Telomerase uses an internal RNA moiety as template for the synthesis of telomere repeats. In Saccharomyces cerevisiae, the telomerase holoenzyme contains the telomerase reverse transcriptase subunit Est2p, the telomerase RNA moiety TLC1, the telomerase associated proteins Est1p and Est3p, and Sm proteins. Here we assess telomerase assembly by determining the localization of telomerase components. We found that Est1p, Est2p and TLC1 can migrate independently of each other to the nucleus. With limiting amounts of TLC1, overexpressed Est1p and Est2p accumulated in the nucleolus, whereas enzymatically active Est2p–TLC1 complexes are distributed over the entire nucleus. The distribution to the nucleoplasm depended on the specific interaction between Est2p and TLC1 but was independent of Est1p and Est3p. Altogether, our results suggest a role of the nucleolus in telomerase biogenesis. We also describe experiments that support a transient cytoplasmic localization of TLC1 RNA.
Telomerase is a ribonucleoprotein (RNP) polymerase that uses an internal RNA moiety as template for the synthesis of telomere repeats (Greider and Blackburn, 1989; Lingner et al., 1997b). In Saccharomyces cerevisiae, telomerase assembly involves association of three Ever Shorter Telomeres (EST1–3) gene products and presumably seven Sm proteins with the telomerase RNA moiety TLC1 (Lundblad and Szostak, 1989; Singer and Gottschling, 1994; Lendvay et al., 1996; Seto et al., 1999; Hughes et al., 2000). It has remained unclear which cellular compartments are involved in the assembly and if it occurs in an ordered manner. In budding yeast, TLC1 is transcribed by RNA polymerase II and polyadenylated (Chapon et al., 1997). Whether the RNA moiety of telomerase is exported from the nucleus to the cytoplasm as other polyadenylated RNAs remained unclear (Huang and Carmichael, 1996). The TLC1 transcript has a trimethyl G cap and is associated with Sm proteins, which also bind to snRNAs essential for intron splicing (Seto et al., 1999). These properties of the TLC1 transcript suggest that it shares the biogenesis pathway with other snRNAs. Most vertebrate snRNAs undergo several steps of maturation in the cytoplasmic compartment (including association with Sm proteins and hypermethylation to a trimethylguanosine cap) before being re‐imported into the nucleus as mature RNPs (Mattaj, 1986; Izaurralde et al., 1995; Huber et al., 1998; Ohno et al., 2000; Will and Luhrmann, 2001). In yeast, hypermethylation of the cap structure may occur in the nucleolus (Mouaikel et al., 2002) and it is uncertain whether maturation of snRNPs involves the cytoplasm.
Here, we report on the subcellular localization of overexpressed telomerase components. We found that Est2p and Est1p localize independently from each other to the nucleolus but that the assembled Est2p–TLC1 complex is mainly nucleoplasmic. We also analyzed the TLC1 distribution in heterokaryons and found that TLC1 is able to migrate from one nucleus to another. In addition, an open reading frame (ORF) was translated when provided as a fusion transcript with TLC1. These results support the notion that TLC1 is exported from the nucleus to the cytoplasm as part of the telomerase biogenesis pathway. We conclude that yeast telomerase biogenesis involves several cellular compartments and nuclear sub‐compartments.
Results and discussion
Localization of telomerase components
In order to determine the localization of telomerase, we replaced the endogenous copy of EST2 with a GFP–EST2 fusion gene whose expression was controlled with the GAL1 promoter to obtain the strain YKF15. Cells carrying the fusion construct did not senesce or show considerable telomere shortening when grown both on glucose (data not shown) or galactose (see below). This shows that the chimeric protein is functional and that low expression due to leakage of GAL1 promoter in glucose is sufficient to prevent the cells from senescing. Nonetheless, detection of GFP–Est2p by fluorescence microscopy was only possible upon induction in galactose. It revealed an enrichment of the protein in a crescent‐shaped zone at the nuclear periphery (Figure 1). This zone was identified as the nucleolus by co‐staining with the nucleolar protein Nop1p (Schimmang et al., 1989). A small amount of GFP–Est2p was detected in the nucleoplasm but not enriched at telomeres that cluster at the nuclear periphery (Figure 1A, telomeres labeled by immunostaining against Rap1p; Gotta et al., 1996). Deletion of the telomerase RNA gene TLC1 or the telomerase protein subunit encoding genes EST1 and EST3 did not affect nuclear import or nucleolar enrichment of Est2p (Figure 2A and B), indicating that this localization is inherent to Est2p itself.
