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 glucocorticoid receptor heterocomplexes is an immunophilin of the FK506 binding class [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 binding proteins for these two potent groups of immunosuppressants in the same molecular complex compels us to ask whether FK506 affects glucocorticoid 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. However, at concentrations in excess of those required to occupy all of its binding sites on hsp56, FK506 does not affect the steroid binding activity of the receptor nor does it stabilize or dissociate the receptor-hsp90 complex. FK506 does not affect steroid-mediated hsp90 dissociation from the receptor in vitro, and it does not affect steroid-mediated nuclear transfer of the receptor or steroid-mediated transcriptional enhancement from a reporter in intact cells. When immunopurified mouse glucocorticoid receptor is reconstituted into a heat shock protein complex by rabbit reticulocyte lysate, hsp56 is present in the reconstituted complex in addition to hsp90 and hsp70. FK506, however, does not affect reconstitution of the complex or return of the receptor to the steroid binding state, a change of conformation that occurs upon receptor association with hsp90. Although the existence of the glucocorticoid receptor and the FK506 immunophilin in the same heteromolecular complex is indeed provocative, neither enhancement nor inhibition of glucocorticoid receptor function by FK506 is observed at the molecular level.