Nuclear bodies are electron dense, spherical structures with a diameter between 0.2 and 1.0 µm. The function of these nuclear domains is unclear. One class of nuclear bodies contains the tumor suppressor protein PML. Besides in the nucleus, a small number of PML-containing structures of about the same size as nuclear bodies is also present in the cytoplasm. We have investigated whether PML is transported from the nucleus to the cytoplasm and/or vice versa. To this end we injected the PML-specific mAb 5E10 into the cytoplasm of living cells. Subsequently, we monitored translocation of the antibody across the nuclear envelope by indirect immunofluorescent microscopy. It is well known that antibodies in the cytoplasm of living cells do not enter the nucleus, unless as a complex with a karyophilic protein. We observed accumulation into PML-containing nuclear bodies of 5E10 microinjected into the cytoplasm. Control rabbit IgG and a mAb specific for lamin B2 were not translocated to the nucleus. All nuclear PML bodies were labeled simultaneously by 5E10 with gradually increased intensity in time. Labeling of PML-containing nuclear bodies by 5E10 microinjected into the cytoplasm was not affected by inhibition of protein synthesis. These results suggest that the 5E10 antigen PML shuttles between the nucleus and cytoplasm, indicating that nuclear bodies are dynamic structures.
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