Résumé :
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Communication n° 383 Mutations of the survival motor neuron (SMN1) gene are responsible for reduced levels of SMN protein causing SMA disease. The ubiquitous SMN protein is part of a large multiprotein complex that participates to the assembly of proteins and their cognate RNAs. Particularly, SMN complex facilitates the assembly of spliceosomal U small nuclear ribonucleoproteins (snRNPs). Nevertheless, the mechanism underlying the neuromuscular defect remains elusive. Mostly detected in the cytoplasm, SMN is also found in the nucleoplasm, nucleoli and nuclear gems/Cajal bodies (CB), which accumulate newly imported U snRNPs. SMN is a modular protein composed of a central Tudor domain, a K-rich sequence in the N-terminal region, a self-association YG-box and an exon 7 encoded C-terminus. Here, to analyse how SMN accumulates in CB, we have studied the localization of a series of fluorescently tagged proteins (FP) corresponding to SMN deletion mutants using transient over-expression in COS cells. We demonstrate that Tudor domain plays a central role in the nuclear localization of SMN and cooperates to the residency of SMN in CB by a mechanism that requires binding of the K-rich sequence and the YG-box region. The K-rich sequence is also a cryptic nucleolar localization signal of SMA mutant SMN472del5. Furthermore, in type I SMA-derived fibroblasts we find that the reduction of SMN levels correlates with the absence of snRNPs in CB. COS cells transfection with FP-SMNDex7, the most frequent mutation in SMA, results in profound reduction of snRNPs and SMN-interacting protein gemin2 in CB, whereas FP-SMN cells accumulate both snRNPs and gemin2 in CB. Biochemical analyses reveal in vivo interaction between FP-SMNDex7 and the endogenous SMN/gemin2 complex. The possibility to mimic the SMA effect in a model system is a step towards a better understanding of the spatio-temporal trafficking of RNPs and molecular characterization of SMA pathological processes.
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