Tel1p preferentially associates with short telomeres to stimulate their elongation

In many organisms, telomeric DNA consists of long tracts of short repeats. Shorter tracts are preferentially lengthened by telomerase, suggesting a conserved mechanism that recognizes and elongates short telomeres. Tel1p, an ATM family checkpoint kinase, plays an important role in telomere elongation, as cells lacking Tel1p have short telomeres and show reduced recruitment of telomerase components to telomeres. We show that Tel1p association increased as telomeres shortened in vivo in the presence or absence of telomerase and that Tel1p preferentially associated with the shortest telomeres. Tel1p association was independent of Tel1p kinase activity and enhanced by Mre11p. Tel1p overexpression simultaneously stimulated telomerase-mediated elongation and Tel1p association with all telomeres. Thus, Tel1p preferentially associates with the shortest telomeres and stimulates their elongation by telomerase.     

Telomerase and ATM/Tel1p protect telomeres from nonhomologous end joining

Telomeres protect chromosome ends from fusing to double-stranded breaks (DSBs). Using a quantitative real-time PCR assay, we show that nonhomologous end joining between a telomere and an inducible DSB was undetectable in wild-type cells, but occurred within a few hours of DSB induction in approximately 1/2000 genomes in telomerase-deficient cells and in >1/1000 genomes in telomerase-deficient cells also lacking the ATM homolog Tel1p. The fused telomeres contained very little telomeric DNA, suggesting that catastrophic telomere shortening preceded fusion. Lengthening of telomeres did not prevent such catastrophic telomere shortening and fusion events. Telomere-DSB fusion also occurred in cells containing a catalytically inactive telomerase and in tel1 mec1 cells where telomerase cannot elongate telomeres. Thus, telomerase and Tel1p function in telomere protection as well as in telomere elongation.     

Tel1ATM dictates the replication timing of short yeast telomeres

Telomerase action is temporally linked to DNA replication. Although yeast telomeres are normally late replicating, telomere shortening leads to early firing of subtelomeric DNA replication origins. We show that double‐strand breaks flanked by short telomeric arrays cause origin firing early in S phase at late‐replicating loci and that this effect on origin firing time is dependent on the Tel1^ATM checkpoint kinase. The effect of Tel1^ATM on telomere replication timing extends to endogenous telomeres and is stronger than that elicited by Rif1 loss. These results establish that Tel1^ATM specifies not only the extent but also the timing of telomerase recruitment.     

S. cerevisiae Tel1p and Mre11p are required for normal levels of Est1p and Est2p telomere association

In diverse organisms, the Mre11 complex and phosphoinositide 3-kinase-related kinases (PIKKs), such as Tel1p and Mec1p from S. cerevisiae, are key mediators of DNA repair and DNA damage checkpoints that also function at telomeres. Here, we use chromatin immunoprecipitation (ChIP) to determine if Mre11p, Tel1p, or Mec1p affects telomere maintenance by promoting recruitment of telomerase subunits to S. cerevisiae telomeres. We find that recruitment of Est2p, the catalytic subunit of telomerase, and Est1p, a telomerase accessory protein, was severely reduced in mre11Delta and tel1Delta cells. In contrast, the levels of Est2p and Est1p binding in late S/G2 phase, the period in the cell cycle when yeast telomerase lengthens telomeres, were indistinguishable in wild-type (WT) and mec1Delta cells. These data argue that Mre11p and Tel1p affect telomere length by promoting telomerase recruitment to telomeres, whereas Mec1p has only a minor role in telomerase recruitment in a TEL1 cell.     

Tel1 and Rif2 oppositely regulate telomere protection at uncapped telomeres in Saccharomyces cerevisiae

