Nucleolin, defective for MPF phosphorylation, localizes normally during mitosis and nucleologenesis

To determine what effect maturation promoting factor (MPE p34(cdc2) kinase/ cyclin B) phosphorylation has on nucleolin's distribution during mitotic nu cleolar disassembly and reassembly, we altered Chinese hamster ovary (CHO) nucleolin (the N protein) such that it cannot be phosphorylated by p34(cdc2 ). As expected, the transiently expressed epitope-tagged N protein showed n o apparent defect in nucleolar localization in interphase CHO cells, even a fter hypotonic shock and recovery to quickly disassemble and then reassembl e interphase nucleoli. In mitotic CHO cells, the N protein localized to the perichromosomal sheath and the cytoplasm, as is typical for nucleolin. Sim ilar to epitope-tagged wild-type nucleolin, the N protein also maintained i ts association with persistent nucleoli characteristic of mitotic Chinese h amster lung (Dede) cells. In synchronized HeLa cells, the N protein again l ocalized to the perichromosomal sheath and the cytoplasm as nucleoli disass embled during prophase. In HeLa cell telophase, the N protein localized nor mally to nucleolus-derived foci within the cytoplasm and prenucleolar bodie s within reforming nuclei. The observations indicate that MPF phosphorylati on is not essential for nucleolin's localizations to the perichromosomal sh eath and the cytoplasm during prophase and metaphase, and that functional M PF phosphorylation sites are not essential for nucleolin's localizations du ring nucleologenesis.