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Conversion of estrogen receptor from a state with low affinity for estradiol into a state of higher affinity does not
Abstract:
This study was undertaken to establish whether the heat-promoted conversion of receptor-estradiol complex (RE2) from a state with fast into a state with slow E2 dissociation requires 8S/4S to 5S transformation. The calf uterine estrogen receptor labeled with [3H]E2 at 0 C (state with low affinity for E2) was immobilized on hydroxylapatite (HAP) in the absence (8S oligomer) or presence (4S monomer) of 0.4 M KCl and heated at 28 C in the presence of unlabeled diethylstilbestrol. Under these conditions, the dissociation of [3H]E2 was biphasic and occurred at rates similar to those obtained with R[3H]E2 free in cytosol. In contrast to the latter, however, the heat-promoted conversion of HAP-immobilized R[3H]E2 into a state of higher affinity for E2 was not accompanied by receptor dimerization, since the HAP eluate (0.4 M phosphate buffer) contained only the 4S monomer; upon reheating or desalting of this eluate, 4S to 5S dimerization occurred. The heat-promoted formation of 4S RE2 monomers with higher affinity for E2 was not due to monomer-HAP interactions, since even after elution from HAP, the 4S RE2 remained in the state of higher affinity for E2. Addition of pyridoxal 5'-phosphate to slow dissociating, high affinity 5S R[3H]E2 dimers free in cytosol induced rapid [3H]E2 dissociation, although the receptor remained unaltered in the transformed dimerized state. The effect of PLP was readily reversed by the addition of lysine. It is proposed that the 4S receptor monomers exist in two conformational states; upon E2 binding to the low affinity state, conformational changes result in stronger interactions between the steroid and the amino acid residues of the estrogen-binding domain; thus, the rate of E2 dissociation decreases. The formation of this 4S RE2 state with higher affinity for E2 is independent from receptor dimerization. A model is presented in which 4S to 5S transformation may lead to stabilization of 4S monomers in the conformation with higher affinity for E2.