(A) Molecular weight and monoclonals anti-ER recognition of [125I]TAZ-labelled ER isoforms extracted from hydroxylapatite (HAP) with KCl. (left) Part of human breast cancer cytosol pool, after labelling with 1 nmol/l [125I]TAZ for 1 h at 0°C in the presence or absence of a 200-fold excess of radioinert oestradiol, was immunoprecipitated with H222, H226 or ER1D5 anti-ER monoclonal antibodies, and then analyzed by SDS-PAGE and autoradiography. (right) Another part of this cytosol pool was adsorbed onto HAP, labelled with 1 nmol/l [125I]TAZ, extracted with 0.5 mol/l KCl, and immunoprecipitated before being subjected to electrophoresis. (B) Presumed structure of ER isoforms extracted from HAP with KCl. Potential sites of covalent attachment of TAZ [40,41] are indicated by open circles; antigenic sites for anti-ER monoclonal antibodies are shown above ER structure. The predicted ER isoforms extracted from HAP as well as their sizes determined by SDS-PAGE are shown below.