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Vol. 19, Issue 4, 1706-1716, April 2008

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A Novel Role for PA28-Proteasome in Nuclear Speckle Organization and SR Protein Trafficking

Véronique Baldin^*, Muriel Militello^*, Yann Thomas^*, Christine Doucet^*, Weronika Fic, Stephanie Boireau, Isabelle Jariel-Encontre, Marc Piechaczyk, Edouard Bertrand, Jamal Tazi, and Olivier Coux^*

*Centre de Recherche de Biochimie Macromoléculaire (CRBM-CNRS UMR 5237) and Institut de Génétique Moléculaire de Montpellier (IGMM-CNRS UMR 5535), IFR122, Universités Montpellier 1 et 2, Montpellier, France

Submitted July 5, 2007; Revised January 22, 2008; Accepted January 30, 2008
Monitoring Editor: A. Gregory Matera

In eukaryotic cells, proteasomes play an essential role in intracellular proteolysis and are involved in the control of most biological processes through regulated degradation of key proteins. Analysis of 20S proteasome localization in human cell lines, using ectopic expression of its CFP-tagged 7 subunit, revealed the presence in nuclear foci of a specific and proteolytically active complex made by association of the 20S proteasome with its PA28 regulator. Identification of these foci as the nuclear speckles (NS), which are dynamic subnuclear structures enriched in splicing factors (including the SR protein family), prompted us to analyze the role(s) of proteasome-PA28 complexes in the NS. Here, we show that knockdown of these complexes by small interfering RNAs directed against PA28 strongly impacts the organization of the NS. Further analysis of PA28-depleted cells demonstrated an alteration of intranuclear trafficking of SR proteins. Thus, our data identify proteasome-PA28 complexes as a novel regulator of NS organization and function, acting most likely through selective proteolysis. These results constitute the first demonstration of a role of a specific proteasome complex in a defined subnuclear compartment and suggest that proteolysis plays important functions in the precise control of splicing factors trafficking within the nucleus.

Address correspondence to: Olivier Coux (olivier.coux{at}crbm.cnrs.fr)