It has been well documented that Tel1 positively regulates telomere-end resection by promoting Mre11-Rad50-Xrs2(MRX) activity, while Rif2 negatively regulates telomere-end resection by inhibiting MRX activity. At uncapped telomeres, whether Tel1 or Rif2 plays any role remains largely unknown. In this work, we examined the roles of Tel1 and Rif2 at uncapped telomeres in yku70△ and/or cdc13-1 mutant cells cultured at non-permissive temperature. We found that deletion of TEL1 exacerbates the temperature sensitivity of both yku70△ and cdc13-1 cells. Further epistasis analysis indicated that MRX and Tel1 function in the same pathway in telomere protection. Consistently, TEL1 deletion increases accumulation of Exo1-dependent telomeric single-stranded DNA(ssDNA) at uncapped telomeres, which stimulates checkpoint-dependent cell cycle arrest. Moreover, TEL1 deletion in yku70△ cells facilitates Rad51-dependent Y0 recombination. In contrast, RIF2 deletion in yku70△ cells decreases the accumulation of telomeric ssDNA after 8 h of incubation at the non-permissive temperature of 37℃ and suppresses the temperature sensitivity of yku70△ cells, likely due to the increase of Mre11 association at telomeres.Collectively, our findings indicate that Tel1 and Rif2 regulate telomere protection at uncapped telomeres via their roles in balancing MRX activity in telomere resection.     

Tel1 kinase and subtelomere-bound Tbf1 mediate preferential elongation of short telomeres by telomerase in yeast

Telomerase enables telomere length homeostasis, exhibiting increasing preference for telomeres as their lengths decline. This regulation involves telomere repeat-bound Rap1, which provides a length-dependent negative feedback mechanism, and the Tel1 and Mec1 kinases, which are positive regulators of telomere length. By analysing telomere elongation of wild-type chromosome ends at single-molecule resolution, we show that in tel1Delta cells the overall frequency of elongation decreases considerably, explaining their short telomere phenotype. At an artificial telomere lacking a subtelomeric region, telomere elongation no longer increases with telomere shortening in tel1Delta cells. By contrast, a natural telomere, containing subtelomeric sequence, retains a preference for the elongation of short telomeres. Tethering of the subtelomere binding protein Tbf1 to the artificial telomere in tel1Delta cells restored preferential telomerase action at short telomeres; thus, Tbf1 might function in parallel to Tel1, which has a crucial role in a TG-repeat-controlled pathway for the activation of telomerase at short telomeres.     

Telomerase Recruitment in Saccharomyces cerevisiae Is Not Dependent on Tel1-Mediated Phosphorylation of Cdc13

