Estrogen receptor transformation to a high-affinity state without subunit-subunit interactions.

We have tested the ability of monomeric rat estrogen receptor (generated by dissociating the native receptor aggregate with KCl and binding to hydroxylapatite) to undergo heat-induced transformation to a state with altered chromatographic properties and estradiol dissociation kinetics. Participation of nonadsorbed cytosolic components in the transformation process was eliminated by washing the adsorbed receptor prior to incubation at 28-30 degrees C for 30 min. We found that monomeric estradiol-receptor complexes can be converted to a form which coelutes with transformed receptor during hydroxylapatite chromatography and which has a slow estradiol dissociation rate characteristic of transformed receptor. Monomeric receptor does not, however, display cooperative ligand-binding behavior, strongly suggesting adsorbed monomers cannot dimerize or interact with other adsorbed cytosol proteins. Maintenance of the monomer form was further verified by the sedimentation of eluted receptor solely as a 4S species. Although conversion of monomers to the slow-dissociating state was incomplete (congruent to 80%) on hydroxylapatite, eluted receptor showed only a slow dissociation rate component. The data are consistent with a model in which receptor monomers are in equilibrium between low-affinity and high-affinity conformations. Dimerization of the high-affinity receptor may shift the equilibrium to greatly favor the high-affinity (transformed) state.