Nuclear localization of an HA‐tagged Est1 was described previously (Zhou et al., 2000), but nuclear versus nucleolar localization was not distinguished. To determine the subcellular localization of Est1p we used the same strategy as for Est2p and found that a functional fusion protein between GFP and Est1p expressed from the GAL1 promoter was also present in the nucleoplasm and enriched in the nucleolus (Figure 1C). To conclude, both Est2p and Est1p were concentrated in the nucleolus when expressed from the GAL1 promoter. Several hypotheses could account for this observation. First, Est2p and Est1p may have accumulated in the nucleolus because their overexpression resulted in an accumulation at an intermediate step of telomerase biogenesis. Secondly, the nucleolus may serve to stockpile mature telomerase and thus participate in the control of the access of telomerase to the telomere. Finally, our finding could simply reflect a general affinity of Est1p and Est2p for a subcellular site containing a high concentration of RNA. The results below indicate that the nucleolus does not stockpile mature telomerase.
In order to detect the RNA subunit of telomerase, we performed in situ hybridization using a DNA‐based probe containing Alexa 546‐derivatized dUTP. We found that overexpressed telomerase RNA was distributed over the entire nucleus with no preferential accumulation in the nucleolus (Figure 1D). The localization of TLC1 did not change upon deletion of EST1, EST2 or EST3 (data not shown).
Active telomerase is localized in the nucleoplasm
Telomerase activity requires Est2p and TLC1, which together make up the catalytic core (Lingner et al., 1997b). We reasoned that overexpression of both moieties would be necessary to see the assembled complex. We therefore transformed the PGAL1‐GFP–EST2 fusion gene harboring strain YKF15 with a plasmid encoding TLC1 under the control of the GAL1 promoter and, as a control, with the empty plasmid pRS314. We observed no difference in growth when both strains were grown in galactose, showing that induction of both GFP–Est2p and TLC1 was not toxic. Direct observation of GFP–Est2p showed that the catalytic subunit was redistributed over the entire nucleus upon TLC1 overexpression (Figure 2A, compare panels 1 and 2). Mutants of TLC1 impaired for binding to Est2p (Livengood et al., 2002) did not mediate the relocalization of Est2p (Figure 2B, panels 1 and 2). Therefore, the redistribution was dependent on the specific interaction between Est2p and TLC1. TLC1 RNA localization remained unchanged under these conditions (data not shown). This suggested that both subunits co‐localize in the nucleoplasm when assembled. To test this hypothesis, we determined in vitro telomerase activity as a measure for assembled telomerase, from strains expressing endogenous levels of telomerase and from strains overexpressing GFP–Est2p, TLC1 or both. Telomerase activity was strongly increased in extracts from the strain overexpressing both Est2p and TLC1 (Figure 2C, lane 4), whereas it was similar to wild type when only one of the components was overexpressed (Figure 2C, lanes 2 and 3). When extract derived from the TLC1 overexpressing strain was mixed with extract from the Est2p overexpressing strain, no increase in telomerase activity was detected, ruling out a productive association of the two subunits in vitro (Figure 2C, compare lanes 2 and 3 with 6). Together, our results indicate that assembled and active Est2p–TLC1 complexes colocalize with each other in the nucleoplasm. However, the increased in vitro telomerase activity was not sufficient to increase telomere length (Figure 2D, compare lane 4 with 1–3). Telomere lengthening may have been limited for example by telomerase recruitment factors such as Est1p or Cdc13p (Evans and Lundblad, 1999), or by components of the telomeric chromatin.
