The assembly of progesterone receptor (PR) heterocomplexes in vitro involves at least eight components of the molecular chaperone machinery, and as earlier reports have shown, these proteins exhibit complex, dynamic, but ordered, interactions with one another and PR. Using the selective hsp90 binding agent geldanamycin (GA), we have found that PR assembly in vitro can be arrested at a previously observed intermediate assembly step. Like mature PR complexes, the intermediate complexes contain hsp90, but they differ from mature complexes by the presence of hsp70, p60, and p48 and the absence of immunophilins and p23. Arrest of PR assembly is likely due to GA's ability to directly block binding of p23 to hsp90. An important functional consequence of GA-mediated assembly arrest in vitro is the inability of the resulting PR complexes to bind progesterone, despite the presence of hsp90 in the receptor complexes. The biological significance of the in vitro observations is demonstrated by GA's ability to (i) rapidly block PR's hormone binding capacity in intact cells and (ii) alter the composition of COS cell PR complexes in a manner similar to that observed during in vitro reconstitutions. An updated model for the cyclic assembly pathway of PR complexes that incorporates the present findings with earlier results is presented.In the absence of hormone, steroid receptors are known to exist in heteromeric complexes containing multiple proteins, including heat shock protein 90 (hsp90) (for reviews see references 20 and 33). In order to understand the role of these proteins in regulating receptor assembly and hormone signalling, we and others have used a cell-free rabbit reticulocyte lysate (RL) system to support assembly of functional progesterone (30) and glucocorticoid (24) receptor complexes in vitro. For progesterone receptor (PR) assembly in vitro, at least eight distinct proteins have been identified as taking part in the ordered assembly of PR complexes (26,31). hsp70 can function as a molecular chaperone in promoting protein folding, and hsp90 may be able to promote folding as well (reviewed in reference 9). The other six proteins have not been shown to have individual chaperone activity, but each commonly associates with hsp70 and/or hsp90, and it is possible that each serves at least as an accessory in chaperone-mediated processes. Thus, all eight receptor-associated proteins can be considered components of the overall molecular chaperone machinery in eukaryotic cytoplasm.Importantly, proper assembly with hsp90 is required to establish and maintain PR's high-affinity progesterone binding state (26), as was previously demonstrated for glucocorticoid receptor (24). In vitro assembly of PR complexes competent for hormone binding requires ATP, Mg 2ϩ , K ϩ , and near-physiological temperature in addition to several protein factors (6-8, 12, 24, 26, 30, 31). A working model outlining the complex and dynamic interactions occurring during PR assembly in vitro was proposed earlier (26). In that model, a transient PR compl...

The ability to bind immunosuppressive drugs such as cyclosporin and FK506 defines the immunophilin family of proteins, and the FK506-binding proteins form the FKBP subfamily of immunophilins. Some FKBPs, notably FKBP12 (the 12-kDa FK506-binding protein), have defined roles in regulating ion channels or cell signaling, and well established structures. Other FKBPs, especially the larger ones, participate in important biological processes, but their exact roles and the structural bases for these roles are poorly defined. FKBP51 (the 51-kDa FKBP) associates with heat shock protein 90 (Hsp90) and appears in functionally mature steroid receptor complexes. In New World monkeys, FKBP51 has been implicated in cortisol resistance. We report here the x-ray structures of human FKBP51, to 2.7 Å, and squirrel monkey FKBP51, to 2.8 Å, by using multiwavelength anomalous dispersion phasing. FKBP51 is composed of three domains: two consecutive FKBP domains and a three-unit repeat of the TPR (tetratricopeptide repeat) domain. This structure of a multi-FKBP domain protein clarifies the arrangement of these domains and their possible interactions with other proteins. The two FKBP domains differ by an insertion in the second that affects the formation of the progesterone receptor complex.

Previous studies on the assembly of progesterone receptor (PR) complexes in vitro have suggested that PR assembly is a dynamic, ordered process involving at least eight nonreceptor proteins. One of these proteins, p60, appears transiently during assembly and is not a component of functionally mature PR complexes. In the present study we observe that a monoclonal antibody specific for p60 can, on the one hand, inhibit formation of mature PR complexes containing heat shock protein 90 (hsp90), p23, and immunophilins and, on the other, enhance recovery of early PR complexes containing hsp70 and Hip (p48). This observation supports a model in which p60 functions at an intermediate stage of PR assembly to facilitate formation of subsequent PR complexes lacking p60. Since p60 is typically found in a complex with hsp90 and hsp70, we have further characterized its interactions with these proteins. P60 can bind either hsp70 or hsp90 independently and in an ATP-independent manner. Since hsp90 and hsp70 do not readily associate on their own, it appears that p60 is the central organizing component of an hsp90-p60-hsp70 complex. Mutational analysis of p60 indicates that the N terminus is required for hsp70 binding, and a central region containing tetratricopeptide repeat motifs is necessary for binding hsp90 and hsp70. The hsp90-p60-hsp70 multichaperone complex is highly dynamic and does not appear to be affected by the hsp90-binding drug geldanamycin. The interactions of hsp70 and hsp90 in intermediate PR complexes are shown to be distinct from their separate interactions in early PR complexes (hsp70) or in mature PR complexes (hsp90). From these results, it appears that p60 is a key mediator in the chaperoned assembly and functional maturation of PR complexes.

