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 are overexpressed in the WCL2 line of Chinese hamster ovary cells are distributed in a nonrandom manner throughout all planes of the nucleus. These untransformed nuclear receptors exist in a heterocomplex containing three heat shock proteins, hsp90, hsp70, and hsp56, the latter being an immunophilin of the FK506 binding type whose cellular function is unknown. Because a knowledge of the cellular distribution of hsp56 could provide important clues to its function in steroid-receptor heterocomplexes, we have examined hsp56 localization in intact cells by indirect immunofluorescence using the UPJ56 antibody. The majority of hsp56 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 colocalize with microtubules. The nuclear hsp56 was examined in the WCL2 cells, where the protein was found by con...