Importance of A/B and C domains of the estrogen receptor for its adsorption to hydroxylapatite.

Regulatory properties of estrogen receptor (ER) result from the existence of functional domains within its primary structure. Thus, A/B and C domains which are rich in tyrosyl residues control gene expression while the E domain confers estrogen binding capacity. Hydroxylapatite (HAP) is known to adsorb ER. Scatchard plot analysis of [3H]estradiol binding patterns of HAP batches to which cytosolic ER had been adsorbed revealed that AB and/or C domains are mainly responsible for this property. Thus, treatment of these batches with the tyrosine reagent tetranitromethane (TNM) led to a dramatic release of adsorbed receptors. This did not occur with ER preparations devoid of exposed ABC domains obtained by selective immunoextraction with H-226 anti-ER monoclonal antibody prior to HAP assay. KC1 treatment (500 mM) of HAP batches also led to a release of bound receptors especially those devoid of exposed ABC domains. Such binding characteristics were also found with full length and truncated ERs produced in yeast: the full length receptor strongly interacted with HAP while the truncated receptor devoid of AB and C domains displayed only a weak adsorption. Additional investigation revealed that estradiol binding to cytosolic ER does not modify its reactivity towards TNM.