A ROLE FOR HDJ-2 HSDJ IN CORRECTING SUBNUCLEAR TRAFFICKING, TRANSACTIVATION, AND TRANSREPRESSION DEFECTS OF A GLUCOCORTICOID RECEPTOR ZINC-FINGER MUTANT/

All steroid receptors possess a bipartite nuclear localization signal sequence (NLS) that localizes within the second zinc finger of their D NA-binding domain. Fine-structure mapping of the rat glucocorticoid re ceptor (rGR) NLS identified a composite signal composed of three disti nct proto-NLSs that function effectively when present in unique pairs. At least one of the rGR proto-NLSs appears to influence receptor traf ficking within the nucleus, as revealed by a unique nuclear staining p attern of receptors possessing a paint mutation (i.e., arginine at pos ition 496; R496), at proto-NLS, pNLS-2. Specifically, carboxyl-termina l-truncated rGRs possessing various point mutations at R496 localized within a limited number of large foci in nuclei of transiently transfe cted Cos-1 cells. R496 mutations did not affect subnuclear targeting w hen present in full-length rGR, reflecting a protective effect of the receptor's ligand-binding domain that can be exerted in cis and in tra ns. The effects of rGR R496 mutations on subnuclear targeting were not autonomous because we also observed a coincident localization of hsp7 0, the 70-kDa heat shock protein, within nuclear foci that include R49 6 mutant receptors. The elimination of R496 mistargeting by overexpres sion of an hsp70 partner (i.e., the DnaJ homologue, HDJ-2/HSDJ) sugges ts that the hsp70/DnaJ chaperone system is mobilized to specific sites within the nucleus in response to inappropriate targeting or folding of specific mutant receptors. HDJ-2/HSDJ overexpression also corrects defective transactivation and transrepression activity of R496 mutant GRs. Thus, molecular chaperones, such as members of the hsp70 and DnaJ families, may survey the nucleus for misfolded proteins and actively participate in their refolding into biologically active conformational states.