NUCLEAR PROGESTERONE-RECEPTOR IS MAINLY HEAT-SHOCK PROTEIN 90-FREE INVIVO

Heat shock protein 90 (hsp'') is associated with many steroid receptor s in tissue homogenates. It is widely accepted that hsp90 regulates th e binding of the receptor to the corresponding gene regulatory element . However there is no unequivocal evidence that steroid receptor-hsp90 complexes are present in the intact cells. We demonstrate here the ab sence of progesterone receptor (PR)-hsp90 complexes in intact target c ell nuclei, using immunohistochemical and biochemical methods to deter mine the location and composition of the nonliganded (aporeceptor) and liganded (holoreceptor) PR complexes. In the chicken oviduct cells, b oth apo- and holoreceptors were nuclear, while hsp90 was exclusively c ytoplasmic. When expressed transiently in HeLa cells, hsp90 was detect ed in the cytoplasm and PR was detected in the nucleus. Their location or staining intensity was not affected when they were coexpressed in the same cells. To confirm that the sensitivity of the immunohistochem ical detection of hsp90 and PR did not differ significantly, a chimeri c hsp90-PR was transiently expressed in HeLa cells. Both hsp90 and PR antigens of the chimera were detected in nuclei with the same intensit y. In homogenates of the same tissue samples that were used for immuno histochemistry, the PR was complexed with hsp90. Hsp90-PR complexes we re formed in vitro when immature bursa of Fabricius, known to contain high levels of hsp90, was homogenized in the presence of hsp90-free ap oreceptor, while holoreceptor did not associate with hsp90. Our data s how that nuclear PR is not complexed with hsp90 in vivo and suggest th at the 8S-PR may be an in vitro artifact generated during tissue proce ssing.