In Saccharomyces cerevisiae, association between the Est1 telomerase subunit and the telomere-binding protein Cdc13 is essential for telomerase to be recruited to its site of action. A current model proposes that Tel1 binding to telomeres marks them for elongation, as the result of phosphorylation of a proposed S/TQ cluster in the telomerase recruitment domain of Cdc13. However, three observations presented here argue against one key aspect of this model. First, the pattern of Cdc13 phosphatase-sensitive isoforms is not altered by loss of Tel1 function or by mutations introduced into two conserved serines (S249 and S255) in the Cdc13 recruitment domain. Second, an interaction between Cdc13 and Est1, as monitored by a two-hybrid assay, is dependent on S255 but Tel1-independent. Finally, a derivative of Cdc13, cdc13–(S/TQ)₁₁→(S/TA)₁₁, in which every potential consensus phosphorylation site for Tel1 has been eliminated, confers nearly wild-type telomere length. These results are inconsistent with a model in which the Cdc13–Est1 interaction is regulated by Tel1-mediated phosphorylation of the Cdc13 telomerase recruitment domain. We propose an alternative model for the role of Tel1 in telomere homeostasis, which is based on the assumption that Tel1 performs the same molecular task at double-strand breaks (DSBs) and chromosome termini.TELOMERE length homeostasis is a highly regulated process that must balance telomere loss (as the result of incomplete replication and/or nucleolytic degradation) with telomeric repeat addition (through the action of telomerase and/or recombination). In the budding yeast Saccharomyces cerevisiae, a key regulatory event is recruitment of telomerase to chromosome ends by the telomere end-binding protein Cdc13 (Nugent et al. 1996; Evans and Lundblad 1999; Pennock et al. 2001; Bianchi et al. 2004; Chan et al. 2008). Recruitment relies on a direct interaction between Cdc13 and the Est1 subunit of telomerase (Pennock et al. 2001), which brings the catalytic core of the enzyme to its site of action. Disruption of this interaction, due to mutations in either CDC13 (cdc13-2) or EST1 (est1-60), results in an Est (ever-shorter-telomere) phenotype, as manifested by progressive telomere shortening and an eventual senescence phenotype. The recruitment activity of Cdc13, which resides in a 15-kDa N-terminal domain (Pennock et al. 2001), is sufficient to direct telomerase even to nontelomeric sites (Bianchi et al. 2004). As predicted by the recruitment model, association of telomerase with telomeres is greatly reduced in strains expressing the recruitment-defective cdc13-2 allele (Chan et al. 2008).Telomerase action at individual telomeres is highly regulated. Using an assay that monitors telomere addition at single nucleotide resolution (single telomere extension, STEX), Lingner and colleagues showed that only ∼7% of telomeres with wild-type (i.e., 300 bp) length are elongated by telomerase during a single cell cycle (Teixeira et al. 2004). However, as telomere length declines, the extension frequency increases: ∼20% of telomeres 200 bp in length and >40% of 100-bp-long telomeres are elongated (Teixeira et al. 2004; Arneric and Lingner 2007). The mechanism by which telomerase distinguishes short from long telomeres has been the subject of intense investigation. Work from a number of laboratories has led to the proposal that Tel1-dependent phosphorylation of Cdc13 at underelongated telomeres mediates the interaction between Cdc13 and the telomerase-associated Est1 protein, thus ensuring that telomerase is directed to the shortest telomeres in a population. In support of this model, the Est1 and Est2 telomerase subunits exhibit enhanced association with telomeres that have been artificially shortened, whereas Cdc13 displays length-independent association with telomeres (Bianchi and Shore 2007; Sabourin et al. 2007). This suggests that the preferential elongation of shorter telomeres is controlled at the level of recruitment of the telomerase holoenzyme by Cdc13. Furthermore, efficient association of Est1 and Est2 with chromosome ends requires Tel1 and Mre11 (which acts in the same pathway as Tel1 for telomere length regulation; Nugent et al. 1998; Ritchie and Petes 2000) but not Mec1 (Takata et al. 2005; Goudsouzian et al. 2006). Tel1 itself is also telomere bound (Takata et al. 2004), with enhanced binding to shorter telomeres (Bianchi and Shore 2007; Hector et al. 2007; Sabourin et al. 2007; Abdallah et al. 2009), although there is considerable controversy over the degree and timing of Tel1 association with chromosome termini during the cell cycle. As expected for a key regulator of the interaction between Cdc13 and a telomerase subunit, a tel1-Δ strain has short telomeres (Lustig and Petes 1986), although telomere length is not impaired enough to confer the Est phenotype displayed by cdc13-2 and est1-60 strains.Implicit in the above proposal is that Cdc13 must be a direct substrate of Tel1. In support of this, Teng and colleagues reported several years ago that the recruitment domain of Cdc13 has a cluster of potential Tel1 (and/or Mec1) phosphorylation sites (Tseng et al. 2006). Substrates of the DNA damage kinases often contain several closely spaced phosphorylation sites, termed S/TQ cluster domains (SCDs), which are considered a structural hallmark of many DNA damage-response proteins (Traven and Heierhorst 2005). On the basis of in vitro kinase assays with GST fusions to 75- to 90-amino-acid portions of the Cdc13 recruitment domain, Tseng et al. 2006 concluded that four SQ sites in the recruitment domain of Cdc13 are overlapping substrates for both Tel1 and Mec1, leading to the proposal that telomerase recruitment in S. cerevisiae is regulated by Tel1-dependent phosphorylation of Cdc13.The above model makes a key prediction: in a tel1-Δ strain, telomerase should no longer exhibit a length-dependent pattern of elongation. However, preferential elongation of short telomeres still occurs at native chromosome ends in the absence of Tel1 (Arneric and Lingner 2007). In addition, Petes and colleagues have argued, on the basis of epistasis data, that Tel1 performs an indirect role in the telomerase pathway, rather than directly targeting a telomerase regulator (Ritchie et al. 1999; Ritchie and Petes 2000). These observations are not easily explained, if preferential recognition of short telomeres by telomerase is mediated by Tel1-dependent phosphorylation of Cdc13. In this current study, we have re-examined the evidence for phosphorylation of Cdc13 as a regulatory mechanism for telomere length homeostasis. We report on a series of observations that indicate that Tel1 contributes to telomere length control through a mechanism other than phosphorylation of the Cdc13 S/TQ cluster.     

