Résumé :
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Infantile muscular atrophy (SMA) is a group of inherited neuromuscular disorders cause by mutations in the Survival Motor Neuron (SMN) gene. SMAs are characterized by the degeneration of the alpha-motorneurons and muscular atrophy. The ubiquitously expressed SMN protein is part of the essential SMN complex forms of Gemin2 to 8 and unrip. The SMN complex is diffuse in the cytoplasm and concentrates in discrete subnuclear domains named Cajal bodies (CBs). CBs are involved in nuclear functions such as telomerase assembly and maintenance, histone gene transcription, histone mRNA processing, RNA polymerase assembly and ribonucleoprotein (RNP) maturation, including the splicing small nuclear RNP (snRNPs). The reduced number of SMN-positive CBs correlates with the severity of disease in SMA patients, indicating that defective CB targeting of SMN is likely to contribute to the disease physiopathology. The SMN complex participates to the biogenesis of snRNPs in the cytoplasm, their nuclear import and localization in CBs for the final steps of maturation. How the cytoplasmic and nuclear activities of SMN complex are related to each other remains unclear. It is to note however that the nucleocytoplasmic distribution of the complex is dependent on its phosphorylation state. Here, we combined cell biology and proteomic approaches to identify modulators of the localization of SMN in CBs from HeLa cells. By coimmunoprecipitation and in vitro binding studies, we showed that protein phosphatase 1 (PP1) associates with SMN complex. Immunofluorescence experiments revealed that RNA interference-mediated depletion of PP1 enhances the localization of SMN complex and snRNPs in CBs. Furthermore, SMN complex showed increased formation in extracts from PP1-depleted cells compared to controls. Finally, the analyses by 2D gel electrophoresis revealed that the reduction in protein levels of PP1 alters the post-translational modification pattern of SMN that is restored upon the overexpression of PP1. Our findings provide a role for dephosphorylation in the regulation of SMN complex formation and localization in CBs and hopefully, may help to identify therapeutic targets in SMA disease.
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