Characterization of two 8 S forms of chick oviduct progesterone receptor

Progesterone receptor from the chick oviduct was characterized in the presence of sodium molybdate, an agent which blocks receptor transformation and maintains the receptor in an aggregated state. In sucrose gradients containing 10 mM molybdate, receptor from total cytosol sedimented with a peak at 8.9 +/- 0.1 S in the presence of 10 mM KCl and at 7.9 +/- 0.1 S in the presence of 300 mM KCl. Two receptor forms could be separated on DEAE-cellulose. One eluted at 100 mM KCl (type 1) and the other at 160 mM KCl (type 2). Both forms sedimented at 7.8 to 7.9 S and gel filtration in the presence of 300 mM KCl showed that type 1 had a Stokes radius of 73 A and type 2, of 76 A. Calculations based on the Stokes radii and sedimentation coefficients yielded estimated molecular weights of 234,000 (type 1) and 244,000 (type 2). These two 8 S receptor forms were analyzed for their content of the smaller A and B receptor components which had been identified previously in cytosol. After removal of molybdate, the type 1 and type 2 receptors were dissociated into 3.5 S forms by treatment with pyridoxal 5'-phosphate followed by borohydride reduction. The smaller forms were then identified by DEAE-cellulose chromatography. The type 2 receptor contained only the B component. Type 1 receptor also yielded only one smaller hormone-binding component. This form resembled the A receptor but appeared to be slightly larger and more acidic. Therefore, under these conditions, the A and B forms of progesterone receptor are not found together in a dimer. Instead, B is found in one 8 S complex and A, or a precursor of A, is found in another 8 S complex.