Tetrahymena mutants with short telomeres.

Telomere length is dynamic in many organisms. Genetic screens that identify mutants with altered telomere lengths are essential if we are to understand how telomere length is regulated in vivo. In Tetrahymena thermophila, telomeres become long at 30 degrees, and growth rate slows. A slow-growing culture with long telomeres is often overgrown by a variant cell type with short telomeres and a rapid-doubling rate. Here we show that this variant cell type with short telomeres is in fact a mutant with a genetic defect in telomere length regulation. One of these telomere growth inhibited forever (tgi) mutants was heterozygous for a telomerase RNA mutation, and this mutant telomerase RNA caused telomere shortening when overexpressed in wild-type cells. Several other tgi mutants were also likely to be heterozygous at their mutant loci, since they reverted to wild type when selective pressure for short telomeres was removed. These results illustrate that telomere length can regulate growth rate in Tetrahymena and that this phenomenon can be exploited to identify genes involved in telomere length regulation.     

Counting of Rif1p and Rif2p on Saccharomyces cerevisiae telomeres regulates telomere length

Telomere length is negatively regulated by proteins of the telomeric DNA-protein complex. Rap1p in Saccharomyces cerevisiae binds the telomeric TG(1-3) repeat DNA, and the Rap1p C terminus interacts with Rif1p and Rif2p. We investigated how these three proteins negatively regulate telomere length. We show that direct tethering of each Rif protein to a telomere shortens that telomere proportionally to the number of tethered molecules, similar to previously reported counting of Rap1p. Surprisingly, Rif proteins could also regulate telomere length even when the Rap1p C terminus was absent, and tethered Rap1p counting was completely dependent on the Rif proteins. Thus, Rap1p counting is in fact Rif protein counting. In genetic settings that cause telomeres to be abnormally long, tethering even a single Rif2p molecule was sufficient for maximal effectiveness in preventing the telomere overelongation. We show that a heterologous protein oligomerization domain, the mammalian PDZ domain, when fused to Rap1p can confer telomere length control. We propose that a nucleation and spreading mechanism is involved in forming the higher-order telomere structure that regulates telomere length.     

Tel1 and Rad51 are involved in the maintenance of telomeres with capping deficiency

Vertebrate-like T₂AG₃ telomeres in tlc1-h yeast consist of short double-stranded regions and long single-stranded overhang (G-tails) and, although based on Tbf1-capping activity, they are capping deficient. Consistent with this idea, we observe Y’ amplification because of homologous recombination, even in the presence of an active telomerase. In these cells, Y’ amplification occurs by different pathways: in Tel1⁺ tlc1h cells, it is Rad51-dependent, whereas in the absence of Tel1, it depends on Rad50. Generation of telomeric G-tail, which is cell cycle regulated, depends on the MRX (Mre11-Rad50-Xrs2) complex in tlc1h cells or is MRX-independent in tlc1h tel1Δ mutants. Unexpectedly, we observe telomere elongation in tlc1h lacking Rad51 that seems to act as a telomerase competitor for binding to telomeric G-tails. Overall, our results show that Tel1 and Rad51 have multiple roles in the maintenance of vertebrate-like telomeres in yeast, supporting the idea that they may participate to evolutionary conserved telomere protection mechanism/s acting at uncapped telomeres.     

Reciprocal association of the budding yeast ATM-related proteins Tel1 and Mec1 with telomeres in vivo

The phosphoinositide (PI)-3-kinase-related kinase (PIKK) family proteins Tel1p and Mec1p have been implicated in the telomere integrity of Saccharomyces cerevisiae. However, the mechanism of PIKK-mediated telomere length control remains unclear. Here, we show that Tel1p and Mec1p are recruited to the telomeres at specific times in the cell cycle in a mutually exclusive manner. In particular, Mec1p interacts with the telomeres during late S phase and is associated preferentially with shortened telomeres. We propose a model in which telomere integrity is maintained by the reciprocal association of PIKKs, and Mec1p acts as a sensor for structural abnormalities in the telomeres. Our study suggests a mechanistic similarity between telomere length regulation and DNA double-strand break repair, both of which are achieved by the direct association of PIKKs.     

