Antibodies to Chicken Progesterone Receptor Peptide 523–536 Recognize a Site Exposed in Receptor-Deoxyribonucleic Acid Complexes but not in Receptor-Heat Shock Protein-90 Complexes*

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Abstract
We have prepared monospecific polyclonal rabbit antibodies to the peptide sequence 523-536 of the chicken progesterone receptor. This region, located between the DNA-binding and hormone-binding domains, is predicted by hydropathic analyses to be on the surface of the protein. The synthetic peptide was coupled to keyhole limpet hemocyanin and injected into rabbits. Three rabbits produced antibodies; all three are specific for progesterone receptors, recognize both native and denatured receptor, and do not interfere with either hormone binding or receptor recognition of its DNA response element in gel retardation assays. However, the antibodies do not interact with cytosolic 8S receptor complexes which contain the heat shock protein hsp90, suggesting that this site is occluded in the 8S complex. In contrast, the antibodies recognize a type of receptor dimer which forms on the DNA response element. Thus, these antibodies are a unique tool for studying receptor protein-protein interactions.