Phosphorylation of NPA58, a rat nuclear pore-associated protein, correlates with its mitotic distribution.

At the onset of mitosis in higher eukaryotic cells, the nuclear envelope and its components including subunits of the nuclear pore complexes are disassembled, and these are reassembled toward the end of mitosis. We have studied the role of protein phosphorylation in this process, by investigating the phosphorylation status of a specific pore-associated protein during mitosis. Using a monoclonal antibody, mAb E2, earlier shown to inhibit nuclear protein import in rat fibroblast cells, we have identified a 58-kDa protein termed NPA58 that is partially associated with nuclear pores based on a high degree of coincident immunofluorescence in dual labeling experiments with mAb 414, a well-studied pore-complex-reactive antibody. NPA58 is specifically phosphorylated during mitosis and dephosphorylated upon release from metaphase arrest. Confocal microscopy analysis shows that NPA58 is dispersed in the cytoplasm early in mitosis when it is phosphorylated, while its relocalization in the reforming nuclear envelope during telophase temporally correlates with its dephosphorylation upon release from metaphase arrest. Our data provide in vivo evidence that the modifications mediated by phosphorylation and dephosphorylation are required for regulating the mitotic localization of a nuclear-pore-associated protein.