IMPORTANCE OF A B-DOMAIN AND C-DOMAIN OF THE ESTROGEN-RECEPTOR FOR ITS ADSORPTION TO HYDROXYLAPATITE/

Regulatory properties of estrogen receptor (ER) result from the existe nce of functional domains within its primary structure. Thus, A/B and C domains which are rich in tyrosyl residues control gene expression w hile the E domain confers estrogen binding capacity. Hydroxylapatite ( HAP) is known to adsorb ER. Scatchard plot analysis of [H-3]estradiol binding patterns of HAP batches to which cytosolic ER had been adsorbe d revealed that AB and/or C domains are mainly responsible for this pr operty. Thus, treatment of these batches with the tyrosine reagent tet ranitromethane (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 a ntibody prior to HAP assay. KCl treatment (500 mM) of HAP batches also led to a release of bound receptors especially those devoid of expose d ABC domains. Such binding characteristics were also found with full length and truncated ERs produced in yeast: the full length receptor s trongly interacted with HAP while the truncated receptor devoid of AB and C domains displayed only a weak adsorption. Additional investigati on revealed that estradiol binding to cytosolic ER does not modify its reactivity towards TNM.