Positive cooperativity of the estrogen receptor.

Abstract

The equilibrium [3H]estradiol binding by the partially purified estrogen receptor from calf uteri was measured at 25.degree. C. The Scatchard plot of the binding data showed a convex curve characteristic of positive cooperativity and a Hill coefficient of 1.58 .+-. 0.21, at receptor concentrations of 1-10 nM. Below a receptor concentration of .apprxeq. 0.3 nM, the Scatchard plot approached linearity, suggesting that the cooperative interactions are dependent upon a monomer-dimer equilibrium. Trypsin pretreatment of the receptor resulted in a loss of dimer formation and of the cooperative interactions. The positive cooperative characteristics of the estrogen receptor were not produced by receptor inactivation, failure to complete the [3H]estradiol-receptor equilibrium reaction or radioimpurity of the [3H]estradiol. The activated 5S estrogen receptor apparently is a homodimer, and its formation is associated with a positive cooperative estradiol-binding reaction.