We have examined the sedimentation properties of MCF-7 cell cytosol estrogen receptors (ERc) and salt-extracted nuclear estrogen receptors (ERn), and the interaction of these receptors with a monoclonal antibody, as assessed on 0.4 M KC1 sucrose gradients prepared in different buffers. ERc labeled with [3H]estradiol (E2) sediments as a 4 S species in all gradient buffers tested, whereas ERn extracted from cells incubated with [3H]E2 sediments as a 4 S species in sucrose gradients prepared in phosphate-thioglycerol-glycerol buffer, but as a 5.4 S species in Tris-EDTA buffered gradients. Interaction of ERc with antibody D547SpY results in formation of a 7.2 S complex, whereas the antibody reacts with ERn to form complexes that are 8.5 S in sucrose gradients prepared in either buffer. These findings are consistent with the hypothesis that estrogen receptors from MCF-7 cells undergo a 4 S to 5.4 S transformation during nuclear interaction, but that the 5.4 S ERn is converted to a 4 S form during centrifugation in phosphate-buffered gradients. Antibody interaction blocks conversion of receptor to the 4 S form, since only 8.5 S complexes are observed, regardless of the gradient buffer composition. These results highlight that the pattern of receptor transformation by E2 in these cancer cells need not differ from that seen in normal estrogen target tissues such as uterus.
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