The molecular properties of uterine estrogen receptors, following limited proteolysis, have been characterized. Mild trypsinization of cytoplasmic estrogen receptor preparations from lamb and rat uterus causes a marked disaggregation of the receptor, giving lower molecular weight forms that retain unimpaired their ligand binding properties. These trypsinized receptors have little tendency to reaggregate and thus are amenable to further purification and to detailed characterization. Sedimentation coefficients obtained by sucrose gradient centrifugation are 3.04 ± 0.04 S for the lamb and 4.01 ± 0.09 S for the rat trypsinized receptors; Stokes radii, determined by gel partition chromatography, are 2.76 ± 0.23 nm for lamb and 2.94 ± 0.25 nm for rat uterine receptor. Discontinuous polyacrylamide gel electrophoresis in two systems (pH 7.8 and 10.2) shows the trypsinized lamb receptor as three distinct components (two major and one minor); the rat receptor is somewhat more disperse, but appears as mainly one component. Molecular weight estimates based on combined sedimentation coefficients and Stokes radii are 35 300 for the lamb components and 49 600 for the rat. Molecular weights estimated by electrophoretic mobility (Ferguson plots) show the rat receptor to be somewhat lighter (41 800 ± 10 000) than the two principal lamb components (46 800 ± 11 000 and 46 400 ± 11 000). Isoelectric focusing in polyacrylamide gels gives apparent pi values of 5.5 and 5.8 (major components), and 6.2 (minor) for the lamb and 6.4 for the rat receptor. These readily characterizable, trypsinized forms of the estrogen receptor may be related to other smaller molecular weight forms of receptors for estrogens and other steroids. Thus, the availability of these receptor preparations should be of major assistance to studies that are aimed at understanding the interaction of estrogenic compounds with the receptor binding site and the relationship between various forms of steroid receptors.
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