The specific, high-affinity binding of the avian oviduct progesterone receptor (PR) with target-cell nuclei and chromatin has been shown to involve DNA complexed with specific chromatin acceptor proteins. One of these chromatin acceptor proteins has been partially purified and found to be a small hydrophobic protein with a broad pI of 5.0-6.0 [Goldberger, A., & Speisberg, T. C., (1988) Biochemistry 27, 2103–2109]. This paper describes the final purification over 100 000-fold to apparent homogeneity of this candidate PR acceptor protein, termed the receptor binding factor 1 (RBF-1). When the avian genomic DNA is bound by RBF-1, saturable, high-affinity (KD ~ 2 × 10−9 M) binding sites for PR are generated. RBF-1 has a unique, hydrophobic N-terminal sequence. The PR binding to the RBF-1-DNA complexes is shown to be dependent on an intact activated PR with which excess nonradiolabeled PR can compete. By use of a new, highly specific monoclonal antibody (mAb) to the RBF-1 with Western immunoblotting, RBF-1 was shown to be localized in the nucleus and to be tissue and species specific. Selective removal of the chromatin proteins containing RBF-1 results in the loss of the highest affinity class of PR binding sites. A second class of residual PR binding sites remains in the nucleoacidic protein (NAP), a complex of proteins more tightly bound to the DNA. This class of PR binding activity has been classified as the RBF-2. The RBF-1 is estimated to be 0.03% of the total chromatin protein with about 1.2 × 105 molecules/diploid cell.
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