Ca²⁺-dependent nuclear export mediated by calreticulin

We have characterized a pathway for nuclear export of the glucocorticoid receptor (GR) in mammalian cells. This pathway involves the Ca²⁺-binding protein calreticulin (CRT), which directly contacts the DNA binding domain (DBD) of GR and facilitates its delivery from the nucleus to the cytoplasm. In the present study, we investigated the role of Ca²⁺ in CRT-dependent export of GR. We found that removal of Ca²⁺ from CRT inhibits its capacity to stimulate the nuclear export of GR in digitonin-permeabilized cells and that the inhibition is due to the failure of Ca²⁺-free CRT to bind the DBD. These effects are reversible, since DBD binding and nuclear export can be restored by Ca²⁺ addition. Depletion of intracellular Ca²⁺ inhibits GR export in intact cells under conditions that do not inhibit other nuclear transport pathways, suggesting that there is a Ca²⁺ requirement for GR export in vivo. We also found that the Ran GTPase is not required for GR export. These data show that the nuclear export pathway used by steroid hormone receptors such as GR is distinct from the Crm1 pathway. We suggest that signaling events that increase Ca²⁺ could positively regulate CRT and inhibit GR function through nuclear export.