PROTEIN PHOSPHATASE-5 IS A MAJOR COMPONENT OF GLUCOCORTICOID RECEPTORHSP90 COMPLEXES WITH PROPERTIES OF AN FK506-BINDING IMMUNOPHILIN

Steroid receptors are recovered from hormone-free cells in multiprotei n complexes containing hsp90, p23, an immunophilin, and often some hsp 70, The immunophilin, which can be of the FK506- or cyclosporin A-bind ing class, binds to hsp90 via its tetratricopeptide repeat (TPR) domai n, and different receptor heterocomplexes exist depending upon which i mmunophilin occupies the TPR-binding region of hsp90, We have recently reported that a protein serine/threonine phosphatase that is designat ed PP5 and contains four TPRs binds to hsp90 and is co-purified with t he glucocorticoid receptor (GR) (Chen, M.-S., Silverstein, A. M., Prat t, W. B., and Chinkers, M. (1996) J. Biol. Chem. 271, 32315-32320). In this work, we show that PP5 is recovered with both GR that is nuclear and GR that is cytoplasmic in hormone-free cells. Approximately one-h alf of the GR hsp90 heterocomplexes in L cell cytosol contains an immu nophilin with high affinity FK506 binding activity, such as FKBP51 or FKBP52, and similar to 35% contains PP5. Only a small (but undetermine d) fraction of the native GR hsp90 heterocomplexes contain the cyclosp orin A-binding immunophilin CyP-40. PP5, FKBP52, and CyP-40 exist in s eparate heterocomplexes with hsp90, and competition binding experiment s with the PP5 TPR domain suggest that the three proteins occupy a com mon binding site on hsp90. A 55-residue connecting region between the N-terminal TPR domain of human PP5 and its C-terminal phosphatase doma in has 50% amino acid homology and 22% identity with the central porti on of the peptidylprolyl isomerase domain of human FKBP52, Of the 9 re sidues in this portion of FKBP52 involved in high affinity interaction s with FK506, 3 residues are retained and 4 have homologous substituti ons in PP5, Although immunoadsorbed PP5 did not bind [H-3]FK506, we fo und that both rabbit PP5 in reticulocyte lysate and purified rat PP5 w ere specifically retained by an FK506 affinity matrix. Thus, we propos e that PP5 possesses properties of an immunophilin with low affinity F K506 binding activity and that it determines a major portion of the na tive GR heterocomplexes in L cell cytosol.