2. Mechanism of steroid action. Immunoreactivity of the core of estrogen receptors found in different subcellular compartments of target cells from a variety of mammalian species.

A goat antiserum was raised by immunization with the endoglycosidase fission product of porcine estrogen receptor. Its IgG fraction was employed for ascertaining the interrelationship of receptor forms extracted from three subcellular compartments of target cells and for comparing receptors from various species. Unspecific alignments of (non-immune) IgGs giving rise to soluble complexes were avoided by removal of the receptor entity responsible with carrier-attached trypsin. Coincubation of the estradiol-tagged tryptic receptor cores and immune IgG resulted in the formation of labelled precipitates and in the release of estradiol from its binding site, in every instance tested. A minimum of three common antigenic determinants must therefore exist not only on all porcine receptor forms, but also on human, ovine, bovine, rabbit, guinea-pig and rat estrogen receptors, pointing to a high degree of phylogenetic conservation.