Cdc13p: a single-strand telomeric DNA-binding protein with a dual role in yeast telomere maintenance

The role of the Mre11-Rad50-Xrs2 complex in telomerase- mediated lengthening of Saccharomyces cerevisiae telomeres

BACKGROUND

The Saccharomyces Mre11p, Rad50p, and Xrs2p proteins form a complex, called the MRX complex, that is required to maintain telomere length. Cells lacking any one of the three MRX proteins and Mec1p, an ATM-like protein kinase, undergo telomere shortening and ultimately die, phenotypes characteristic of cells lacking telomerase. The other ATM-like yeast kinase, Tel1p, appears to act in the same pathway as MRX: mec1 tel1 cells have telomere phenotypes similar to those of telomerase-deficient cells, whereas the phenotypes of tel1 cells are not exacerbated by the loss of a MRX protein.

RESULTS

The nuclease activity of Mre11p was found to be dispensable for the telomerase-promoting activity of the MRX complex. The association of the single-stranded TG1-3 DNA binding protein Cdc13p with yeast telomeres occurred efficiently in the absence of Tel1p, Mre11p, Rad50p, or Xrs2p. Targeting of catalytically active telomerase to the telomere suppressed the senescence phenotype of mec1 mrx or mec1 tel1 cells. Moreover, when telomerase was targeted to telomeres, telomere lengthening was robust in mec1 mrx and mec1 tel1 cells.

CONCLUSIONS

These data rule out models in which the MRX complex is necessary for Cdc13p binding to telomeres or in which the MRX complex is necessary for the catalytic activity of telomerase. Rather, the data suggest that the MRX complex is involved in recruiting telomerase activity to yeast telomeres.

Exonuclease activity is required for sequence addition and Cdc13p loading at a de novo telomere

The Saccharomyces cerevisiae Mre11p/Rad50p/Xrs2p (MRX) complex is evolutionarily conserved and functions in DNA repair and at telomeres [1-3]. In vivo, MRX is required for a 5' --> 3' exonuclease activity that mediates DNA recombination at double-strand breaks (DSBs). Paradoxically, abolition of this exonuclease activity in MRX mutants results in shortened telomeric DNA tracts. To further explore the role of MRX at telomeres, we analyzed MRX mutants in a de novo telomere addition assay in yeast cells [4]. We found that the MRX genes were absolutely required for telomerase-mediated addition in this assay. Furthermore, we found that Cdc13p, a single-stranded telomeric DNA binding protein essential for telomere DNA synthesis and protection [5], was unable to bind to the de novo telomeric DNA substrate in cells lacking Rad50p. Based on the results from this model system, we propose that the MRX complex helps to prepare telomeric DNA for the loading of Cdc13p, which then protects the chromosome from further degradation and recruits telomerase and other DNA replication components to synthesize telomeric DNA.

Specific binding of single-stranded telomeric DNA by Cdc13p of Saccharomyces cerevisiae

Cdc13p is a single strand telomere-binding protein of Saccharomyces cerevisiae; its telomere-binding region is within amino acids 451-693, Cdc13(451-693)p. In this study, we used purified Cdc13p and Cdc13(451-693)p to characterize their telomere binding activity. We found that the binding specificity of single-stranded TG(1-3) DNA by these two proteins is similar. However, the affinity of Cdc13(451-693)p to DNA was slightly lower than that of Cdc13p. The binding of telomeric DNA by these two proteins was disrupted at NaCl concentrations higher than 0.3 m, indicating that electrostatic interaction contributed significantly to the binding process. Because both proteins bound to strand TG(1-3) DNA positioned at the 3' end, the 5' end, or in the middle of the oligonucleotide substrates, our results indicated that the location of TG(1-3) in single-stranded DNA does not appear to be important for Cdc13p binding. Moreover, using DNase I footprint analysis, the structure of the telomeric DNA complexes of Cdc13p and Cdc13(451-693)p was analyzed. The DNase I footprints of these two proteins to three different telomeric DNA substrates were virtually identical, indicating that the telomere contact region of Cdc13p is within Cdc13(451-693)p. Together, the binding properties of Cdc13p and its binding domain support the theory that the specific binding of Cdc13p to telomeres is an important feature of telomeres that regulate telomerase access and/or differentiate natural telomeres from broken ends.

Renaturation and stabilization of the telomere-binding activity of Saccharomyces Cdc13(451-693)p by L-arginine

Production of recombinant proteins can be valuable in studying their biological functions. However, recombinant proteins expressed in Escherichia coli sometimes form undesirable insoluble aggregates. Solubilization and renaturation of these aggregates becomes a problem that one needs to solve. Here we used recombinant Cdc13(451-693)p as example to show the presence of l-arginine during renaturation greatly enhanced the renaturation efficiency. Cdc13p is the single-stranded telomere-binding protein of yeast Saccharomyces cerevisiae. The telomere-binding domain has been mapped within amino acids 451-693 of Cdc13p, Cdc13(451-693)p. Recombinant Cdc13(451-693)p was expressed in E. coli as insoluble protein aggregates. Purification of insoluble Cdc13(451-693)p was achieved by denaturing the protein with 6 M guanidine-HCl and followed by Ni-nitrilotriacetic acid agarose column chromatography. Renaturation of Cdc13(451-693)p to the active form was achieved by dialyzing denatured protein in the presence of l-arginine. Moreover, the presence of l-arginine was also helped in maintaining the telomere-binding activity of Cdc13(451-693)p. Taking together, l-arginine might have a general application in renaturation of insoluble aggregates.

