In budding yeast (Saccharomyces cerevisiae), the majority of box H/ACA small nucleolar RNPs (snoRNPs) have been shown to direct site-specific pseudouridylation of rRNA. Among the known protein components of H/ACA snoRNPs, the essential nucleolar protein Cbf5p is the most likely pseudouridine (⌿) synthase. Cbf5p has considerable sequence similarity to Escherichia coli TruBp, a known ⌿ synthase, and shares the "KP" and "XLD" conserved sequence motifs found in the catalytic domains of three distinct families of known and putative ⌿ synthases. To gain additional evidence on the role of Cbf5p in rRNA biosynthesis, we have used in vitro mutagenesis techniques to introduce various alanine substitutions into the putative ⌿ synthase domain of Cbf5p. Yeast strains expressing these mutated cbf5 genes in a cbf5⌬ null background are viable at 25°C but display pronounced cold-and heat-sensitive growth phenotypes. Most of the mutants contain reduced levels of ⌿ in rRNA at extreme temperatures. Substitution of alanine for an aspartic acid residue in the conserved XLD motif of Cbf5p (mutant cbf5D95A) abolishes in vivo pseudouridylation of rRNA. Some of the mutants are temperature sensitive both for growth and for formation of ⌿ in the rRNA. In most cases, the impaired growth phenotypes are not relieved by transcription of the rRNA from a polymerase II-driven promoter, indicating the absence of polymerase I-related transcriptional defects. There is little or no abnormal accumulation of pre-rRNAs in these mutants, although preferential inhibition of 18S rRNA synthesis is seen in mutant cbf5D95A, which lacks ⌿ in rRNA. A subset of mutations in the ⌿ synthase domain impairs association of the altered Cbf5p proteins with selected box H/ACA snoRNAs, suggesting that the functional catalytic domain is essential for that interaction. Our results provide additional evidence that Cbf5p is the ⌿ synthase component of box H/ACA snoRNPs and suggest that the pseudouridylation of rRNA, although not absolutely required for cell survival, is essential for the formation of fully functional ribosomes.In eukaryotes the biosynthesis of rRNA occurs in a specialized organelle known as the nucleolus (33,41,46,56). rRNA is transcribed by RNA polymerase I (Pol I) as a single large precursor, which undergoes a series of endo-and exonucleolytic cleavages to produce mature rRNA species. In the yeast Saccharomyces cerevisiae, the 35S pre-rRNA precursor is processed to produce mature 18S, 5.8S, and 25S RNAs (54). The 5S rRNA and ribosomal proteins are imported into the nucleolus for assembly into precursors of the 40S and 60S ribosomal subunits before their export to the cytoplasm (16,41,46). An interesting feature of rRNA maturation is the extensive modification the 35S precursor undergoes prior to subsequent cleavage events (29,40,39). One such modification, isomerization of uridine to pseudouridine (⌿), is by far the most abundant posttranscriptional modification of rRNA (29,40,39). Formation of ⌿ is also known to occur in tRNAs (49), small nuclear RNAs (sn...
Tankyrase (TANK1) is a human telomere-associated poly(ADP-ribose) polymerase (PARP) that binds the telomere-binding protein TRF1 and increases telomere length when overexpressed. Here we report characterization of a second human tankyrase, tankyrase 2 (TANK2), which can also interact with TRF1 but has properties distinct from those of TANK1. TANK2 is encoded by a 66-kilobase pair gene (TNKS2) containing 28 exons, which express a 6.7-kilobase pair mRNA and a 1166-amino acid protein. The protein shares 85% amino acid identity with TANK1 in the ankyrin repeat, sterile ␣-motif, and PARP catalytic domains but has a unique N-terminal domain, which is conserved in the murine TNKS2 gene. TANK2 interacted with TRF1 in yeast and in vitro and localized predominantly to a perinuclear region, similar to the properties of TANK1. In contrast to TANK1, however, TANK2 caused rapid cell death when highly overexpressed. TANK2-induced death featured loss of mitochondrial membrane potential, but not PARP1 cleavage, suggesting that TANK2 kills cells by necrosis. The cell death was prevented by the PARP inhibitor 3-aminobenzamide. In vivo, TANK2 may differ from TANK1 in its intrinsic or regulated PARP activity or its substrate specificity.Telomeres are the repetitive DNA sequences and specialized proteins that cap the ends of linear chromosomes and protect them from end-to-end fusion. In mammalian cells, loss or disruption of a telomere can cause cellular senescence, cell death, or genomic instability, depending on the genotype and cell context. A variety of