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The Small Nucle(ol)ar RNA Cap Trimethyltransferase Is Required for Ribosome Synthesis and Intact Nucleolar Morphology

Fonds National de la Recherche Scientifique, Université Libre de Bruxelles, Institut de Biologie et de Médecine Moléculaires, Charleroi-Gosselies,¹ Fonds National de la Recherche Scientifique, Laboratoire de Biologie Cellulaire et Tissulaire, Université de Liège, Liège, Belgium,² CNRS-UMR5535, IFR122, Institut de Génétique Moléculaire, Montpellier, France³

Received 3 May 2004/ Returned for modification 27 May 2004/ Accepted 17 June 2004

Nucleolar morphogenesis is a poorly defined process. Here we report that the Saccharomyces cerevisiae nucleolar trimethyl guanosine synthase I (Tgs1p), which specifically selects the m⁷G cap structure of snRNAs and snoRNAs for m^2,2,7G conversion, is required not only for efficient pre-mRNA splicing but also for pre-rRNA processing and small ribosomal subunit synthesis. Mutational analysis indicates that the requirement for Tgs1p in pre-mRNA splicing, but not its involvement in ribosome synthesis, is dependent upon its function in cap trimethylation. In addition, we report that cells lacking Tgs1p showed a striking and unexpected loss of nucleolar structural organization. Tgs1p is not a core component of the snoRNP proteins; however, in vitro, the protein interacts with the KKD/E domain present at the carboxyl-terminal ends of several snoRNP proteins. Strains expressing versions of the snoRNPs lacking the KKD/E domain were also defective for nucleolar morphology and showed a loss of nucleolar compaction. We propose that the transient and functional interactions of Tgs1p with the abundant snoRNPs, through presumed interactions with the KKD/E domain of the snoRNP proteins, contribute substantially to the coalescence of nucleolar components. This conclusion is compatible with a model of self-organization for nucleolar assembly.

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