Interaction of calf uterus estradiol receptor with erythrocyte cytoskeleton

The relationship of the cytosolic estradiol receptor with membranous and cytoskeletal structures of the cell matrix has been studied using a model system formed by the erythrocyte membrane. Extraction of erythrocyte ghosts with various procedures and also with the nonionic detergent Triton X-100 under conditions that yield a cytoskeletal matrix reveals the presence of binding sites for the soluble estradiol receptor of calf uterus. The interaction between the estradiol receptor and the cytoskeleton is critically dependent on temperature which is required for the interaction itself and not for a modification of the receptor molecule. Ionic strength and cations, with selectivity for Mg2+, also influence the interaction which has an optimum at pH 7.5. Saturation experiments reveal the presence of a limited number (less than 100) of sites per cell having high affinity ( K d ⋍ 10 −9 M ) for the estradiol-receptor complex. The affinity of the receptor for the steroid does not change after it has bound to the cytoskeleton. Limited proteolysis of the receptor leads to a loss of its binding capacity for the cytoskeleton without altering its estradiol binding properties, indicating that the cytoskeletal binding domain is different from the steroid binding domain. We suggest that the estradiol receptor, generally considered ‘soluble’ in the cytoplasm, might be physiologically associated with cytoskeletal components of the cell cytoplasm.