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J. Biol. Chem., Vol. 281, Issue 48, 37009-37016, December 1, 2006

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Gemin8 Is Required for the Architecture and Function of the Survival Motor Neuron Complex^*

From the Dulbecco Telethon Institute, Institute of Cell Biology, Consiglio Nazionale delle Ricerche, Monterotondo Scalo, Rome 00016, Italy

The biogenesis of spliceosomal small nuclear ribonucleoproteins (snRNPs) in higher eukaryotes requires the functions of several cellular proteins and includes nuclear as well as cytoplasmic phases. In the cytoplasm, a macromolecular complex containing the survival motor neuron (SMN) protein, Gemin2-8 and Unrip mediates the ATP-dependent assembly of Sm proteins and snRNAs into snRNPs. To carry out snRNP assembly, the SMN complex binds directly to both Sm proteins and snRNAs; however, the contribution of the individual components of the SMN complex to its composition, interactions, and function is poorly characterized. Here, we have investigated the functional role of Gemin8 using novel monoclonal antibodies against components of the SMN complex and RNA interference experiments. We show that Gemin6, Gemin7, and Unrip form a stable cytoplasmic complex whose association with SMN requires Gemin8. Gemin8 binds directly to SMN and mediates its interaction with the Gemin6/Gemin7 heterodimer. Importantly, loss of Gemin6, Gemin7, and Unrip interaction with SMN as a result of Gemin8 knockdown affects snRNP assembly by impairing the SMN complex association with Sm proteins but not with snRNAs. These results reveal the essential role of Gemin8 for the proper structural organization of the SMN complex and the involvement of the heteromeric subunit containing Gemin6, Gemin7, Gemin8, and Unrip in the recruitment of Sm proteins to the snRNP assembly pathway.

Received for publication, August 7, 2006 , and in revised form, October 4, 2006.

^* This work was supported by Telethon-Italy (TCP 02011), "Compagnia di San Paolo" and by a grant from the Muscular Dystrophy Association (U.S.A.). 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.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1 and S2.

¹ To whom correspondence should be addressed: Dulbecco Telethon Institute, Institute of Cell Biology, Consiglio Nazionale delle Ricerche, Via E. Ramarini 32, Monterotondo Scalo, Rome, Italy 00016. Tel.: 39-06-90091326; Fax: 39-06-90091259; E-mail: livio.pellizzoni{at}ibc.cnr.it.

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