After exposure to ligand at 0-4 °C, estrogen receptors from mouse uteri characteristically eluted between thyroglobulin (Mr 669000) and ferritin (Mr 443000) during size-exclusion HPLC. However, when preparations were warmed with ligand under mild activating conditions, most or all of the receptor was observed as a much larger complex, which eluted between dextran blue 2000 and thyroglobulin. Formation of the large complex required ligand, was inhibited by molybdate, and occurred even in 0.4 M KC1. Slower ligand dissociation characterized the large complex, indicating that activated receptors were included preferentially. This large complex did not form when charged cytosols were aged, concentrated, or precipitated, indicting that formation was not the result of random aggregation. After exposure to conditions commonly used for activation (25 °C, 60 min), most receptor existed as a very large, monodisperse complex of finite size, predicting an ordered structure for these large complexes that should be useful for defining the types of proteins which can interact with estrogen receptors. Formation of the large complex was not impeded or disrupted by EDTA, RNase, DNase I, thiourea, or mercaptoethanol; however, the capacity to form this large complex was not demonstrated by preparations that had been exposed to trypsin or by the small receptor forms obtained after salt extraction. Proteolytic sensitivity and lack of sensitivity to RNase or DNase indicate that interactions between receptors and other proteins are involved in peak A formation. Since the formation of this large complex was observed in preparations obtained from CF-1 mouse uteri, pituitaries, and hypothalami, nude mouse uteri, and MCF-7 human breast cancer cells, this capacity appears well distributed across biological sources. Multiple characterizations described here indicate that the formation of this large complex is distinct from the broader consideration termed receptor aggregation. Since the formation of this large complex conforms with receptor activation, this complex is a direct indication that through activation estrogen receptors acquire the capacity to engage in intermolecular associations with other proteins. Since this type of structural engagement could provide a means through which estrogen receptors unite regulatory elements for the initiation of the complex series of events associated with hormone action, it may be helpful for identifying these types of individual regulatory components.
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