FK506 BINDING TO THE 56-KILODALTON IMMUNOPHILIN (HSP56) IN THE GLUCOCORTICOID RECEPTOR HETEROCOMPLEX HAS NO EFFECT ON RECEPTOR FOLDING OR FUNCTION

It has recently been reported that the hsp56 component of glucocortico id receptor heterocomplexes is an immunophilin of the FK506 binding cl ass [Yem, A. W., Tomasselli, A. G., Heinrikson, R. L., Zurcher-Neely, H., Ruff, V. A., Johnson, R. A., & Deibel, M. R. (1992) J. Biol. Chem. 267, 2868-2871; Tai, P. K., Albers, M. W., Chang, H., Faber, L. E., & Schreiber, S. L. (1992) Science 256, 1315-1318]. The existence of bin ding proteins for these two potent groups of immunosuppressants in the same molecular complex compels us to ask whether FK506 affects glucoc orticoid receptor function. We show here that hsp56 is a component of the native L-cell glucocorticoid receptor heterocomplex and that [H-3] FK506 binds to the immunopurified, untransformed receptor complex. How ever, at concentrations in excess of those required to occupy all of i ts binding sites on hsp56, FK506 does not affect the steroid binding a ctivity of the receptor nor does it stabilize or dissociate the recept or-hsp90 complex. FK506 does not affect steroid-mediated hsp90 dissoci ation from the receptor in vitro, and it does not affect steroid-media ted nuclear transfer of the receptor or steroid-mediated transcription al enhancement from a reporter in intact cells. When immunopurified mo use glucocorticoid receptor is reconstituted into a heat shock protein complex by rabbit reticulocyte lysate, hsp56 is present in the recons tituted complex in addition to hsp90 and hsp70. FK506, however, does n ot affect reconstitution of the complex or return of the receptor to t he steroid binding state, a change of conformation that occurs upon re ceptor association with hsp90. Although the existence of the glucocort icoid receptor and the FK506 immunophilin in the same heteromolecular complex is indeed provocative, neither enhancement nor inhibition of g lucocorticoid receptor function by FK506 is observed at the molecular level.