Immunological similarity between the chick oviduct progesterone receptor forms A and B

A large-scale purification of the progesterone receptor from laying hens is described which yields apparently homogeneous form A and form B receptor in denatured form. The purification procedure is based initially on differential DNA affinity chromatography of both form A and B receptors. Under the conditions of preparation and activation described, progesterone receptor form B binds to DNA-cellulose even in the presence of 100 mM salt. This binding cannot be observed after thermal activation. Receptors obtained at 5% purity using conventional chromatographic purification steps were covalently cross-linked with radioactive ligand by photoaffinity labeling and purified to homogeneity using preparative gel electrophoresis systems under denaturing conditions. This material has been successfully used to generate polyclonal antibodies in rabbits. Immunoblots demonstrated a high degree of cross-reaction between anti-A antibodies and progesterone receptor form B, as well as between anti-B antibodies and progesterone receptor form A, using homogeneous as well as 5% pure receptors as probes. Implications of the immunological data and the novel DNA-binding characteristics of form B are discussed with respect to topological conformation of the progesterone receptor and the structural similarity between forms A and B.