A monoclonal antibody (mAB 1C4C10) that reacts specifically with human nuclear proteins IEF 8Z30 and 8Z31 (charge variants; HeLa protein catalogue number; Bravo, R., and J. E. Celis, 1982, Clin. Chem., 28:766-781) has been microinjected into the cytoplasm of cultured cells that either express (primates) or lack these proteins (at least having similar molecular weights and pIs; other species), and its cellular localization has been determined by indirect immunofluorescence. Nuclear localization (nucleolar and nucleoplasmic) of the antibody was observed only in cells expressing these antigens, suggesting that a determinant present in IEF 8Z30 and 8Z31 is required for cytoplasm-nuclear translocation. Nuclear migration was not inhibited by cycloheximide, implying that these proteins may shuttle between nucleus and cytoplasm. The results assumed to support the signal rather than the free diffusion model are further supported by microinjection experiments using antibodies (proliferating cell nuclear antigen/cyclin, DNA) that react with nuclear components but do not recognize cytoplasmic antigens. Furthermore, they raise the possibility that some nonnuclear proteins may be transported to the nucleus by interacting with proteins harboring nuclear location signals.

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