A re-examination of the interaction of estradiol with target cell receptors

Two intranuclear estradiol receptor complexes can be distinguished by sucrose density gradient ultracentrifugation of the immature rat uterus following either in vivo or in vitro administration of radiolabelled estradiol. Time course studies demonstrate that the earliest detectable complex sediments as a 4 S form whereas a 5 S form accumulates during prolonged exposure to hormone (> 10 min). Pulse chase experiments suggest that the 5 S complex arises from the lighter 4 S complex in a precursor-product manner. The properties of both the intranuclear 4 S and 5 S forms are shown to be those of authentic estrogen receptors. These observations are not in agreement with the generally held view that receptor 4 S to 5 S transformation occurs in the cytoplasm and is a necessary prerequisite to nuclear translocation.