Nucleocytoplasmic trafficking of steroid-free glucocorticoid receptor

Glucocorticoid receptor (GR) recycles between an inactive form complexed wi th heat shock proteins (hsps) and localized to the cytoplasm and a free Lig anded form that regulates specific gene transcription in the nucleus. We re port here that, contrary to previous assumptions, association of GR into hs p-containing complexes is not sufficient to prevent the shuttling or traffi cking of the GR across the nuclear membrane. Following the withdrawal of tr eatment with cortisol or the hormone antagonist RU486, GRs recycled rapidly into hsp associated, hormone-responsive complexes. However, cortisol-withd rawn receptors redistributed to the cytoplasm very slowly (t(1/2) = 8-9 h) and RU486-withdrawn receptors not at all. Persistent localization of these GRs to the nucleus was not due to a gross defect in export, since in both i nstances the complexed nuclear GRs transferred efficiently between heteroka ryon nuclei. Moreover, the addition of a nuclear retention signal to the N terminus of GR induced the transfer of naive receptor to the nucleus in the absence of steroid. These results suggest that the localization of GR to t he cytoplasm is determined by fine control of the rates of transfer of GR a cross the nuclear membrane and/or by active retention that occurs independe ntly from the association of GR with hsps.