PETER PAN, A NEW LINK BETWEEN RIBOSOME BIOGENESIS and Cell Cycle Control

The nucleolus is the nuclear compartment for ribosome biogenesis in eukaryotes. Inhibition of ribosome biogenesis induces the tumor suppressor p53 and inhibits cell cycle progression. In this thesis the molecular linkage between ribosome biogenesis and cell cycle control was studied. Recent work identified the PeBoW complex, composed of Pes1, Bop1, and WDR12, as an essential factor for the processing of the ribosomal 28S RNA and cell cycle progression. To further elucidate the function of PeBoW in cell cycle control, its cellular interaction partners were investigated. In this work Ppan was identified as PeBoW binding factor. Knock-down of Ppan affected cell proliferation and processing of the large ribosomal subunit RNA. The cellular interactions and localization of Ppan and Ppan deletion mutants in response to various kinds of stress were studied in detail. The σ70 and coil-coil domains in Ppan are required for nucleolar targeting, while the N-terminal domain of Ppan can be masked and unmasked, depending on the stress conditions. Strikingly, while most nucleolar factors translocate to the nucleoplasm or into so-called nucleolar caps under stress, Ppan remains in the nucleolar core, possibly involved in the maintenance of a basal nucleolar structure. Finally, with co-immunoprecipitation experiments and the Bimolecular Fluorescence Complementation assay (BiFC) the binding of Hdm2, a p53 specific ubiquitin ligase, to Ppan could be demonstrated. This observation links Ppan to p53-dependent cell cycle checkpoints. Zellzyklus-Kontrollstellen.