SMN-independent subunits of the SMN complex - Identification of a small nuclear ribonucleoprotein assembly intermediate

Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6148, USA.
Journal of Biological Chemistry (Impact Factor: 4.57). 10/2007; 282(38):27953-9. DOI: 10.1074/jbc.M702317200
Source: PubMed


The survival of motor neurons (SMN) complex is essential for the biogenesis of small nuclear ribonucleoprotein (snRNP) complexes
in eukaryotic cells. Reduced levels of SMN cause the motor neuron degenerative disease, spinal muscular atrophy. We identify
here stable subunits of the SMN complex that do not contain SMN. Sedimentation and immunoprecipitation experiments using cell
extracts reveal at least three complexes composed of Gemin3, -4, and -5; Gemin6, -7, and unrip; and SMN with Gemin2, as well
as free Gemin5. Complexes containing Gemin3-Gemin4-Gemin5 and Gemin6-Gemin7-unrip persist at similar levels when SMN is reduced.
In cells, immunofluorescence microscopy shows differential localization of Gemin5 after cell stress. We further show that
the Gemin5-containing subunits bind small nuclear RNA independently of the SMN complex and without a requirement for exogenous
ATP. ATP hydrolysis is, however, required for displacement of small nuclear RNAs from the Gemin5-containing subunits and their
assembly into snRNPs. These findings demonstrate a modular nature of the SMN complex and identify a new intermediate in the
snRNP assembly process.

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    • "Experimental insights into these potential mechanisms come from the mass spectrometry and confocal data which show that REIIBP has a higher affinity for GEMIN3, 4 and 5 compared to other components of the SMN complex (Table 1). This trio of proteins interact strongly [52]: GEMIN5 is essential for binding and loading the snRNA [53], GEMIN3 acts as an RNA helicase giving access to the RNA, while the function of GEMIN4 is unknown. It is thus plausible that REIIBP catalytic activity could control the loading of snRNAs onto the complex, or alternatively it could diminish the number of functional SMN complex molecules. "
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    ABSTRACT: The chromosomal translocation t(4;14) deregulates MMSET (WHSC1/NSD2) expression and is a poor prognostic factor in multiple myeloma (MM). MMSET encodes two major protein isoforms. We have characterized the role of the shorter isoform (REIIBP) in myeloma cells and identified a clear and novel interaction of REIIBP with members of the SMN (survival of motor neuron) complex that directly affects the assembly of the spliceosomal ribonucleic particles. Using RNA-seq we show that REIIBP influences the RNA splicing pattern of the cell. This new discovery provides novel insights into the understanding of MM pathology, and potential new leads for therapeutic targeting.
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    • "Key Gemin members of the SMN-Gemins complex have been reported to form part of additional multiprotein complexes in which they function independently of the SMN complex [21], [71]. For instance, Gemin3 and Gemin4 form a less abundant complex that co-sediments with polyribosomes and contains Argonaute2 (AGO2) as well as numerous microRNAs (miRNAs) [72], [73], [74]. "
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    ABSTRACT: Membership of the survival motor neuron (SMN) complex extends to nine factors, including the SMN protein, the product of the spinal muscular atrophy (SMA) disease gene, Gemins 2-8 and Unrip. The best-characterised function of this macromolecular machine is the assembly of the Sm-class of uridine-rich small nuclear ribonucleoprotein (snRNP) particles and each SMN complex member has a key role during this process. So far, however, only little is known about the function of the individual Gemin components in vivo. Here, we make use of the Drosophila model organism to uncover loss-of-function phenotypes of Gemin2, Gemin3 and Gemin5, which together with SMN form the minimalistic fly SMN complex. We show that ectopic overexpression of the dead helicase Gem3(ΔN) mutant or knockdown of Gemin3 result in similar motor phenotypes, when restricted to muscle, and in combination cause lethality, hence suggesting that Gem3(ΔN) overexpression mimics a loss-of-function. Based on the localisation pattern of Gem3(ΔN), we predict that the nucleus is the primary site of the antimorphic or dominant-negative mechanism of Gem3(ΔN)-mediated interference. Interestingly, phenotypes induced by human SMN overexpression in Drosophila exhibit similarities to those induced by overexpression of Gem3(ΔN). Through enhanced knockdown we also uncover a requirement of Gemin2, Gemin3 and Gemin5 for viability and motor behaviour, including locomotion as well as flight, in muscle. Notably, in the case of Gemin3 and Gemin5, such function also depends on adequate levels of the respective protein in neurons. Overall, these findings lead us to speculate that absence of any one member is sufficient to arrest the SMN-Gemins complex function in a nucleocentric pathway, which is critical for motor function in vivo.
    Full-text · Article · Dec 2013 · PLoS ONE
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    • "The SMN complex plays a critical role in the assembly of diverse ribonucleoprotein complexes in the nervous system. The functions of Gemin3 and Gemin4 are speculated to be involved in target mRNA recognition and translational repression (Dostie et al., 2003; Battle et al., 2007). RISC also recruits P-100. "
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