Progesterone Receptors of Chick Oviduct

In the presence of several divalent cations, progesterone receptors of estrogen-treated chick oviducts release a sub-unit characterized by a sedimentation coefficient of 2.6 S, Stokes radius of 21 A, and apparent molecular weight of about 20,000. Charcoal-dextran assay of bound steroid reveals the same order of affinities for the subunit as for intact receptors: progesterone (Kd ≃ 4 x 10-10 m) g deoxycorticosterone g testosterone g corticosterone g 17β-estradiol g cortisol. Among the cations tested, the order of effectiveness for subunit formation is Ca2+ g Mn2+ ≃ Sr2+ g Ba2+. Mg2+ is ineffective. Subunit can be obtained by CaCl2 treatment of receptors partially purified by density gradient centrifugation (4 and 5 S components), isoelectric focusing, or polycation precipitation. Subunit is formed in the presence or absence of steroid or of diisopropylfluorophosphate, a protease inhibitor. These observations and the results of preliminary studies of 45Ca2+-binding are consistent with a direct interaction of Ca2+ and similar ions with a component of the receptor complex.