Analysis of chicken progesterone receptor structure using a spontaneous sheep antibody

A spontaneous sheep antibody to chick progesterone receptor was characterized and used as a tool to study the receptor structure. The antibody, which is present to some extent in sera from about one-third of the sheep tested, binds to Staphylococcus aureus protein A--Sepharose and therefore appears to be an Igg. It is specific for the chick progesterone receptor and does not react with free progesterone or with any of the other proteins tested, including other receptors and corticosteroid binding globulin. The antibody os nonprecipitating and has a vary low titer (equivalence point = 2.5 pmol of receptor/mL of serum). The interaction of the receptor with the antibody was measured, and an apparent dissociation constant of 2 x 10(-9) M was determined from these studies. The antibody reacts equally well with the two receptor subunits A and B but does not appear to react with the native aggregate form found in the cytosol. Thus, the immunologic site is occluded in the aggregate, and therefore the antibody will be a useful probe for this important region of the proteins. The antibody recognition sites on the receptors were further characterized by analysis of a proteolytic digest of receptors by using an endogenous Ca2+-activated neutral protease. Competition studies using native receptor and receptor digests demonstrated that the antigenic site was not destroyed in the digest and was separated from the hormone binding fragment. We conclude that receptor subunits A and B have a cross-reactive immunologic site on a portion of the molecule other than the hormone binding domain.