38 research outputs found

    Nucleolus: the fascinating nuclear body

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    Nucleoli are the prominent contrasted structures of the cell nucleus. In the nucleolus, ribosomal RNAs are synthesized, processed and assembled with ribosomal proteins. RNA polymerase I synthesizes the ribosomal RNAs and this activity is cell cycle regulated. The nucleolus reveals the functional organization of the nucleus in which the compartmentation of the different steps of ribosome biogenesis is observed whereas the nucleolar machineries are in permanent exchange with the nucleoplasm and other nuclear bodies. After mitosis, nucleolar assembly is a time and space regulated process controlled by the cell cycle. In addition, by generating a large volume in the nucleus with apparently no RNA polymerase II activity, the nucleolus creates a domain of retention/sequestration of molecules normally active outside the nucleolus. Viruses interact with the nucleolus and recruit nucleolar proteins to facilitate virus replication. The nucleolus is also a sensor of stress due to the redistribution of the ribosomal proteins in the nucleoplasm by nucleolus disruption. The nucleolus plays several crucial functions in the nucleus: in addition to its function as ribosome factory of the cells it is a multifunctional nuclear domain, and nucleolar activity is linked with several pathologies. Perspectives on the evolution of this research area are proposed

    Functional ultrastructure of the plant nucleolus

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    Nucleolin, a major conserved multifunctional nucleolar phosphoprotein of proliferating cells

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    10 p.-3 fig.Nucleolin is the major nucleolar protein of animal, plant and yeast proliferating cells. It is enriched in the most soluble nuclear or nucleolar protein extract, containing ribonucleoproteins, from which it has been purified. It has a tripartite structure in which each domain accounts for different functions. Despite its multifunctionality, the best characterized role of nucleolin is in the primary cleavage of pre-rRNA, an early step of ribosome biogenesis. In the nucleolus of proliferating cells, nucleolin is mostly located in the dense fibrillar component, following a vectorial pattern, from the periphery of fibrillar centers outwards. This pattern is lost in quiescent cells in which nucleolin is present in low levels. Nucleolin is the most phosphorylated protein of the soluble nuclear extract. It is phosphorylated by casein kinase II and CDKA, and phosphorylation is closely associated with the role of nucleolin in proliferating cells. During mitosis, nucleolin is transported from the mother to the daughter cell nucleolus in the form of processing particles, together with pre-rRNA precursors and other nucleolar proteins. It forms part of prenucleolar bodies and plays a role in nucleologenesis. Recent studies on the nucleolin function, carried out on samples with inactivated nucleolin genes (siRNA downregulated or mutants) have evidenced that nucleolin is absolutely essential for cell proliferation, for the organization of the nucleolus and for transcription and processing of pre-rRNA. In plants, nucleolin controls the auxin responsiveness, thus being involved in the regulation of plant development.Work performed in the authors’ laboratory was supported by different Grants of the Spanish “PlanNacional de Investigación Científica y Tecnológica”,and particularly by the Grant No.ESP2006-13600-C02-02.Peer reviewe
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