Dimerization of the RU486-bound calf uterine progesterone receptor.

Dimerization of the RU486-bound progesterone receptor was studied by measuring the Hill coefficient of RU486 binding in calf uterine cytosol at receptor concentrations between 0.3 and 15 nM. The limiting value of the Hill coefficient at high receptor concentrations was 1.38 +/- 0.01. The limiting value of the Hill coefficient at low receptor concentrations was 1.05 +/- 0.03. The dimerization constant, defined as the concentration of receptor at which the Hill coefficient was midway between the limiting values, was 2.6 +/- 0.1 nM. In contrast, the dimerization constant of the progesterone-bound receptor, which was measured using the same approach, is 7 nM [Skafar, D. F. Biochemistry 30 (1991) 6148-6154]. The results presented here support and quantify the observation that the RU486-bound progesterone receptor will dimerize at lower receptor concentrations than the progesterone-bound receptor.