New function of CDC13 in positive telomere length regulation

Two roles for the Saccharomyces cerevisiae Cdc13 protein at the telomere have previously been characterized: it recruits telomerase to the telomere and protects chromosome ends from degradation. In a synthetic lethality screen with YKU70, the 70-kDa subunit of the telomere-associated Yku heterodimer, we identified a new mutation in CDC13, cdc13-4, that points toward an additional regulatory function of CDC13. Although CDC13 is an essential telomerase component in vivo, no replicative senescence can be observed in cdc13-4 cells. Telomeres of cdc13-4 mutants shorten for about 150 generations until they reach a stable level. Thus, in cdc13-4 mutants, telomerase seems to be inhibited at normal telomere length but fully active at short telomeres. Furthermore, chromosome end structure remains protected in cdc13-4 mutants. Progressive telomere shortening to a steady-state level has also been described for mutants of the positive telomere length regulator TEL1. Strikingly, cdc13-4/tel1Delta double mutants display shorter telomeres than either single mutant after 125 generations and a significant amplification of Y' elements after 225 generations. Therefore CDC13, TEL1, and the Yku heterodimer seem to represent distinct pathways in telomere length maintenance. Whereas several CDC13 mutants have been reported to display elongated telomeres indicating that Cdc13p functions in negative telomere length control, we report a new mutation leading to shortened and eventually stable telomeres. Therefore we discuss a key role of CDC13 not only in telomerase recruitment but also in regulating telomerase access, which might be modulated by protein-protein interactions acting as inhibitors or activators of telomerase activity.

Multiple pathways cooperate in the suppression of genome instability in Saccharomyces cerevisiae

Gross chromosome rearrangements (GCRs), such as translocations, deletion of a chromosome arm, interstitial deletions and inversions, are often observed in cancer cells. Spontaneous GCRs are rare in Saccharomyces cerevisiae; however, the existence of mutator mutants with increased genome instability suggests that GCRs are actively suppressed. Here we show by genetic analysis that these genome rearrangements probably result from DNA replication errors and are suppressed by at least three interacting pathways or groups of proteins: S-phase checkpoint functions, recombination proteins and proteins that prevent de novo addition of telomeres at double-strand breaks (DSBs). Mutations that inactivate these pathways cause high rates of GCRs and show synergistic interactions, indicating that the pathways that suppress GCRs all compete for the same DNA substrates.

Cell cycle control of septin ring dynamics in the budding yeast

Septins constitute a cytoskeletal structure that is conserved in eukaryotes. In Saccharomyces cerevisiae, the Cdc3, Cdc10, Cdc11, Cdc12 and Shs1/Sep7 septins assemble as a ring that marks the cytokinetic plane throughout the budding cycle. This structure participates in different aspects of morphogenesis, such as selection of cell polarity, localization of chitin synthesis, the switch from hyperpolar to isotropic bud growth after bud emergence and the spatial regulation of septation. The septin cytoskeleton assembles at the pre-bud site before bud emergence, remains there during bud growth and duplicates at late mitosis eventually disappearing after cell separation. Using a septin-GFP fusion and time-lapse confocal microscopy, we have determined that septin dynamics are maintained in budding zygotes and during unipolar synchronous growth in pseudohyphae. By means of specific cell cycle arrests and deregulation of cell cycle controls we show that septin assembly is dependent on G1 cyclin/Cdc28-mediated cell cycle signals and that the small GTPase Cdc42, but not Rho1, are essential for this event. However, during bud growth, the septin ring shapes a bud-neck-spanning structure that is unaffected by failures in the regulation of mitosis, such as activation of the DNA repair or spindle assembly checkpoints or inactivation of the anaphase-promoting complex (APC). At the end of the cell cycle, the splitting of the ring into two independent structures depends on the function of the mitotic exit network in which the protein phosphatase Cdc14 participates. Our data support a role of cell cycle control mechanisms in the regulation of septin dynamics to accurately coordinate morphogenesis throughout the budding process in yeast.

Turning telomeres off and on

We envision multiple steps in telomere maintenance, based largely on genetic data from budding yeast. First, the telomere must unfold or open itself such that the free end is accessible to the appropriate enzymatic machinery. Second, telomerase must be recruited, together with the DNA replication machinery that synthesizes the C-rich strand. The processivity of telomerase is regulated both by a length-sensing feedback mechanism and by second-strand synthesis. Finally, the telosome refolds into a protective end structure. If telomerase is nonfunctional, recombination may occur once telomeres are open. Multiple pathways regulate these different steps, producing a highly dynamic chromosomal cap.