U3 snoRNP and Rrp5p associate independently with Saccharomyces cerevisiae 35S pre-rRNA, but Rrp5p is essential for association of Rok1p

Biogenesis of eukaryotic ribosomal subunits proceeds via a series of precursor ribonucleoprotein particles that correspond to different stages in the maturation pathway. The different pre-ribosomal particles each contain a distinct complement of non-ribosomal, trans-acting factors that are crucial for correct and efficient progress of the maturation process. Although in recent years we have gained considerable insight into the composition of the pre-ribosomal particles, our knowledge how the ordered association with and their dissociation from the pre-ribosome of these trans-acting factors is controlled is still quite limited. Here, we have studied the mutual dependence between three of these factors, Rrp5p, U3 snoRNP and Rok1p, all essential for the early stages of pre-rRNA processing/assembly, for association with the 35S pre-rRNA in Saccharomyces cerevisiae. Using co-immunoprecipitation assays, we show that Rrp5p and U3 snoRNP associate independently of each other and that the two factors do not detectably interact prior to incorporation into the pre-ribosome. In contrast, association of the putative RNA helicase Rok1p, which is known to genetically interact with Rrp5p, is absolutely dependent on the presence of the latter protein but does not require U3.     

Multiple ATR-Chk1 pathway proteins preferentially associate with checkpoint-inducing DNA substrates

The ATR-Chk1 DNA damage checkpoint pathway is a critical regulator of the cellular response to DNA damage and replication stress in human cells. The variety of environmental, chemotherapeutic, and carcinogenic agents that activate this signal transduction pathway do so primarily through the formation of bulky adducts in DNA and subsequent effects on DNA replication fork progression. Because there are many protein-protein and protein-DNA interactions proposed to be involved in activation and/or maintenance of ATR-Chk1 signaling in vivo, we systematically analyzed the association of a number of ATR-Chk1 pathway proteins with relevant checkpoint-inducing DNA structures in vitro. These DNA substrates included single-stranded DNA, branched DNA, and bulky adduct-containing DNA. We found that many checkpoint proteins show a preference for single-stranded, branched, and bulky adduct-containing DNA in comparison to undamaged, double-stranded DNA. We additionally found that the association of checkpoint proteins with bulky DNA damage relative to undamaged DNA was strongly influenced by the ionic strength of the binding reaction. Interestingly, among the checkpoint proteins analyzed the checkpoint mediator proteins Tipin and Claspin showed the greatest differential affinity for checkpoint-inducing DNA structures. We conclude that the association and accumulation of multiple checkpoint proteins with DNA structures indicative of DNA damage and replication stress likely contribute to optimal ATR-Chk1 DNA damage checkpoint responses.     

Evidence that the S.cerevisiae Sgs1 protein facilitates recombinational repair of telomeres during senescence

RecQ DNA helicases, including yeast Sgs1p and the human Werner and Bloom syndrome proteins, participate in telomere biology, but the underlying mechanisms are not fully understood. Here, we explore the protein sequences and genetic interactors of Sgs1p that function to slow the senescence of telomerase (tlc1) mutants. We find that the S-phase checkpoint function of Sgs1p is dispensable for preventing rapid senescence, but that Sgs1p sequences required for homologous recombination, including the helicase domain and topoisomerase III interaction domain, are essential. sgs1 and rad52 mutations are epistatic during senescence, indicating that Sgs1p participates in a RAD52-dependent recombinational pathway of telomere maintenance. Several mutations that are synthetically lethal with sgs1 mutation and which individually lead to genome instability, including mus81, srs2, rrm3, slx1 and top1, do not speed the senescence of tlc1 mutants, indicating that the rapid senescence of sgs1 tlc1 mutants is not caused by generic genome instability. However, mutations in SLX5 or SLX8, which encode proteins that function together in a complex that is required for viability in sgs1 mutants, do speed the senescence of tlc1 mutants. These observations further define roles for RecQ helicases and related proteins in telomere maintenance.     

Tethered Sir3p nucleates silencing at telomeres and internal loci in Saccharomyces cerevisiae.

