MECHANISMS OF PROGESTERONE-RECEPTOR EXPORT FROM NUCLEI - ROLE OF NUCLEAR-LOCALIZATION SIGNAL, NUCLEAR EXPORT SIGNAL, AND RAN GUANOSINE TRIPHOSPHATE

Steroid hormone receptors are, in most cases, mainly nuclear proteins that undergo a continuous nucleocytoplasmic shuttling. The mechanism o f the nuclear export of these proteins remains largely unknown. To app roach this problem experimentally in vivo, we have prepared cell lines permanently coexpressing the wild-type nuclear progesterone receptor (PR) and a cytoplasmic receptor mutant deleted of its nuclear localiza tion signal (NLS) [(Delta NLS)PR]. Each receptor species was deleted f rom the epitope recognized by a specific monoclonal antibody, thus all owing separated observation of the two receptor forms in the same cell s. Administration of hormone provoked formation of heterodimers during nucleocytoplasmic shuttling and import of (Delta NLS)PR into the nucl eus. Washing out of the hormone allowed us to follow the export of (De lta NLS)PR into the cytoplasm. Microinjection of BSA coupled to a NLS inhibited the export of (Delta NLS)PR. On the contrary, microinjection of BSA coupled to a nuclear export signal (NES) was without effect. M oreover, leptomycin B, which inhibits NES-mediated export, was also wi thout effect. tsBN2 cells contain a thermosensitive RCC1 protein (Ran GTP exchange protein). At the nonpermissive temperature, the nuclear e xport of (Delta NLS)PR could be observed, whereas the export of NES-BS A was suppressed. Microinjection of GTP gamma S confirmed that the exp ort of (Delta NLS)PR was not dependent on GTP hydrolysis. These experi ments show that the nuclear export of PR is not NES mediated but proba bly involves the NLS. It does not involve Ran GTP, and it is not depen dent on the hydrolysis of GTP. The nucleocytoplasmic shuttling of ster oid hormone receptors thus appears to utilize mechanisms different fro m those previously described for some viral, regulatory, and heterogen eous ribonuclear proteins.