Immunoanalysis of calf uterine progesterone receptor: modulation of receptor-associated 90 kDa heat-shock protein. f.

We have examined the influence of transforming agents on the in vitro modulation of the 90 kDa heat-shock protein (hsp-90) associated with the calf uterine progesterone receptor (PR). This analysis was facilitated by the use of alpha PR6 (Sullivan et al. 1986) (anti-PR monoclonal antibody that recognizes 110 kDa protein of chicken PR, subunit PR-B), which was seen to shift the rate of sedimentation of the untransformed (8S) and thermally transformed (4S) [3H]R5020-receptor complexes from the calf uterine cytosol toward the bottom of the tube. Silver staining of the alpha PR6-purified calf uterine cytosol revealed the presence of two major proteins with Mr 114 kDa and 90 kDa. Affinity-labeling of uterine cytosol with [3H]R5020, however, yielded only one major protein of 114 kDa. Incubation of uterine cytosol with AC88 (Riehl et al. 1985), a monoclonal antibody that recognizes hsp-90, resulted in a precipitation of a single 90 kDa protein which showed electrophoretic mobility similar to the second protein precipitated with alpha PR6. Western blot analysis confirmed that alpha PR6 interacts only with the 114 kDa cytosol protein representing the calf uterine PR. Incubation of PR complexes at 23 degrees C or at 0 degrees C with 0.3 M KCl or 10 mM ATP also caused the dissociation of hsp-90 from the 114 kDa PR protein, although thermal transformation was less effective in dissociating hsp-90 from PR when the ligand binding site was occupied by the antiprogestin RU486. The presence of iodoacetamide (IA) stabilized the nontransformed RU486-bound PR against thermal transformation while there was dissociation of hsp-90 from the R5020-receptor complexes. Results of our study demonstrate that calf uterine PR is represented by a major steroid binding protein of 114 kDa that exists in association with hsp-90. Exposure to transforming conditions leads to dissociation of receptor-associated hsp-90. Furthermore, the inability of IA-treated RU486-occupied PR to transform suggests that transformation of agonist-bound PR involves SH-groups which must be protected from the inactivating influence of IA.