The nucleolus: an organelle formed by the act of building a ribosome

Identification of Myb-binding Protein 1A (MYBBP1A) as a Novel Substrate for Aurora B Kinase

“…Although long dubbed "an organelle formed by the act of building a ribosome" (55), the nucleolus has now been shown to contain many proteins unrelated to ribosome biogenesis, including proteins involved in DNA replication and repair, telomere maintenance, protein degradation, and cell cycle regulation, supporting the hypothesis that this organelle fulfills several additional functions (54).…”

Nop58p is a common component of the box C+D snoRNPs that is required for snoRNA stability

“…Eukaryotic ribosomal RNAs (rRNAs) are synthesized from precursor rRNAs (pre-rRNAs) through a complex processing pathway (Fig+ 1; see Eichler & Craig, 1994;Venema & Tollervey, 1995;Sollner-Webb et al+, 1996;Tollervey, 1996 for recent reviews)+ While these processing reactions take place, the pre-rRNAs are covalently modified on both the sugar residues (29-O-methylation) and bases (pseudouridine formation and base methylation) (Maden, 1990;Maden & Hughes, 1997) and assemble with the ribosomal proteins into ribonucleoprotein (RNP) particles (Warner, 1989;Raué & Planta, 1991)+ Most of these steps occur in the nucleolus, a specialized subnuclear compartment (Reeder, 1990;Hernandez-Verdun, 1991;Mélèse & Xue, 1995)+ Eukaryotic nucleoli contain a large number of small, metabolically stable RNAs known collectively as the small nucleolar RNAs (snoRNAs) (reviewed in Fournier & Maxwell, 1993;Bachellerie et al+, 1995;Maxwell & Fournier, 1995); some 150 snoRNA species are predicted to be present in human cells+ Recently, it has become apparent that these snoRNAs fall into two classes that are structurally and functionally distinct (Balakin et al+, 1996;Ganot et al+, 1997b;Tollervey & Kiss, 1997;reviewed in Lafontaine & Tollervey, 1998)+ These are designated the box CϩD and the box HϩACA snoRNAs after conserved sequence elements that are believed to be sites of RNA-protein interactions+ The only exception is the RNA component of the endonuclease RNase MRP, which is related to RNase P (Forster & Altman, 1990;Lygerou et al+, 1994; reviewed in Morrissey & Tollervey, 1995)+ Within each major family of snoRNAs, two functionally distinct groups can be discerned+ A small number of snoRNA species-the box HϩACA snoRNA snR30 and the box CϩD snoRNAs U3 and U14-are required for cleavage of the pre-rRNA at the early processing sites, A 0 , A 1 , and A 2 (Fig+ 1; Li et al+, 1990;Hughes & Ares, 1991;Morrissey & Tollervey, 1993)+ Since these cleavages are required for synthesis of the 18S rRNA, this group of snoRNAs are essential for viability+ In contrast, the vast majority of snoRNAs function as guide RNAs for the covalent modification of the pre-...…”

Two mutant forms of the S1/TPR-containing protein Rrp5p affect the 18S rRNA synthesis in Saccharomyces cerevisiae

“…The nucleolus compartment of eukaryotic cells is devoted to the assembly of large and small ribosomal subunits (reviewed by Mélèse & Xue, 1995)+ The association of almost 80 ribosomal proteins into 60S and 40S subunits occurs concomitantly with the maturation of ribosomal RNA (rRNA) molecules+ The rRNAs have a mixed transcriptional origin+ The nucleolar RNA polymerase I transcribes a single, large rRNA precursor 35S which is processed into three mature rRNAs (18S, 5+8S and 25S)+ The fourth eukaryotic rRNA molecule (5S) is transcribed independently by RNA polymerase III+ The 5S, 5+8S and 25S rRNA species constitute the RNA molecules of mature 60S subunits whereas mature 40S subunits contain only 18S rRNA+ The maturation of the pre-rRNA 35S polymerase I transcript is largely studied in yeast and numerous small nucleolar RNAs (snoRNAs) and protein components involved in this process have been characterized (Eichler & Craig, 1994;Venema & Tollervey, 1995;Tollervey & Kiss, 1997)+ The 35S primary transcript is successively matured by both endonucleolytic and exonucleolytic cleavages that generate stable rRNA precursors whose identification by in vivo labeling or by Northern hybridization has led to the processing pattern drawn in Figure 1+ The nucleic cleavages contribute to a precise excision of both external (59 and 39 ETS) and internal transcribed spacers (ITS1 and ITS2) that interrupt the 35S rRNA sequence+ Aberrations in any of the cleavage steps result in a defective maturation that modifies the reference pattern shown in Figure 1+ In such cases, an increase or decrease in intermediate precursors is evident whereas some aberrant and sometimes deadend pre-rRNA species are detected+ This observation of altered profiles is classically used to determine at which step either a given mutation or the depletion of a cellular component will affect the rRNA maturation process+ These approaches have indeed revealed that a large number of trans-acting factors are required for the same processing step (Venema & Tollervey, 1995)+ For example, the snoRNAs U3, U14, snR30, and snR10 are all essential (or at least important in the case of snR10) for efficient cleavage at sites A 0 , A 1 , and A 2 (Tollervey, 1987;Li et al+, 1990;Hughes & Ares, 1991;Morrissey & Tollervey, 1993; see Fig+ 1B)+ The snoRNAs are believed to function as small nucleolar ribonucleoprotein (snoRNP) complexes rather than as free RNAs (Maxwell & Fournier, 1995) and some nucleolar proteins were shown to be physically associated with snoRNAs (Maxwell & Fournier, 1995;Smith & Steitz, 1997)+ Two RNP particles involved in the yeast rRNA processing are considered in this article: the abovementioned snoRNP complex (Venema & Tollervey, 1995) and the RNase MRP complex …”