THE HSP56 IMMUNOPHILIN COMPONENT OF UNTRANSFORMED STEROID-RECEPTOR COMPLEXES IS LOCALIZED BOTH TO MICROTUBULES IN THE CYTOPLASM AND TO THE SAME NONRANDOM REGIONS WITHIN THE NUCLEUS AS THE STEROID-RECEPTOR

In their unliganded state, mouse glucocorticoid receptors (GR) that ar e overexpressed in the WCL2 line of Chinese hamster ovary cells are di stributed in a nonrandom manner throughout all planes of the nucleus. These untransformed nuclear receptors exist in a heterocomplex contain ing three heat shock proteins, hsp90, hsp70, and hsp56, the latter bei ng an immunophilin of the FK506 binding type whose cellular function i s unknown. Because a knowledge of the cellular distribution of hsp56 c ould provide important clues to its function in steroid-receptor heter ocomplexes, we have examined hsp56 localization in intact cells by ind irect immunofluorescence using the UPJ56 antibody. The majority of hsp 56 is located in the nucleus, with substantial amounts also visualized in the cytoplasm of intact cells. The cytoplasmic hsp56 was examined in rat pulmonary endothelial cells where the protein was found to colo calize with microtubules. The nuclear hsp56 was examined in the WCL2 c ells, where the protein was found by confocal imaging to colocalize th roughout all planes of the nucleus in the same mottled pattern as the overexpressed GR. Like the GR, the nuclear hsp56 is recovered largely in the cytosolic fraction after hypotonic rupture of WCL2 cells. An ob servation potentially related to the microtubule-associated fraction o f hsp56 is that immunoadsorption of hsp56 from WCL2 cytosol is accompa nied by coadsorption of the microtubule-associated protein-1C complex. These observations are discussed with respect to the possible biologi cal functions of hsp56 in the folding and/or cytoplasmic-nuclear traff icking of the receptor.