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      The SMN Complex: An Assembly Machine for RNPs

      Cold Spring Harbor symposia on quantitative biology
      Cold Spring Harbor Laboratory Press

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          The survival motor neuron protein in spinal muscular atrophy.

          The 38 kDa survival motor neuron (SMN) protein is encoded by two ubiquitously expressed genes: telomeric SMN (SMN(T)) and centromeric SMN (SMN(C)). Mutations in SMN(T), but not SMN(C), cause proximal spinal muscular atrophy (SMA), an autosomal recessive disorder that results in loss of motor neurons. SMN is found in the cytoplasm and nucleus. The nuclear form is located in structures termed gems. Using a panel of anti-SMN antibodies, we demonstrate that the SMN protein is expressed from both the SMN(T) and SMN(C) genes. Western blot analysis of fibroblasts from SMA patients with various clinical severities of SMA showed a moderate reduction in the amount of SMN protein, particularly in type I (most severe) patients. Immunocytochemical analysis of SMA patient fibroblasts indicates a significant reduction in the number of gems in type I SMA patients and a correlation of the number of gems with clinical severity. This correlation to phenotype using primary fibroblasts may serve as a useful diagnostic tool in an easily accessible tissue. SMN is expressed at high levels in brain, kidney and liver, moderate levels in skeletal and cardiac muscle, and low levels in fibroblasts and lymphocytes. In SMA patients, the SMN level was moderately reduced in muscle and lymphoblasts. In contrast, SMN was expressed at high levels in spinal cord from normals and non-SMA disease controls, but was reduced 100-fold in spinal cord from type I patients. The marked reduction of SMN in type I SMA spinal cords is consistent with the features of this motor neuron disease. We suggest that disruption of SMN(T) in type I patients results in loss of SMN from motor neurons, resulting in the degeneration of these neurons.
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            Spliceosomal UsnRNP biogenesis, structure and function.

            Significant advances have been made in elucidating the biogenesis pathway and three-dimensional structure of the UsnRNPs, the building blocks of the spliceosome. U2 and U4/U6*U5 tri-snRNPs functionally associate with the pre-mRNA at an earlier stage of spliceosome assembly than previously thought, and additional evidence supporting UsnRNA-mediated catalysis of pre-mRNA splicing has been presented.
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              Methylation of Sm proteins by a complex containing PRMT5 and the putative U snRNP assembly factor pICln.

              Seven Sm proteins, termed B/B', D1, D2, D3, E, F, and G, assemble in an ordered manner onto U snRNAs to form the Sm core of the spliceosomal snRNPs U1, U2, U4/U6, and U5. The survival of motor neuron (SMN) protein binds to Sm proteins and mediates in the context of a macromolecular (SMN-) complex the assembly of the Sm core. Binding of SMN to Sm proteins is enhanced by modification of specific arginine residues in the Sm proteins D1 and D3 to symmetrical dimethylarginines (sDMAs), suggesting that assembly might be regulated at the posttranslational level. Here we provide evidence that the previously described pICln-complex, consisting of Sm proteins, the methyltransferase PRMT5, pICln, and two novel factors, catalyzes the sDMA modification of Sm proteins. In vitro studies further revealed that the pICln complex inhibits the spontaneous assembly of Sm proteins onto a U snRNA. This effect is mediated by pICln via its binding to the Sm fold of Sm proteins, thereby preventing specific interactions between Sm proteins required for the formation of the Sm core. Our data suggest that the pICln complex regulates an early step in the assembly of U snRNPs, possibly the transfer of Sm proteins to the SMN-complex.
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                Journal
                10.1101/sqb.2006.71.001

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