title: A novel association of the SMN protein with two major non-ribosomal nucleolar proteins and its implication in spinal muscular atrophy.
abstract: Spinal muscular atrophy (SMA) is caused by the loss of functional survival motor neuron 1 (SMN1) protein. This ubiquitously expressed protein is a component of a novel complex immunodetected in both the cytoplasm and the nucleus, which is associated with complexes involved in mRNA splicing, ribosome biogenesis and transcription. Here, we study a mutant protein corresponding to the N-terminal half of the protein that is encoded by the SMA frameshift mutation SMN 472del5. We show by confocal microscopy that the resulting mutant protein exhibits various distribution patterns in different transiently transfected COS cells. The mutant distributes into the nucleoplasm and/or the nucleolus, whereas the normal SMN protein accumulates at discrete nucleocytoplasmic dot-like structures previously named gems/Cajal bodies. The cell population with the nucleolar distribution is enriched upon treatment with mimosine, a synchronizing drug in late G(1) phase. Co-immunoprecipitation studies carried out on nuclear extracts reveal that both the endogenous SMN and mutant proteins are associated with complexes containing two major non-ribosomal nucleolar proteins, namely nucleolin and protein B23, and that the association is mediated, by among other things, RNA moieties. Both the association of the SMN protein with nucleolin-containing complexes and the nucleolin/B23 complex are disrupted in fibroblasts derived from a type I SMA patient harboring a homozygous SMN1 gene deletion. These findings suggest that altered assembly and/or stability of ribonucleoprotein complexes may contribute to the pathophysiological processes in SMA.
authors.affiliation: INSERM U393, IRNEM Institute, Hôpital Necker-Enfants Malades, 149 Rue de Sèvres, 75743 Paris Cédex 15, France. lefebvre@necker.fr
authors.forename: Suzie
authors.initials: S
authors.lastname: Lefebvre
authors.forename: Philippe
authors.initials: P
authors.lastname: Burlet
authors.forename: Louis
authors.initials: L
authors.lastname: Viollet
authors.forename: Solange
authors.initials: S
authors.lastname: Bertrandy
authors.forename: Céline
authors.initials: C
authors.lastname: Huber
authors.forename: Caroline
authors.initials: C
authors.lastname: Belser
authors.forename: Arnold
authors.initials: A
authors.lastname: Munnich
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology; D019556:COS Cells / Q000378:metabolism; D002466:Cell Nucleolus / Q000502:physiology; D002478:Cells, Cultured; D017362:Cyclic AMP Response Element-Binding Protein; D005347:Fibroblasts / Q000378:metabolism; D019084:Fluorescent Antibody Technique, Indirect; D016368:Frameshift Mutation; D006801:Humans; D015151:Immunoblotting; D018613:Microscopy, Confocal; D008898:Mimosine / Q000494:pharmacology; D009134:Muscular Atrophy, Spinal / Q000378:metabolism*; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D015739:Nocodazole / Q000494:pharmacology; D009687:Nuclear Proteins / Q000378:metabolism*; D009697:Nucleolus Organizer Region / Q000378:metabolism; D000090243:Nucleophosmin; D010750:Phosphoproteins / Q000378:metabolism*; D011233:Precipitin Tests; D016601:RNA-Binding Proteins / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D055532:SMN Complex Proteins; D055533:Survival of Motor Neuron 1 Protein; D014162:Transfection; D000097796:Nucleolin
chemical_list: D000970:Antineoplastic Agents; D017362:Cyclic AMP Response Element-Binding Protein; D008898:Mimosine; D009419:Nerve Tissue Proteins; D015739:Nocodazole; D009687:Nuclear Proteins; D000090243:Nucleophosmin; D010750:Phosphoproteins; D016601:RNA-Binding Proteins; D011993:Recombinant Fusion Proteins; D055532:SMN Complex Proteins; D055533:Survival of Motor Neuron 1 Protein; D000097796:Nucleolin; C526471:SMN1 protein, human