Telomeres and their control

Telomeres are DNA and protein structures that form complexes protecting the ends of chromosomes. Understanding of the mechanisms maintaining telomeres and insights into their function have advanced considerably in recent years. This review summarizes the currently known components of the telomere/telomerase functional complex, and focuses on how they act in the control of processes occurring at telomeres. These include processes acting on the telomeric DNA and on telomeric proteins. Key among them are DNA replication and elongation of one telomeric DNA strand by telomerase. In some situations, homologous recombination of telomeric and subtelomeric DNA is induced. All these processes act to replenish or restore telomeres. Conversely, degradative processes that shorten telomeric DNA, and nonhomologous end-joining of telomeric DNA, can lead to loss of telomere function and genomic instability. Hence they too must normally be tightly controlled.

Ten1 functions in telomere end protection and length regulation in association with Stn1 and Cdc13

In Saccharomyces cerevisiae, Cdc13 has been proposed to mediate telomerase recruitment at telomere ends. Stn1, which associates with Cdc13 by the two-hybrid interaction, has been implicated in telomere maintenance. Ten1, a previously uncharacterized protein, was found to associate physically with both Stn1 and Cdc13. A binding defect between Stn1-13 and Ten1 was responsible for the long telomere phenotype of stn1-13 mutant cells. Moreover, rescue of the cdc13-1 mutation by STN1 was much improved when TEN1 was simultaneously overexpressed. Several ten1 mutations were found to confer telomerase-dependent telomere lengthening. Other, temperature-sensitive, mutants of TEN1 arrested at G(2)/M via activation of the Rad9-dependent DNA damage checkpoint. These ten1 mutant cells were found to accumulate single-stranded DNA in telomeric regions of the chromosomes. We propose that Ten1 is required to regulate telomere length, as well as to prevent lethal damage to telomeric DNA.

Cdc13 both positively and negatively regulates telomere replication

Cdc13 is a single-strand telomeric DNA-binding protein that positively regulates yeast telomere replication by recruiting telomerase to chromosome termini through a site on Cdc13 that is eliminated by the cdc13-2 mutation. Here we show that Cdc13 has a separate role in negative regulation of telomere replication, based on analysis of a new mutation, cdc13-5. Loss of this second regulatory activity results in extensive elongation of the G strand of the telomere by telomerase, accompanied by a reduced ability to coordinate synthesis of the C strand. Both the cdc13-5 mutation and DNA polymerase alpha mutations (which also exhibit elongated telomeres) are suppressed by increased expression of the Cdc13-interacting protein Stn1, indicating that Stn1 coordinates action of the lagging strand replication complex with the regulatory activity of CDC13. However, the association between Cdc13 and Stn1 is abolished by cdc13-2, the same mutation that eliminates the interaction between Cdc13 and telomerase. We propose that Cdc13 participates in two regulatory steps-first positive, then negative-as a result of successive binding of telomerase and the negative regulator Stn1 to overlapping sites on Cdc13. Thus, Cdc13 coordinates synthesis of both strands of the telomere by first recruiting telomerase and subsequently limiting G-strand synthesis by telomerase in response to C-strand replication.

RAD50 function is essential for telomere maintenance in Arabidopsis

We have identified and characterized an Arabidopsis thaliana rad50 mutant plant containing a T-DNA insertion in the AtRAD50 gene and showing both meiotic and DNA repair defects. We report here that rad50/rad50 mutant cells show a progressive shortening of telomeric DNA relative to heterozygous rad50/RAD50 controls and that the mutant cell population rapidly enters a crisis, with the majority of the cells dying. Surviving rad50 mutant cells have longer telomeres than wild-type cells, indicating the existence in plants of an alternative RAD50-independent mechanism for telomere maintenance. These results report the role of a protein essential for double-strand break repair in telomere maintenance in higher eukaryotes.

Cdc13 delivers separate complexes to the telomere for end protection and replication

In Saccharomyces cerevisiae, the telomere binding protein Cdc13 mediates telomere replication by recruiting telomerase, and also performs an essential function in chromosome end protection. We show here that delivery of the Stn1 protein to the telomere, by fusing the DNA binding domain of Cdc13 (DBD(CDC13)) to Stn1, is sufficient to rescue the lethality of a cdc13 null strain and, hence, provide end protection. Telomere replication is still defective in this strain, but can be restored by delivering telomerase to the telomere as a DBD(CDC13)-telomerase fusion. These results establish Stn1 as the primary effector of chromosome end protection, whereas the principal function of Cdc13 is to provide a loading platform to recruit complexes that provide end protection and telomere replication.

t-loops at trypanosome telomeres

Mammalian telomeres form large duplex loops (t-loops) that may sequester chromosome ends by invasion of the 3' TTAGGG overhang into the duplex TTAGGG repeat array. Here we document t-loops in Trypanosoma brucei, a kinetoplastid protozoan with abundant telomeres due to the presence of many minichromosomes. These telomeres contained 10-20 kb duplex TTAGGG repeats and a 3' TTAGGG overhang. Electron microscopy of psoralen/UV cross-linked DNA revealed t-loops in enriched telomeric restriction fragments and at the ends of isolated minichromosomes. In mammals, t-loops are large (up to 25 kb), often comprising most of the telomere. Despite similar telomere lengths, trypanosome t-loops were much smaller (approximately 1 kb), indicating that t-loop sizes are regulated. Coating of non-cross-linked minichromosomes with Escherichia coli single-strand binding protein (SSB) often revealed 3' overhangs at both telomeres and several cross-linked minichromosomes had t-loops at both ends. These results suggest that t-loops and their prerequisite 3' tails can be formed on the products of both leading and lagging strand synthesis. We conclude that t-loops are a conserved feature of eukaryotic telomeres.

