Evidence that the FK506-binding immunophilin heat shock protein 56 is required for trafficking of the glucocorticoid receptor from the cytoplasm to the nucleus.

The FK506-binding immunophilin hsp56 (FKBP52) is one of several chaperone proteins associated with untrasformed steroid receptors in a multiprotein heterocomplex. The function of heat shock protein 56 (hsp56) with respect to receptor action is unknown. hsp56 is not required for glucocorticoid receptor heterocomplex assembly or for proper folding of the receptor hormone-binding domain into a high affinity steroid-binding conformation. In intact cells, the majority of the hsp56 is located in the nucleus, with a minority colocalizing with microtubules in the cytoplasm. hsp56 contains a conserved negatively charged domain that we speculate might serve as a nuclear localization signal recognition sequence. Here we show that injection of an antibody raised against this negative sequence into intact L cells impedes subsequent dexamethasone-mediated shift of the glucocorticoid receptor into the nucleus. Nonimmune rabbit serum and an antibody raised against another site on hsp56 do not affect receptor movement. Inhibition of receptor movement by the 419 antibody against the negative sequence is blocked by preincubation with purified hsp56, but not by preincubation with purified hsp90, hsp70, or BSA. These observations are consistent with the possibility that hsp56 is involved in receptor trafficking to the nucleus, possibly functioning as the nuclear localization signal recognition protein. Receptor trafficking to the nucleus is not affected by FK506, indicating that the peptidylprolyl isomerase activity of hsp56 is not involved.