The small nucle(ol)ar RNA cap trimethyltransferase is required for ribosome synthesis and intact nucleolar morphology.
Nucleolar morphogenesis is a poorly defined process. Here we report that the Saccharomyces cerevisiae nucleolar trimethyl guanosine synthase I (Tgs1p), which specifically selects the m(7)G cap structure of snRNAs and snoRNAs for m(2,2,7)G 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.
Colau
Geoffroy
G
Fonds National de la Recherche Scientifique, Université Libre de Bruxelles, Institut de Biologie et de Médecine Moléculaires, Charleroi-Gosselies, Belgium.
Thiry
Marc
M
Leduc
Vivian
V
Bordonné
Rémy
R
Lafontaine
Denis L J
DL
D001483:Base Sequence; D002466:Cell Nucleolus / Q000378:metabolism* / Q000648:ultrastructure*; D005800:Genes, Fungal; D008780:Methyltransferases / Q000235:genetics / Q000378:metabolism*; D008854:Microscopy, Electron; D017434:Protein Structure, Tertiary; D012315:RNA Caps / Q000378:metabolism*; D012323:RNA Processing, Post-Transcriptional; D012331:RNA, Fungal / Q000378:metabolism*; D020718:Ribonucleoproteins, Small Nucleolar / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012270:Ribosomes / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism / Q000648:ultrastructure; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017384:Sequence Deletion
D012315:RNA Caps; D012331:RNA, Fungal; D020718:Ribonucleoproteins, Small Nucleolar; D029701:Saccharomyces cerevisiae Proteins; D008780:Methyltransferases; C456271:trimethylguanosine synthase