Tof1p regulates DNA damage responses during S phase in Saccharomyces cerevisiae

A tof1 mutant was recovered in a screen aimed at identifying genes involved specifically in the S phase branch of the MEC1-dependent DNA damage response pathway. The screen was based on the observation that mutants missing this branch are particularly dependent on the cell cycle-wide branch and, therefore, on RAD9, for surviving DNA damage. tof1 and rad9 conferred synergistic sensitivity to MMS, UV, and HU, and the double mutant was incapable of slowing S phase in response to MMS, inducing RNR3 transcription in response to UV, and phosphorylating Rad53p in response to HU. TOF1's contribution to DNA damage response appeared to be restricted to S phase, since TOF1 did not contribute to UV-induced transcription during G1 or to the cdc13-1-induced block to anaphase in G2/M. I suggest a model in which Tof1p functions to link Mec1p with Rad53p.

Telomere-binding and Stn1p-interacting activities are required for the essential function of Saccharomyces cerevisiae Cdc13p

Yeast Saccharomyces cerevisiae Cdc13p is the telomere-binding protein that protects telomeres and regulates telomere length. It is documented that Cdc13p binds specifically to single-stranded TG(1-3) telomeric DNA sequences and interacts with Stn1p. To localize the region for single-stranded TG(1-3) DNA binding, Cdc13p mutants were constructed by deletion mutagenesis and assayed for their binding activity. Based on in vitro electrophoretic mobility shift assay, a 243-amino-acid fragment of Cdc13p (amino acids 451-693) was sufficient to bind single-stranded TG(1-3) with specificity similar to that of the native protein. Consistent with the in vitro observation, in vivo one-hybrid analysis also indicated that this region of Cdc13p was sufficient to localize itself to telomeres. However, the telomere-binding region of Cdc13p (amino acids 451-693) was not capable of complementing the growth defects of cdc13 mutants. Instead, a region comprising the Stn1p-interacting and telomere-binding region of Cdc13p (amino acids 252-924) complemented the growth defects of cdc13 mutants. These results suggest that binding to telomeres by Cdc13p is not sufficient to account for the cell viability, interaction with Stn1p is also required. Taken together, we have defined the telomere-binding domain of Cdc13p and showed that both binding to telomeres and Stn1p by Cdc13p are required to maintain cell growth.

The Bfa1/Bub2 GAP complex comprises a universal checkpoint required to prevent mitotic exit

At the end of the cell cycle, cyclin-dependent kinase (CDK) activity is inactivated to allow mitotic exit [1]. A protein phosphatase, Cdc14, plays a key role during mitotic exit in budding yeast by activating the Cdh1 component of the anaphase-promoting complex to degrade cyclin B (Clb) and inducing the CDK inhibitor Sic1 to inactivate Cdk1 [2]. To prevent mitotic exit when the cell cycle is arrested at G2/M, cells must prevent CDK inactivation. In the spindle checkpoint pathway, this is accomplished through Bfa1/Bub2, a heteromeric GTPase-activating protein (GAP) that inhibits Clb degradation by keeping the G protein Tem1 inactive [3-5]. Tem1 is required for Cdc14 activation. Here we show that in budding yeast, BUB2 and BFA1 are also required for the maintenance of G2/M arrest in response to DNA damage and to spindle misorientation. cdc13-1 bub2 and cdc13-1 bfa1 but not cdc13-1 mad2 double mutants rebud and reduplicate their DNA at the restrictive temperature. We also found that the delay in mitotic exit in mutants with misoriented spindles depended on BUB2 and BFA1, but not on MAD2. We propose that Bfa1/Bub2 checkpoint pathway functions as a universal checkpoint in G2/M that prevents CDK inactivation in response to cell-cycle delay in G2/M.

Cdc13 cooperates with the yeast Ku proteins and Stn1 to regulate telomerase recruitment

The Saccharomyces cerevisiae CDC13 protein binds single-strand telomeric DNA. Here we report the isolation of new mutant alleles of CDC13 that confer either abnormal telomere lengthening or telomere shortening. This deregulation not only depended on telomerase (Est2/TLC1) and Est1, a direct regulator of telomerase, but also on the yeast Ku proteins, yKu70/Hdf1 and yKu80/Hdf2, that have been previously implicated in DNA repair and telomere maintenance. Expression of a Cdc13-yKu70 fusion protein resulted in telomere elongation, similar to that produced by a Cdc13-Est1 fusion, thus suggesting that yKu70 might promote Cdc13-mediated telomerase recruitment. We also demonstrate that Stn1 is an inhibitor of telomerase recruitment by Cdc13, based both on STN1 overexpression and Cdc13-Stn1 fusion experiments. We propose that accurate regulation of telomerase recruitment by Cdc13 results from a coordinated balance between positive control by yKu70 and negative control by Stn1. Our results represent the first evidence of a direct control of the telomerase-loading function of Cdc13 by a double-strand telomeric DNA-binding complex.

