Human Molecular Genetics, 2002, Vol. 11, No. 9 1017-1027
© 2002 Oxford University Press
A novel association of the SMN protein with two major non-ribosomal nucleolar proteins and its implication in spinal muscular atrophy
INSERM U393, IRNEM Institute, Hôpital Necker-Enfants Malades, 149 Rue de Sèvres, 75743 Paris Cédex 15, France
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 G1 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.
* To whom correspondence should be addressed at: INSERM U393, IRNEM Institute, 149 Rue de Sèvres, 75743 Paris Cédex 15, France. Tel: +33 1 44 49 40 00 ext 97831; Fax: +33 1 47 34 85 14; Email: lefebvre{at}necker.fr
Present address: Laboratoire de Biologie Cellulaire des Membranes, Département de Biologie Cellulaire, Institut Jacques Monod, UMR 7592, CNRS/Universités Paris 6 et Paris 7, 2 Place Jussieu, 75251 Paris Cedex 5, France.
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