Rap1p binds to sites embedded within the Saccharomyces cerevisiae telomeric TG1-3 tract. Previous studies have led to the hypothesis that Rap1p may recruit Sir3p and Sir3p-associating factors to the telomere. To test this, we tethered Sir3p adjacent to the telomere via LexA binding sites in the rap1-17 mutant that truncates the Rap1p C-terminal 165 amino acids thought to contain sites for Sir3p association. Tethering of LexA-Sir3p adjacent to the telomere is sufficient to restore telomeric silencing, indicating that Sir3p can nucleate silencing at the telomere. Tethering of LexA-Sir3p or the LexA-Sir3p(N2O5) gain-of-function protein to a telomeric LexA site hyperrepresses an adjacent ADE2 gene in wild-type cells. Hence, Sir3p recruitment to the telomere is limiting in telomeric silencing. In addition, LexA-Sir3p(N2O5) hyperrepresses telomeric silencing when tethered to a subtelomeric site 3.6 kb from the telomeric tract. This hyperrepression is dependent on the C terminus of Rap1p, suggesting that subtelomeric LexA-Sir3p(N205) can interact with Rap1p-associated factors at the telomere. We also demonstrate that LexA-Sir3p or LexA-Sir3p(N205) tethered in cis with a short tract of telomeric TG1-3 sequences is sufficient to confer silencing at an internal chromosomal position. Internal silencing is enhanced in rap1-17 strains. We propose that sequestration of silencing factors at the telomere limits the efficiency of internal silencing.     

Mec1p associates with functionally compromised telomeres

In many organisms, telomere DNA consists of simple sequence repeat tracts that are required to protect the chromosome end. In the yeast Saccharomyces cerevisiae, tract maintenance requires two checkpoint kinases of the ATM family, Tel1p and Mec1p. Previous work has shown that Tel1p is recruited to functional telomeres with shorter repeat tracts to promote telomerase-mediated repeat addition, but the role of Mec1p is unknown. We found that Mec1p telomere association was detected as cells senesced when telomere function was compromised by extreme shortening due to either the loss of telomerase or the double-strand break binding protein Ku. Exonuclease I effects the removal of the 5' telomeric strand, and eliminating it prevented both senescence and Mec1p telomere association. Thus, in contrast to Tel1p, Mec1p associates with short, functionally compromised telomeres.     

Telomerase reverses epidermal hair follicle stem cell defects and loss of long-term survival associated with critically short telomeres

Organ homeostasis and organismal survival are related to the ability of stem cells to sustain tissue regeneration. As a consequence of accelerated telomerase shortening, telomerase-deficient mice show defective tissue regeneration and premature death. This suggests a direct impact of telomere length and telomerase activity on stem cell biology. We recently found that short telomeres impair the ability of epidermal stem cells to mobilize out of the hair follicle (HF) niche, resulting in impaired skin and hair growth and in the suppression of epidermal stem cell proliferative capacity in vitro. Here, we demonstrate that telomerase reintroduction in mice with critically short telomeres is sufficient to correct epidermal HF stem cell defects. Additionally, telomerase reintroduction into these mice results in a normal life span by preventing degenerative pathologies in the absence of increased tumorigenesis.     

The dynamin-like GTPase Sey1p mediates homotypic ER fusion in S. cerevisiae

The endoplasmic reticulum (ER) forms a network of tubules and sheets that requires homotypic membrane fusion to be maintained. In metazoans, this process is mediated by dynamin-like guanosine triphosphatases (GTPases) called atlastins (ATLs), which are also required to maintain ER morphology. Previous work suggested that the dynamin-like GTPase Sey1p was needed to maintain ER morphology in Saccharomyces cerevisiae. In this paper, we demonstrate that Sey1p, like ATLs, mediates homotypic ER fusion. The absence of Sey1p resulted in the ER undergoing delayed fusion in vivo and proteoliposomes containing purified Sey1p fused in a GTP-dependent manner in vitro. Sey1p could be partially replaced by ATL1 in vivo. Like ATL1, Sey1p underwent GTP-dependent dimerization. We found that the residual ER-ER fusion that occurred in cells lacking Sey1p required the ER SNARE Ufe1p. Collectively, our results show that Sey1p and its homologues function analogously to ATLs in mediating ER fusion. They also indicate that S. cerevisiae has an alternative fusion mechanism that requires ER SNAREs.