The results above indicated that the Est2p–TLC1 telomerase core complex is mainly nucleoplasmic, whereas unassembled Est2p accumulates in the nucleolus. To examine the role of Est1p or Est3p in Est2p–TLC1 assembly or its relocalization, EST1 and EST3 were deleted separately in the strain overexpressing both Est2p and TLC1 and in the strain overexpressing only Est2p. All resultant strains showed a senescence phenotype, yet direct observation under the microscope revealed that redistribution of GFP–Est2p from nucleolus to nucleoplasm upon TLC1 overexpression did not depend on the presence of Est1p (Figure 2A, panels 3 and 4) or Est3p (Figure 2A, panels 5 and 6). Furthermore, deletion of the Est1p binding site in TLC1 did not affect the TLC1‐mediated relocalization of Est2p to the nucleoplasm (Figure 2B, panels 3 and 4). Thus, Est1p and Est3p do not influence the steady‐state localization of Est2p and TLC1 or their association, which is consistent with previous studies showing that Est1p and Est3p are not required for telomerase activity in vitro (Cohn and Blackburn, 1995; Lingner et al., 1997a).
Nuclear export and re‐import of TLC1
In order to test whether yeast telomerase RNA is exported from the nucleus to the cytoplasm, we analyzed TLC1 RNA redistribution in yeast heterokaryons containing nuclei that overexpress TLC1 and nuclei that express TLC1 at endogenous levels. In yeast, heterokaryons can be obtained by mating two strains of appropriate genotype and by preventing nuclear fusion in the resulting zygote with the dominant kar1‐Δ15 allele (Vallen et al., 1992; Flach et al., 1994). We therefore mated one strain carrying a plasmid encoding TLC1 under the control of the GAL1 promoter with another strain carrying the kar1‐Δ15 allele and the GFP–NOP1 marker (Figure 3A). The population obtained contained up to 25% of zygotes (determined cytologically). It was subsequently incubated for 12 h in selective medium containing galactose. This prevented growth of the parental strains and allowed expression of TLC1 from the GAL1 promoter. In accordance with previous results (Conde and Fink, 1976; Vallen et al., 1992), we obtained a final population with ∼15% of multinucleated cells. Only 2–3% of diploids arose due to leakage of the kar1‐Δ15 allele. The population was fixed and prepared to allow TLC1 detection by in situ hybridization. Multinucleated cells derived from zygotes were unambiguously identified by containing at least one nucleus that stained positive for TLC1 and GFP–Nop1p. In Figure 3B, two representative heterokaryons are shown. The top row shows a heterokaryon (representing 10–60% of identified heterokaryons, depending on the experiment) in which TLC1 RNA was present in all the nuclei of the heterokaryon. Thus, overexpressed TLC1 RNA was able to migrate from one nucleus to another. Furthermore, our results show that both export and import pathways for TLC1 RNA exist. The re‐import of TLC1 into the nucleus argues that trafficking may also occur at physiological levels and that the export of overexpressed TLC1 was not solely due to the saturation of a putative nuclear retention signal.
Our inability to detect TLC1 RNA at endogenous levels precludes performing this experiment without overexpression of TLC1. Therefore, we undertook an independent approach to determine whether TLC1 RNA is exported to the cytoplasm. We reasoned that a transcript present transiently in the cytoplasmic compartment would be translatable by ribosomes. In this case, an ORF provided as a fusion transcript with the TLC1 RNA would allow production of a functional protein. Our results presented as Supplementary data show that a chimeric transcript containing an ORF embedded in TLC1 is translatable. This suggests that endogenous TLC1 may also be exported from the nucleus to the cytoplasm as part of its life cycle. However, we cannot rule out the possibility that the ORF itself recruited the mRNA export machinery (Ohno et al., 2002) or that translation occurred in the nucleus, although to date translation in the nucleus has only been demonstrated for very small peptides (Iborra et al., 2001). Taken together with the heterokaryon analysis, our results support the notion that TLC1 RNA maturation involves a step in the cytoplasm. However, more sensitive detection methods to localize endogenous TLC1 are required to clarify the issue.