events can lead to dysfunctional telomeres. Telomeres can be damaged directly by genotoxic agents and/or faulty DNA repair processes. In addition, the telomeric structure can be disrupted by changes in the expression or function of certain telomere-associated proteins. Finally, telomeres can erode owing to the biochemistry of DNA replication, which leaves 50 -200 bp 1 of 3Ј-telomeric DNA unreplicated upon completion of each S phase. Thus, in the absence of the enzyme telomerase, or another mechanism to replenish telomeric DNA, proliferating cells progressively lose telomeric DNA and eventually acquire one or more critically short or dysfunctional telomeres (1-3).Most normal mammalian cells respond to a critically short or dysfunctional telomere by undergoing cellular senescence (4 -7). This process results in an irreversible arrest of cell proliferation and striking changes in cell function (8). Dysfunctional telomeres can also induce apoptotic cell death, particularly in cells that harbor mutations in one or more cell cycle or DNA damage checkpoints (9 -11). Very little is known about how telomeres signal cells to undergo senescence or apoptosis. However, the recent discovery of a telomere-associated poly(ADPribose) polymerase (PARP) (12) provides a potential mechanism by which telomeres transmit signals to cellular proteins that regulate the senescence and apoptotic responses.PARPs catalyze the formation of branched chains of ADPribose polymers on selected proteins, using NAD ϩ ...
Carbon, Mol. Cell. Biol. 13:4884-4893, 1993). Cbf5p also binds microtubules in vitro and interacts genetically with two known centromere-related protein genes (NDC10/CBF2 and MCK1). However, Cbf5p was found to be nucleolar and is highly homologous to the rat nucleolar protein NAP57, which coimmunoprecipitates with Nopp140 and which is postulated to be involved in nucleolarcytoplasmic shuttling (U. T. Meier, and G. Blobel, J. Cell Biol. 127:1505-1514, 1994). The temperaturesensitive cbf5-1 mutant demonstrates a pronounced defect in rRNA biosynthesis at restrictive temperatures, while tRNA transcription and pre-rRNA and pre-tRNA cleavage processing appear normal. The cbf5-1 mutant cells are deficient in cytoplasmic ribosomal subunits at both permissive and restrictive temperatures. A high-copy-number yeast genomic library was screened for genes that suppress the cbf5-1 temperature-sensitive growth phenotype. SYC1 (suppressor of yeast cbf5-1) was identified as a multicopy suppressor of cbf5-1 and subsequently was found to be identical to RRN3, an RNA polymerase I transcription factor. A cbf5⌬ null mutant is not rescued by plasmid pNOY103 containing a yeast 35S rRNA gene under the control of a Pol II promoter, indicating that Cbf5p has one or more essential functions in addition to its role in rRNA transcription.Cbf5p of the yeast Saccharomyces cerevisiae was originally isolated as one of the major low-affinity centromeric DNA (CEN) binding proteins (23). CBF5 is an essential gene encoding a highly charged protein with a domain containing ten tandem KKE/D repeats. These repeats are homologous to portions of microtubule-associated proteins 1A and 1B in the domain responsible for microtubule binding (40), and Cbf5p has been shown to bind microtubules in vitro (23). Yeast cells containing C-terminal truncated CBF5 genes delay, with replicated genomes, at the G 2 /M phase of the cell cycle, with the replicated DNA being located at the bud junction (23). Overexpression of Cbf5p suppresses the ndc10-1 temperature-sensitive (ts) mutation in the gene specifying the 110-kDa subunit (Cbf2p/Ndc10p) of the multisubunit yeast centromere DNA binding complex, CBF3, and overexpression of meiosis and centromere regulatory kinase Mck1p suppresses a ts mutation in the gene for either Cbf2p or Cbf5p (22). These genetic interactions support a direct or indirect link between Cbf5p and centromeres. However, both Cbf5p and a homologous protein (NAP57) from rats were found to be nucleolar proteins (21,33). While a functional relationship between a nucleolar protein and centromeres is not obvious, some proteins are associated with both the nucleolus and the centromere in higher eukaryotes. It has been known for some time that centromere autoantigens associate with the nucleolus (43), and experiments with autoimmune sera have also shown that a set of nucleolar proteins and ribonuclear proteins relocate around chromosomes during mitosis (16,17).The nucleolar location of Cbf5p may be indicative of a function unrelated to centromeres and chromos...
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