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J. Biol. Chem., Vol. 282, Issue 38, 27953-27959, September 21, 2007

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SMN-independent Subunits of the SMN Complex

IDENTIFICATION OF A SMALL NUCLEAR RIBONUCLEOPROTEIN ASSEMBLY INTERMEDIATE^*

From the Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6148

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.

Received for publication, March 16, 2007 , and in revised form, July 11, 2007.

^* This work was supported by the Association Française Contre les Myopathies (to A. F. M.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: 415 Curie Blvd., 328 CRB, Philadelphia, PA 19147. Tel.: 215-898-0398; Fax: 215-573-2000; E-mail: gdreyfuss{at}hhmi.upenn.edu.

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