Progesterone-binding components of chick oviduct. Biochemical characterization of purified oviduct progesterone receptor B subunit

A number of physical and chemical properties of pure hen oviduct progesterone receptor B subunit have been determined. The molecule consists of a single polypeptide chain with a molecular weight of 115,000 g/mol as determined by gel filtration in the presence of 6 M guanidine hydrochloride and by gel electrophoresis in sodium dodecyl sulfate. The labeled subunit has retained the biologically important properties which it displayed in cruder preparations: it binds to nuclei (Kd = 1 X 10(-9) M) and chromatin (Kd = 1.5 X 10(-9) M) but does not bind to DNA. Reaction of the purified subunit with dansyl (5-dimethylaminonaphthalene-1-sulfonyl) chloride revealed a single NH2-terminal lysine. The amino acid composition has been determined and has been shown to be distinct from that of other steroid-binding proteins and consistent with the known properties of the molecule. In addition, no evidence for carbohydrate or phosphorylated amino acids was observed. The protein contains about 12% alpha helix as determined by circular dichroism. The ultraviolet spectrum of intact steroid receptor complexes revealed that the purified subunit had no pyridine nucleotide cofactor or nucleic acid, and that each receptor molecule contains a single hormone binding site. Electron microscopic analysis confirms the prolate-ellipsoid shape of the protein, with a long axis of 114 A. The purified protein isolated as described in a companion paper is shown here to have the characteristics of the crude receptor subunit B. Due to the apparent role in the hormone response, this protein has been named progestophilin B.