Heat shock protein 90 and the nuclear transport of progesterone receptor.

Steroid receptors exist as large oligomeric complexes in hypotonic cell extracts. In the present work, we studied the nuclear transport of the 2 major components of the oligomeric complex, the receptor itself and the heat shock protein 90 (Hsp90), by using different in vitro transport systems: digitonin permeabilized cells and purified nuclei. We demonstrate that the stabilized oligomeric complex of progesterone receptor (PR) cannot be transported into the nucleus and that unliganded PR salt dissociated from Hsp90 is transported into the nucleus. When nonstabilized PR oligomer was introduced into the nuclear transport system, the complex dissociated and the PR but not the Hsp90 was transported into the nucleus. If PR exists as an oligomeric form after synthesis, as suggested by the experiments with reticulocyte lysate, the present results suggest that the complex is short-lived and is dissociated before or during nuclear transport. Thus, the role of Hsp90 in PR action is likely to reside in the Hsp90-assisted chaperoning process of PR preceding nuclear transport of the receptor.