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Underphosphorylated retinoblastoma (Rb) protein inhibits progression around the cell cycle by binding to transcription factors like E2F; subsequent hyperphosphorylation of Rb protein releases E2F from the complex so that it can then drive the cell into S phase. We immunolocalized Rb protein in human cells during the cell cycle. Rb protein translocated into nucleoli after DNA replication completed, and the nucleolar Rb was shown to be in the hyperphosphorylated form by immunoblotting. This form, but not its underphosphorylated counterpart, interacted with the nucleolar protein nucleophosmin/B23. The two formed a salt-resistant complex in vitro, and the two could be immunoprecipitated together from nucleolar extracts. These results suggest that hyperphosphorylated Rb protein is imported into nucleoli late in S or G2 phase with nucleophosmin/B23. Analysis of the nucleolar location of Rb protein using various deletion mutants tagged with the green fluorescent protein implicated pocket A of Rb protein as the region responsible for nucleolar targeting; this region also interacted with nucleophosmin/B23. Nucleolar translocation of Rb mutant was inhibited by introducing nucleophosmin/B23 antisense oligomer. These results suggest that nucleolar translocation of Rb protein is promoted by the binding with nucleophosmin/B23 via the pocket A region.

Original publication




Journal article


Exp Cell Res

Publication Date





233 - 241


Active Transport, Cell Nucleus, Cell Cycle, Cell Cycle Proteins, Cell Nucleolus, Cells, Cultured, DNA Replication, Eukaryotic Cells, G2 Phase, Gene Expression Regulation, Humans, Macromolecular Substances, Molecular Conformation, Nuclear Proteins, Oligonucleotides, Antisense, Phosphorylation, Protein Binding, Protein Structure, Tertiary, Retinoblastoma Protein, S Phase