As an approach to the mechanism of the nuclear translocation of estrogen receptor, the estradiol nuclear receptor (RN) of lamb endometrium was extracted with micrococcal nuclease at 2–4° and compared to the “native” 8S and to the Ca2+-transformed cytosol receptors. After extensive digestion of chromatin, giving up to 10% perchloric acid-soluble DNA and a majority of nucleosome monomers, up to 80% of the RN was extracted under low ionic strength. This RN was found to be completely different from the partially proteolyzed Ca2+-transformed cytosol receptor. It migrated with a sedimentation constant of 4 and 6 S. The Stokes radius of the predominant form as determined by ACA 34 chromatography was 5.3 nm. The calculated apparent molecular weights were 130,000 and 90,000, respectively. The RN was able to bind DNA and was eluted from a diethylaminoethyl cellulose column at 0.23 and 0.30 m KCI. We conclude that the mechanism proposed by Puca et al., according to which the Ca2+-transformed cytosol receptor is split by a Ca2+ receptor-transforming factor into a smaller form able to cross the nuclear membrane, is very unlikely.

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This work was supported by the Institut National de la Santé et de la Recherche Medicale, the Fondation pour la Recherche Medicale, and the Délégation Generale a la Recherche Scientifique et Technique.

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