Immunologic detection of a protein homologous to chicken progesterone receptor B subunit.

A monoclonal antibody described previously by us (Edwards, D. P., Weigel, N. L., Schrader, W. T., O'Malley, B. W., and McGuire, W. L. (1984) Biochemistry 23, 4427-4435) was used to study progesterone receptor B subunits of chick and hen oviduct. We find that the antibody does not recognize the form of receptor B able to bind [3H]progesterone in vitro. Rather, it reacts exclusively with a homologous protein of the same molecular weight, termed B antigen. The antigen is present in both immature estrogen-treated chicks and in egg-laying hens. This antigen is indistinguishable from the hormone-binding receptor species (termed receptor B) as shown by peptide mapping techniques using either Staphylococcus aureus V8 protease or trypsin. The B antigen and the hormone binder can be resolved by ion-exchange chromatography. Sedimentation velocity data show that the two proteins are present in distinct, separable cytosolic entities. The functional relationship between the two proteins has not been established.