Projet ANR-07-BLAN-0062 – NUCLEOLIN : Analyse de nouvelles fonctions de la nucléoline in vivo
Programme : Blanc (BLANC) 2007
Année de financement : 2007
Coordinateur du projet : Monsieur Philippe BOUVET (CNRS DELEGATION REGIONALE RHONE AUVERGNE)
Partenaires : Centre national de la recherche scientifique
Montant : 250000.0 euros
Date de début : 2007
Durée : 48 mois
Résumé : Besides the well known role of the nucleolus in ribosome biogenesis, nucleoli play important roles in the regulation of many fundamental cellular processes, including cell cycle regulation, apoptosis, telomerase production, RNA processing and therefore it is not surprising that many nucleolar proteins appear to be multifunctional proteins. -  Nucleolin, one of the most abundant non-ribosomal proteins of the nucleolus has been the focus of many studies since it was first described 34 years ago. It seems to be involved in many aspects of DNA metabolism, chromatin regulation and appeared to be a good pharmacological target for drug development in addition to its role in RNA polymerase I transcription and pre-ribosomal processing and assembly in pre-ribosomes. -  This 77 KDa protein is highly conserved in higher eukaryotes and analogous proteins could be found in plants and yeast. Nucleolin is composed of three main domains: an N-terminal domain rich in acidic residues; a central domain containing 4 RNA recognition motifs (RBD or RRM motif) and a C-terminal domain rich in arginine and glycine residues (RGG or GAR domain). -  -  The analysis of nucleolin functions is especially challenging because of its astonishingly broad range of localization in nucleoli, nucleoplasm and on the cell surface and of mechanisms in which the protein is believed to be involved (many aspects of DNA and RNA metabolism).  -  -  Several observations suggest that nucleolin is a major actor in promoting cell proliferation. First of all, its amount is correlated with the proliferative status of the cell: nucleolin level is higher in tumours and actively dividing cells and is widely used as a marker of cell proliferation. Secondly, over expression of nucleolin cooperates with oncogenic mutant Ras in a rat embryonic fibroblast transformation assay. -  -  Although the higher level of nucleolin in tumours and actively dividing cells suggests that nucleolin could play a positive role in the production of ribosomal RNA, most of the studies rather show a repressive effect of nucleolin on RNA polymerase I transcription. A strong link between the phosphorylation of nucleolin, its proteolysis and the production of ribosomal RNA has been observed. On the other side, it has been demonstrated that nucleolin phosphorylation and rRNA transcription go hand in hand. Nucleolin phosphorylation could be triggered by a variety of stimuli like androgens and growth factors and the phosphorylation is invariably accompanied by increased rRNA transcription and cell proliferation. -  -  A recent report indicates that nucleolin possesses a histone chaperone activity and helps the transcription through nucleosomes. Histone chaperones are key factors for the dynamic organization of chromatin template. They are not only involved in histone storage, their translocation to the nucleus, histone exchange and deposition onto the DNA for replication dependant chromatin assembly but also increasing number of reports suggest that histone chaperones have also significant contribution in chromatin reorganization during transcription initiation as well as during elongation. -  The recent data showing that nucleolin possesses the properties to activates the remodeling of nucleosomes by SWI/SNF and ACF, to destabilize the nucleosomal structure facilitating the passage of the polymerase and its ability to favor the deposition of histone and the formation of nucleosomal particles (chaperone activity) shed new light on the role played in transcription by this multifunctional factor and might be a key mechanism to explain the different functions related to DNA metabolism attributed to nucleolin. -  -  Many questions about how nucleolin functions in these processes and on the role of histone chaperones are still left open and it would especially interesting to determine if in vivo nucleolin acts as a general factor that modulate chromatin structure or if it acts on specific genes. What are the exact roles