Telomere fusions caused by mutating the terminal region of telomeric DNA

Mutations in the template region of a telomerase RNA gene can lead to the corresponding sequence alterations appearing in newly synthesized telomeric repeats. We analyzed a set of mutations in the template region of the telomerase RNA gene (TER1) of the budding yeast Kluyveromyces lactis that were predicted to lead to synthesis of mutant telomeric repeats with disrupted binding of the telomeric protein Rap1p. We showed previously that mutating the left side of the 12-bp consensus Rap1p binding site led to immediate and severe telomere elongation. Here, we show that, in contrast, mutating either the right side of the site or both sides together leads initially to telomere shortening. On additional passaging, certain mutants of both categories exhibit telomere-telomere fusions. Often, six new Bal-31-resistant, telomere repeat-containing bands appeared, and we infer that each of the six K. lactis chromosomes became circularized. These fusions were not stable, appearing occasionally to resolve and then reform. We demonstrate directly that a linear minichromosome introduced into one of the fusion mutant strains circularized by means of end-to-end fusions of the mutant repeat tracts. In contrast to the chromosomal circularization reported previously in Schizosaccharomyces pombe mutants defective in telomere maintenance, the K. lactis telomere fusions retained their telomeric DNA repeat sequences.

Positive and negative regulation of telomerase access to the telomere

The protective caps on chromosome ends - known as telomeres - consist of DNA and associated proteins that are essential for chromosome integrity. A fundamental part of ensuring proper telomere function is maintaining adequate length of the telomeric DNA tract. Telomeric repeat sequences are synthesized by the telomerase reverse transcriptase, and, as such, telomerase is a central player in the maintenance of steady-state telomere length. Evidence from both yeast and mammals suggests that telomere-associated proteins positively or negatively control access of telomerase to the chromosome terminus. In yeast, positive regulation of telomerase access appears to be achieved through recruitment of the enzyme by the end-binding protein Cdc13p. In contrast, duplex-DNA-binding proteins assembled along the telomeric tract exert a feedback system that negatively modulates telomere length by limiting the action of telomerase. In mammalian cells, and perhaps also in yeast, binding of these proteins probably promotes a higher-order structure that renders the telomere inaccessible to the telomerase enzyme.

Phosphorylation of the replication protein A large subunit in the Saccharomyces cerevisiae checkpoint response

The checkpoint mechanisms that delay cell cycle progression in response to DNA damage or inhibition of DNA replication are necessary for maintenance of genetic stability in eukaryotic cells. Potential targets of checkpoint-mediated regulation include proteins directly involved in DNA metabolism, such as the cellular single-stranded DNA (ssDNA) binding protein, replication protein A (RPA). Studies in Saccharomyces cerevisiae have revealed that the RPA large subunit (Rfa1p) is involved in the G1 and S phase DNA damage checkpoints. We now demonstrate that Rfa1p is phosphorylated in response to various forms of genotoxic stress, including radiation and hydroxyurea exposure, and further show that phosphorylation of Rfa1p is dependent on the central checkpoint regulator Mec1p. Analysis of the requirement for other checkpoint genes indicates that different mechanisms mediate radiation- and hydroxyurea-induced Rfa1p phosphorylation despite the common requirement for functional Mec1p. In addition, experiments with mutants defective in the Cdc13p telomere-binding protein indicate that ssDNA formation is an important signal for Rfa1p phosphorylation. Because Rfa1p contains the major ssDNA binding activity of the RPA heterotrimer and is required for DNA replication, repair and recombination, it is possible that phosphorylation of this subunit is directly involved in modulating RPA activity during the checkpoint response.

Analysis of the G-overhang structures on plant telomeres: evidence for two distinct telomere architectures

Telomeres are highly conserved structures essential for maintaining the integrity of eukaryotic genomes. In yeast, ciliates and mammals, the G-rich strand of the telomere forms a 3' overhang on the chromosome terminus. Here we investigate the architecture of telomeres in the dicot plants Silene latifolia and Arabidopsis thaliana using the PENT (primer extension/nick translation) assay. We show that both Arabidopsis and Silene telomeres carry G-overhangs longer than 20-30 nucleotides. However, in contrast to yeast and ciliate telomeres, only half of the telomeres in Silene seedlings possess detectable G-overhangs. PENT reactions using a variety of primers and reaction conditions revealed that the remaining fraction of Silene telomeres carries either no overhangs or overhangs less than 12 nucleotides in length. G-overhangs were observed in Silene seeds and leaves, tissues that lack telomerase activity. These findings suggest that incomplete DNA replication of the lagging strand, rather than synthesis by telomerase, is the primary mechanism for G-overhang synthesis in plants. Unexpectedly, we found that the fraction of telomeres with detectable G-overhangs decreased from 50% in seedlings to 35% in leaves. The difference may reflect increased susceptibility of the G-overhangs to nuclease attack in adult leaves, an event that could act as a precursor for the catabolic processes accompanying leaf senescence

In vitro properties of the conserved mammalian protein hnRNP D suggest a role in telomere maintenance

Mammalian chromosomes terminate with a 3' tail which consists of reiterations of the G-rich repeat, d(TTAGGG). The telomeric tail is the primer for replication by telomerase, and it may also invade telomeric duplex DNA to form terminal lariat structures, or T loops. Here we show that the ubiquitous and highly conserved mammalian protein hnRNP D interacts specifically with the G-rich strand of the telomeric repeat. A single gene encodes multiple isoforms of hnRNP D. All isoforms bind comparably to the G-rich strand, and certain isoforms can also bind tightly and specifically to the C-rich telomeric strand. G-rich telomeric sequences readily form structures stabilized by G-G pairing, which can interfere with telomere replication by telomerase. We show that hnRNP D binding to the G-rich strand destabilizes intrastrand G-G pairing and that hnRNP D interacts specifically with telomerase in human cell extracts. This biochemical analysis suggest that hnRNP D could function in vivo to destabilize structures formed by telomeric G-rich tails and facilitate their extension by telomerase.

