. L ong interspersed nuclear element 1, known as LINE-1 or L1, is a highly successful retrotransposon of mammalian genomes, comprising 17% of the human genome and 19% of the mouse genome, and likely generating an additional 8-13% in the form of short interspersed nuclear elements (SINEs) and processed pseudogenes (1). L1 belongs to a larger group of mobile elements known as the non-LTR retrotransposons, which are widely distributed throughout eukaryotes (2).Non-LTR retrotransposons replicate by reverse transcription of an RNA intermediate, employing a unique mechanism known as target-site primed reverse transcription, or TPRT. In mammalian L1s, two element-encoded proteins are required for retrotransposition: ORF2p, which provides the crucial enzymatic activities for TPRT, endonuclease, and reverse transcriptase (reviewed in ref. 3); and ORF1p, which is a single-stranded nucleic acid binding protein (4, 5) that also acts as a nucleic acid chaperone (6).The calculated mass of the protein encoded by ORF1 in the retrotransposition-competent element, L1 spa (7), is 42,921 Da, and a 43-kDa ORF1p is detected in extracts prepared from a subset of mouse cells by Western blotting. Immunocytochemistry with the same anti-ORF1p Ab reveals a punctate cytoplasmic staining pattern in the two embryonal carcinoma cell types, F9 and JC44, where ORF1p is detected by Western blotting (8), as well as in prepachytene germ cells of the testes (9) and ovary (10). p43 is coenriched with full-length L1 RNA in ribonucleoprotein particles that can be fractionated from F9 cells (11,12). Taken together, these observations suggest that L1 RNA is bound with many molecules of ORF1p during the cytoplasmic phase of the L1 replication cycle, perhaps playing a structural role in protecting the RNA and organiz...

Steroid receptor antagonists, such as the antiestrogen tamoxifen or the antiprogestin RU486, can have inappropriate agonist-like effects in tissues and tumors. To explain this paradox we postulated that coactivators are inadvertently brought to the promoters of DNA-bound, antagonist-occupied receptors. The human (h) progesterone receptor (PR) hinge-hormone binding domain (H-HBD) was used as bait in a two-hybrid screen of a HeLa cDNA library, in which the yeast cells were treated with RU486. We have isolated and characterized two interesting steroid receptor-interacting proteins that regulate transcription in opposite directions. The first is L7/SPA, a previously described 27-kDa protein containing a basic region leucine zipper domain, having no known nuclear function. When coexpressed with tamoxifen-occupied estrogen receptors (hER) or RU486-occupied hPR or glucocorticoid receptors (hGR), L7/SPA increases the partial agonist activity of the antagonists by 3- to 10-fold, but it has no effect on agonist-mediated transcription. The interaction of L7/SPA with hPR maps to the hinge region, and indeed, the hPR hinge region squelches L7/SPA-dependent induction of antagonist-mediated transcription. Interestingly, pure antagonists that lack partial agonist effects, such as the antiestrogen ICI164,384 or the antiprogestin ZK98299, cannot be up-regulated by L7/SPA. We also isolated, cloned, and sequenced the human homolog (hN-CoR) of the 270-kDa mouse (m) thyroid/retinoic acid receptor corepressor. Binding of hN-CoR maps to the hPR-HBD. mN-CoR, and a related human corepressor, SMRT, suppress RU486 or tamoxifen-mediated partial agonist activity by more than 90%. This suppression is completely squelched by overexpression of the hPR H-HBD. Additionally, both corepressors reverse the antagonist-dependent transcriptional up-regulation produced by L7/SPA. Our data suggest that the direction of transcription by antagonist-occupied steroid receptors can be controlled by the ratio of coactivators to corepressors recruited to the transcription complex by promoter-bound receptors. In normal tissues and in hormone-resistant breast cancers in which the agonist activity of mixed antagonists predominates, steroid receptors may be preferentially bound by coactivators. This suggests a strategy by which such partial agonist activity can be eliminated and by which candidate receptor ligands can be screened for this activity.