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J. Biol. Chem., Vol. 283, Issue 4, 2060-2069, January 25, 2008

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Characterization of a Short Isoform of Human Tgs1 Hypermethylase Associating with Small Nucleolar Ribonucleoprotein Core Proteins and Produced by Limited Proteolytic Processing^*

Cyrille Girard, Céline Verheggen, Henry Neel, Anne Cammas, Stephan Vagner, Johann Soret, Edouard Bertrand, and Rémy Bordonné¹

From the Institut de Génétique Moléculaire de Montpellier, UMR 5535, IFR 122, CNRS et Université de Montpellier, 1919 Route de Mende, 34293 Montpellier Cedex 5, France and INSERM U563, Université Toulouse III Paul Sabatier, 31052 Toulouse, France

Tgs1 is the hypermethylase responsible for m₃G cap formation of U small nuclear RNAs (U snRNAs) and small nucleolar RNAs (snoRNAs). In vertebrates, hypermethylation of snRNAs occurs in the cytoplasm, whereas this process takes place in the nucleus for snoRNAs. Accordingly, the hypermethylase is found in both compartments with a diffuse localization in the cytoplasm and a concentration in Cajal bodies in the nucleoplasm. In this study, we report that the Tgs1 hypermethylase exists as two species, a full-length cytoplasmic isoform and a shorter nuclear isoform of 65–70 kDa. The short isoform exhibits methyltransferase activity and associates with components of box C/D and H/ACA snoRNPs, pointing to a role of this isoform in hypermethylation of snoRNAs. We also show that production of the short Tgs1 isoform is inhibited by MG132, suggesting that it results from proteasomal limited processing of the full-length Tgs1 protein. Together, our results suggest that proteasome maturation constitutes a mechanism regulating Tgs1 function by generating Tgs1 species with different substrate specificities, subcellular localizations, and functions.

Received for publication, May 22, 2007 , and in revised form, November 16, 2007.

^* This work was supported by the Association pour la Recherche contre le Cancer, the Ligue Nationale contre le Cancer, and CNRS. 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. Fax: 33-467-040-231; E-mail: remy.bordonne{at}igmm.cnrs.fr.

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