Telomerase activity reconstituted in vitro with purified human telomerase reverse transcriptase and human telomerase RNA component

Telomerase is a specialized reverse transcriptase that catalyzes elongation of the telomeric tandem repeat, TTAGGG, by addition of this sequence to the ends of existing telomeres. Human telomerase reverse transcriptase (hTERT) has been identified as a catalytic enzyme involved in telomere elongation that requires telomerase RNA, human telomerase RNA component (hTR), as an RNA template. We established a new method to express and purify soluble insect-expressed recombinant hTERT. The partially purified FLAG-hTERT retained the catalytic activity of telomerase in a complementation assay in vitro to exhibit telomerase activity in telomerase-negative TIG3 cell extract and in a reconstitution assay with FLAG-hTERT and purified hTR in vitro. FLAG-hTERT (D712A) with a mutation in the VDV motif exhibited no telomerase activity, confirming the authentic catalytic activity of FLAG-hTERT. The reconstituted complex of FLAG-hTERT and hTR in vitro was detected by electrophoretic mobility shift assay, and its activity was stimulated by more than 30-fold by TIG3 cell extract. This suggested that some cellular component(s) in the extract facilitated the reconstituted telomerase activity in vitro. Geldanamycin had no effect on the reconstituted activity but partially reduced the stimulated activity of the reconstituted telomerase by the TIG3 cell extract, suggesting that Hsp90 may contribute to the stimulatory effect of the cellular components.

The Saccharomyces telomere-binding protein Cdc13p interacts with both the catalytic subunit of DNA polymerase α and the telomerase-associated Est1 protein

Saccharomyces telomeres consist of ∼350 bp of C1-3A/TG1-3 DNA. Most of this ∼350 bp is replicated by standard, semiconservative DNA replication. After conventional replication, the C1-3A strand is degraded to generate a long single strand TG1-3 tail that can serve as a substrate for telomerase. Cdc13p is a single strand TG1-3DNA-binding protein that localizes to telomeres in vivo. Genetic data suggest that the Cdc13p has multiple roles in telomere replication. We used two hybrid analysis to demonstrate that Cdc13p interacted with both the catalytic subunit of DNA polymerase α, Pol1p, and the telomerase RNA-associated protein, Est1p. The association of these proteins was confirmed by biochemical analysis using full-length or nearly full-length proteins. Point mutations in either CDC13 orPOL1 that reduced the Cdc13p–Pol1p interaction resulted in telomerase mediated telomere lengthening. Over–expression of the carboxyl terminus of Est1p partially suppressed the temperature sensitive lethality of a cdc13-1 strain. We propose that Cdc13p's interaction with Est1p promotes TG1-3 strand lengthening by telomerase and its interaction with Pol1p promotes C1-3A strand resynthesis by DNA polymerase α.

Telomerase-dependent repeat divergence at the 3' ends of yeast telomeres

Yeast telomeres consist of approximately 300 nt of degenerate repeats with the consensus sequence G(2-3)(TG)(1-6). We developed a method for the amplification of a genetically marked telomere by PCR, allowing precise length and sequence determination of the G-rich strand including the 3' terminus. We examined wild-type cells, telomerase RNA deficient cells and a strain deleted for YKU70, which encodes for a protein involved in telomere maintenance and DNA double strand break repair. The 3' end of the G-rich strand was found to be at a variable position within the telomeric repeat. No preference for either thymine or guanine as the 3' base was detected. Comparison of telomere sequences from clonal populations revealed that telomeres consist of a centromere-proximal region of stable sequence and a distal region with differing degenerate repeats. In wild-type as well as yku70-Delta cells, variation in the degenerate telomeric repeats was detected starting 40-100 nt from the 3' end. Sequence divergence was abolished after deletion of the telomerase RNA gene. Thus, this region defines the domain where telomere shortening and telomerase-mediated extension occurs. Since this domain is much larger than the number of nucleo-tides lost per generation in the absence of telomerase, we propose that telomerase does not extend a given telomere in every cell cycle.

Telomere shortening is proportional to the size of the G-rich telomeric 3'-overhang

Most normal diploid human cells do not express telomerase activity and are unable to maintain telomere length with ongoing cell divisions. We show that the length of the single-stranded G-rich telomeric 3'-overhang is proportional to the rate of shortening in four human cell types that exhibit different rates of telomere shortening in culture. These results provide direct evidence that the size of the G-rich overhang is not fixed but subject to regulation. The potential ability to manipulate this rate has profound implications both for slowing the rate of replicative aging in normal cells and for accelerating the rate of telomere loss in cancer cells in combination with anti-telomerase therapies.

