INTERNUCLEAR MIGRATION OF CHICKEN PROGESTERONE-RECEPTOR, BUT NOT SIMIAN VIRUS-40 LARGE TUMOR-ANTIGEN, IN TRANSIENT HETEROKARYONS

The chicken progesterone receptor (PR) is a transcriptional regulatory protein that localizes predominantly within the nucleus of hormone-treated and untreated cells. Transient heterokaryons were generated between PR-expressing Cos-1 cells and PR-negative NIH3T3 cells to examine whether PRs are confined to the nucleus or are capable of bidirectionally traversing the nuclear envelope. Migration of PR from Cos-1 to NIH3T3 nuclei was observed in both the presence and absence of hormone. Since de novo PR synthesis was inhibited in heterokaryons with cycloheximide treatment, PRs that localize within NIH3T3 nuclei of heterokaryons must derive from preexisting receptors that were exported from Cos-1 nuclei. Thus, PR, like some nucleolar and heat shock proteins, appears to be capable of shuttling between the nuclear and cytoplasmic compartments. Not all proteins that enter the nucleus exhibit this trait, since simian virus-40 large tumor antigen, endogenously expressed in Cos-1 cells, does not efficiently translocate to NIH3T3 nuclei of heterokaryons, which support internuclear migration of PR. Thus, proteins that may use analogous or identical mechanisms for nuclear import may differentially interact with the nuclear export machinery. Furthermore, the fact that PR and simian virus-40 large tumor antigen localization within nuclei is not identical, as revealed by laser scanning confocal microscopy, supports the notion that nuclear export may be influenced by subnuclear compartmentalization.