A variety of regulatory proteins, including different classes of transcription factors and protein kinases, have been identified in complexes with Hsp90. On careful examination of unactivated progesterone receptor complexes, eight different protein participants have been identified, and each can be considered a component of the cytoplasmic molecular chaperone machinery. These proteins are Hsp90, Hsp70, Hip, p60, p23, FKBP51, FKBP52 and Cyp40. Studies in a cell-free assembly system have helped to define a highly ordered, dynamic pathway for assembly of progesterone receptor complexes. In the present study, target proteins other than progesterone receptor were used in this cell-free system to assemble complexes in vitro and to compare the composition of resulting complexes. Targets used were human estrogen receptor, human Fes protein-tyrosine kinase, human heat shock transcription factor Hsf1, and human aryl hydrocarbon receptor. The striking similarity of resulting target complexes with previously characterized progesterone receptor complexes suggest that each of these targets undergoes a common assembly pathway involving multiple chaperone components in addition to Hsp90.

A cDNA for human FKBP51 has been cloned and sequenced, and protein products have been expressed in both in vitro and bacterial systems. The deduced amino acid sequence for human FKBP51 is 90% identical to sequences of recently described murine proteins and is 55% identical to the sequence of human FKBP52. Human FKBP51 mRNA is expressed in a wide range of tissues, and the protein has peptidylprolyl isomerase activity that is inhibited by FK506 but not cyclosporine. FKBP51 is the same as a previously described progesterone receptor-associated immunophilin that, similar to FKBP52 and cyclophilin 40, is an Hsp90-binding protein and appears in functionally mature steroid receptor complexes along with Hsp90 and p23. Each of the three receptor-associated immunophilins displays interactions with progesterone receptor that are more dynamic than Hsp90-receptor interactions. Whereas FKBP52 and FKBP51 compete about equally well for binding to Hsp90 in a purified system, FKBP51 accumulates preferentially in progesterone receptor complexes assembled in a cell-free system. This observation provides a precedent for differential interactions between Hsp90-associated immunophilins and target proteins such as steroid receptors.

FKBP51, FKBP52, and Cyp40 bind competitively to Hsp90 through their respective tetratricopeptide repeat (TPR) domains, and any one of the three immunophilins can be isolated in mature steroid receptor complexes. During cell-free assembly reactions, FKBP51 associates preferentially with progesterone and glucocorticoid receptors, but less preference is observed in FKBP51 association with estrogen receptor. A number of mutant FKBP forms were generated to map sequences responsible for FKBP51's preferred association with progesterone receptor. A double-point mutation in the peptidyl prolyl isomerase domain of FKBP51 that reduces enzymatic activity by greater than 90% had no observed effect on FKBP51 interactions with progesterone receptor or Hsp90. Coprecipitation of FKBP51 and FKBP52 truncation mutants with Hsp90 indicated that sequences both upstream and downstream of the TPR domain are necessary for Hsp90 binding. FKBP chimeric constructs were also generated and tested for Hsp90 binding and receptor association. The TPR domain of FKBP51 required appropriate downstream sequences for Hsp90 binding, but FKBP52's TPR domain did not. The C-terminal region of FKBP51 that functionally interacts with the TPR domain to permit Hsp90 binding also conferred preferential association with PR. In conclusion, despite the overall similarity of FKBP51 and FKBP52, these two immunophilins associate differentially with steroid receptors, and the difference relates to both the Hsp90-binding TPR domain and to poorly conserved C-terminal sequences.

No abstract

The hsp70-interacting protein Hip participates in the assembly pathway for progesterone receptor complexes. During assembly, Hip appears at early assembly stages in a transient manner that parallels hsp70 interactions. In this study, a cDNA for human Hip was used to develop various mutant Hip forms in the initial mapping of functions to particular Hip structural elements. Hip regions targeted for deletion and/or truncation included the C-terminal region (which has some limited homology with Saccharomyces cerevisiae Sti1 and its vertebrate homolog p60), a glycine-glycine-methionine-proline (GGMP) tandem repeat, and a tetratricopeptide repeat (TPR). Binding of Hip to hsp70's ATPase domain was lost with deletions from the TPR and from an adjoining highly charged region; correspondingly, these Hip mutant forms were not recovered in receptor complexes. Truncation of Hip's Sti1-related C terminus resulted in Hip binding to hsp70 in a manner suggestive of a misfolded peptide substrate; this hsp70 binding was localized to the GGMP tandem repeat. Mutants lacking either the C terminus or the GGMP tandem repeat were still recovered in receptor complexes. Truncations from Hip's N terminus resulted in an apparent loss of Hip homo-oligomerization, but these mutants retained association with hsp70 and were recovered in receptor complexes. This mutational analysis indicates that Hip's TPR is required for binding of Hip with hsp70's ATPase domain. In addition, some data suggest that hsp70's peptide-binding domain may alternately or concomitantly bind to Hip's GGMP repeat in a manner regulated by Sti1-related sequences.