DNA ends: maintenance of chromosome termini versus repair of double strand breaks

This review focuses on the factors that define the differences between the two types of DNA ends encountered by eukaryotic cells: telomeres and double strand breaks (DSBs). Although these two types of DNA termini are functionally distinct, recent studies have shown that a number of proteins is shared at telomeres and sites of DSB repair. The significance of these common components is discussed, as well as the types of DNA repair events that can compensate for a defective telomere.

The Est3 protein is a subunit of yeast telomerase

EST1, EST2, EST3 and TLC1 function in a single pathway for telomere replication in the yeast Saccharomyces cerevisiae [1] [2], as would be expected if these genes all encode components of the same complex. Est2p, the reverse transcriptase protein subunit, and TLC1, the templating RNA, are subunits of the catalytic core of yeast telomerase [3] [4] [5]. In contrast, mutations in EST1, EST3 or CDC13 eliminate telomere replication in vivo [1] [6] [7] [8] but are dispensable for in vitro telomerase catalytic activity [2] [9]. Est1p and Cdc13p, as components of telomerase and telomeric chromatin, respectively, cooperate to recruit telomerase to the end of the chromosome [7] [10]. However, Est3p has not yet been biochemically characterized and thus its specific role in telomere replication is unclear. We show here that Est3p is a stable component of the telomerase holoenzyme and furthermore, association of Est3p with the enzyme requires an intact catalytic core. As predicted for a telomerase subunit, fusion of Est3p to the high affinity Cdc13p telomeric DNA binding domain greatly increases access of telomerase to the telomere. Est1p is also tightly associated with telomerase; however, Est1p is capable of forming a stable TLC1-containing complex even in the absence of Est2p or Est3p. Yeast telomerase therefore contains a minimum of three Est proteins for which there is both in vivo and in vitro evidence for their role in telomere replication as subunits of the telomerase complex.

Identification of the single-strand telomeric DNA binding domain of the Saccharomyces cerevisiae Cdc13 protein

The CDC13 gene of Saccharomyces cerevisiae is required both to protect telomeric DNA and to ensure proper function of yeast telomerase in vivo. We have previously demonstrated that Cdc13p has a high affinity single-strand telomeric DNA binding activity, although the primary amino acid sequence of Cdc13p has no previously characterized DNA binding motifs. We report here mapping of the Cdc13 DNA binding domain by a combination of proteolysis mapping and deletion cloning. The DNA binding domain maps to residues 557-694 of the 924-amino acid Cdc13 polypeptide, within the most basic region of Cdc13p. A slightly larger version of this domain can be efficiently expressed in Escherichia coli as a soluble small protein, with DNA binding properties comparable to those of the full-length protein. A single amino acid missense mutation within this domain results in thermolabile DNA binding and conditional lethality in yeast, consistent with the prediction that DNA binding should be essential for CDC13 function. These results show that Cdc13p contains a discrete substructure responsible for DNA binding and should facilitate structural characterization of this telomere binding protein.

Heterogeneous nuclear ribonucleoprotein A1 and UP1 protect mammalian telomeric repeats and modulate telomere replication in vitro

The heterogeneous nuclear ribonucleoprotein A1 protein and a shortened derivative (UP1) promote telomere elongation in mammalian cells. To gain insights into the function of A1/UP1 in telomere biogenesis, we have investigated the binding properties of recombinant A1/UP1 and derivatives to single-stranded DNA oligonucleotides. Our results indicate that UP1 prefers to bind to DNA carrying single-stranded telomeric extensions at the 3' terminus. The RNA recognition motif 1 is sufficient for strong and specific binding to oligomers carrying vertebrate telomeric repeats. We find that the binding of A1/UP1 protects telomeric sequences against degradation by endo- and exonucleases. Moreover, A1/UP1 binding prevents extension by telomerase and terminal deoxynucleotidyltransferase and inhibits rNTP-dependent DNA synthesis in vitro. These observations are consistent with the hypothesis that A1/UP1 is a telomere end-binding protein that plays a role in the maintenance of long 3' overhangs.

Cell cycle restriction of telomere elongation

Telomere elongation by telomerase balances the progressive shortening of chromosome ends due to the succession of replication cycles [1] [2]. Telomerase activity is regulated in vivo at its site of action by the telomere itself. In yeast and human cells, the mean telomere length is maintained at a constant value through a cis-inhibition of telomerase by factors specifically bound to the telomeric DNA [3] [4] [5] [6] [7]. Here, we address an unexplored aspect of telomerase regulation by testing the link between telomere dynamics and cell cycle progression in the budding yeast Saccharomyces cerevisiae. We followed the elongation of an abnormally shortened telomere and observed that, like telomere shortening in the absence of telomerase, telomere elongation is linked to the succession of cell divisions. In cells progressing synchronously through the cell cycle, telomere elongation coincided with the time of telomere replication. On a minichromosome, a replication defect partially suppressed telomere elongation, suggesting a coupling between in vivo telomerase activity and conventional DNA replication.

G-quadruplex DNA: a potential target for anti-cancer drug design

In addition to the familiar duplex DNA, certain DNA sequences can fold into secondary structures that are four-stranded; because they are made up of guanine (G) bases, such structures are called G-quadruplexes. Considerable circumstantial evidence suggests that these structures can exist in vivo in specific regions of the genome including the telomeric ends of chromosomes and oncogene regulatory regions. Recent studies have demonstrated that small molecules can facilitate the formation of, and stabilize, G-quadruplexes. The possible role of G-quadruplex-interactive compounds as pharmacologically important molecules is explored in this article.

