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EMBO reports 6, 1, 70–76 (2005)
doi:10.1038/sj.embor.7400301
Published online: 10 December 2004
Phosphorylation regulates the activity of the SMN complex during assembly of spliceosomal U snRNPs
Matthias Grimmler1, Liane Bauer1, Marjaana Nousiainen2, Roman Körner2, Gunter Meister3 & Utz Fischer1
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1 Institute of Biochemistry, Biocenter at the University of Würzburg, Am Hubland, 97074 Würzburg, Germany
2 Max-Planck-Institute of Biochemistry, Am Klopferspitz 18a, 82152 Martinsried, Germany
3 Rockefeller University, 1230 York Avenue, New York, New York 10021, USA
To whom correspondence should be addressed
Utz Fischer Tel: +49 931 888 4029; Fax: +49 931 888 4028; E-mail: utz.fischer@biozentrum.uni-wuerzburg.de
Received 12 July 2004; Accepted 27 October 2004; Published online 10 December 2004.
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Abstract
The assembly of spliceosomal U-rich small nuclear ribonucleoproteins (U snRNPs) is an ATP-dependent process mediated by the coordinated action of the SMN and the PRMT5 complex. Here, we provide evidence that the activity of this assembly machinery is regulated by means of post-translational modification. We show that two main components of the SMN/PRMT5 system, namely the survival motor neuron (SMN) protein (reduced levels thereof causing spinal muscular atrophy) and pICln, are phosphorylated in vivo. Both proteins share a previously unknown motif containing either one or two phosphoserines. Alteration of these residues in SMN (serines 28 and 31) significantly impairs the activity of the SMN complex. Despite the presence of SMN in both the nucleus and cytoplasm, we find that only the latter promotes efficient SMN-mediated U snRNP assembly activity. As cytoplasmic SMN is phosphorylated to a much larger extent, we hypothesize that this modification is a key activator of the SMN complex.
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