In this report, we provide evidence that telomerase biogenesis in S. cerevisiae involves several distinct subcellular compartments: i.e. the nucleolus, the nucleoplasm and the cytoplasm. Intracellular trafficking supports a multistep character of telomerase enzyme assembly. The RNA subunit is subject to several maturation and assembly steps involving polyadenylation (Chapon et al., 1997), hypermethylation at the 5′ end, association with the Sm proteins (Seto et al., 1999) and association with Est proteins. Our results suggest that some of these maturation steps may occur in the cytoplasm. In addition, assembly steps that occur in the nucleolus are more clearly emerging. The association of the telomerase RNA and the catalytic moiety may take place in the nucleolus, as supported by the nucleolar accumulation of overexpressed Est2p and Est1p. The assembly of telomerases from other species is a chaperone‐assisted process (Holt et al., 1999; Licht and Collins, 1999). Since the nucleolus is considered to be a site with a high concentration of trans‐acting factors required for the assembly of ribosomes and other RNPs (Pederson, 1998; Sleeman and Lamond, 1999; Venema and Tollervey, 1999), it may provide factors that assist in the assembly of telomerase. One of these factors could be the recently identified methyltransferase responsible for the cap hypermethylation of snRNAs and snoRNAs (Mouaikel et al., 2002). In support of this, it was shown that vertebrate telomerase RNA contains an H/ACA motif that targets the RNA to nucleoli where it was hypothesized to associate with the catalytic subunit of telomerase (Mitchell et al., 1999; Lukowiak et al., 2001). We found that the assembled Est2p–TLC1 telomerase core is localized in the nucleoplasm. Thus, association of Est2p with TLC1 may trigger its relocation to or its retention in the nucleoplasm. This localization does not depend on the telomerase associated proteins Est1p or Est3p. Thus it is possible that both of these proteins modulate telomerase activity downstream of the relocation of the assembled complex. It is already known that Est1p is required for telomerase recruitment to telomeres (Evans and Lundblad, 1999). In the case of Est3p, it is at present unclear how it affects the action of telomerase at telomeres in vivo.
Immunostaining and TLC1 detection.
Rap1p and Nop1p immunostaining was performed as described previously (Gotta et al., 1996; Teixeira et al., 1997). The probe for in situ hybridization was created by incorporating Alexa Fluor 546‐dUTP (Molecular Probes) in a PCR using pRS314‐TLC1 as a template and oligonucleotides o1 and o2 as primers. The product was purified from unincorporated nucleotides and digested with RsaI and MboI. Fixation and spheroplasting of cells was performed as described previously (Teixeira et al., 1997). Slides were treated for in situ hybridization as described in Gotta et al. (1999) without the RNase treatment. Images were acquired on Zeiss LSM410 or LSM510 confocal microscopes.
Mating experiments were performed essentially as described previously (Flach et al., 1994). Mata parental cells (5 × 106), grown in raffinose tryptophan‐dropout medium, were mixed in equal amounts with the Matα cells and concentrated on 0.8 μm pore size filters. The filters were placed on galactose‐containing media and incubated at 30°C for 5.5 h. The cells were recovered by vortexing the filter in 10 ml of synthetic medium containing galactose and lacking leucine and tryptophan. The suspension was then incubated for 12 h at 30°C with agitation before in situ hybridization.
We thank T. Laroche and P. Heun for advice, and M. Rose, A. Thierry, B. Dujon, D. Gottschling, V. Doye, A. Livengood and T. Cech for sharing strains and plasmids. M.T.T. is a recipient of a postdoctoral fellowship from the Human Frontiers Science Program and K.F. was supported by a PhD fellowship from the Boehringer Ingelheim Fonds. This work was supported by grants from the Swiss National Science Foundation and the Human Frontiers Science Program.
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