Telomerase RNA template mutations reveal sequence-specific requirements for the activation and repression of telomerase action at telomeres

Telomeric DNA is maintained within a length range characteristic of an organism or cell type. Significant deviations outside this range are associated with altered telomere function. The yeast telomere-binding protein Rap1p negatively regulates telomere length. Telomere elongation is responsive to both the number of Rap1p molecules bound to a telomere and the Rap1p-centered DNA-protein complex at the extreme telomeric end. Previously, we showed that a specific trinucleotide substitution in the Saccharomyces cerevisiae telomerase gene (TLC1) RNA template abolished the enzymatic activity of telomerase, causing the same cell senescence and telomere shortening phenotypes as a complete tlc1 deletion. Here we analyze effects of six single- and double-base changes within these same three positions. All six mutant telomerases had in vitro enzymatic activity levels similar to the wild-type levels. The base changes predicted from the mutations all disrupted Rap1p binding in vitro to the corresponding duplex DNAs. However, they caused two classes of effects on telomere homeostasis: (i) rapid, RAD52-independent telomere lengthening and poor length regulation, whose severity correlated with the decrease in in vitro Rap1p binding affinity (this is consistent with loss of negative regulation of telomerase action at these telomeres; and (ii) telomere shortening that, depending on the template mutation, either established a new short telomere set length with normal cell growth or was progressive and led to cellular senescence. Hence, disrupting Rap1p binding at the telomeric terminus is not sufficient to deregulate telomere elongation. This provides further evidence that both positive and negative cis-acting regulators of telomerase act at telomeres.

Telomere transitions in yeast: the end of the chromosome as we know it

Telomere functions vary as the cell cycle progresses. Recent results highlight fluctuating associations between telomeres and DNA polymerases, DNA-damage repair proteins, and centrosome components. These associations reflect diverse roles of telomeres in chromosome maintenance and in the orchestration of chromosome movements during meiosis.

Identification of two human nuclear proteins that recognise the cytosine-rich strand of human telomeres in vitro

Most studies on the structure of DNA in telomeres have been dedicated to the double-stranded region or the guanosine-rich strand and consequently little is known about the factors that may bind to the telomere cytosine-rich (C-rich) strand. This led us to investigate whether proteins exist that can recognise C-rich sequences. We have isolated several nuclear factors from human cell extracts that specifically bind the C-rich strand of vertebrate telomeres [namely a d(CCCTAA)(n)repeat] with high affinity and bind double-stranded telomeric DNA with a 100xreduced affinity. A biochemical assay allowed us to characterise four proteins of apparent molecular weights 66-64, 45 and 35 kDa, respectively. To identify these polypeptides we screened alambdagt11-based cDNA expression library, obtained from human HeLa cells using a radiolabelled telomeric oligonucleotide as a probe. Two clones were purified and sequenced: the first corresponded to the hnRNP K protein and the second to the ASF/SF2 splicing factor. Confirmation of the screening results was obtained with recombinant proteins, both of which bind to the human telomeric C-rich strand in vitro.

Subtelomeric repeat amplification is associated with growth at elevated temperature in yku70 mutants of Saccharomyces cerevisiae

Inactivation of the Saccharomyces cerevisiae gene YKU70 (HDF1), which encodes one subunit of the Ku heterodimer, confers a DNA double-strand break repair defect, shortening of and structural alterations in the telomeres, and a severe growth defect at 37 degrees. To elucidate the basis of the temperature sensitivity, we analyzed subclones derived from rare yku70 mutant cells that formed a colony when plated at elevated temperature. In all these temperature-resistant subclones, but not in cell populations shifted to 37 degrees, we observed substantial amplification and redistribution of subtelomeric Y' element DNA. Amplification of Y' elements and adjacent telomeric sequences has been described as an alternative pathway for chromosome end stabilization that is used by postsenescence survivors of mutants deficient for the telomerase pathway. Our data suggest that the combination of Ku deficiency and elevated temperature induces a potentially lethal alteration of telomere structure or function. Both in yku70 mutants and in wild type, incubation at 37 degrees results in a slight reduction of the mean length of terminal restriction fragments, but not in a significant loss of telomeric (C(1-3)A/TG(1-3))(n) sequences. We propose that the absence of Ku, which is known to bind to telomeres, affects the telomeric chromatin so that its chromosome end-defining function is lost at 37 degrees.

The Est1 subunit of yeast telomerase binds the Tlc1 telomerase RNA

Est1 is a component of yeast telomerase, and est1 mutants have senescence and telomere loss phenotypes. The exact function of Est1 is not known, and it is not homologous to components of other telomerases. We previously showed that Est1 protein coimmunoprecipitates with Tlc1 (the telomerase RNA) as well as with telomerase activity. Est1 has homology to Ebs1, an uncharacterized yeast open reading frame product, including homology to a putative RNA recognition motif (RRM) of Ebs1. Deletion of EBS1 results in short telomeres. We created point mutations in a putative RRM of Est1. One mutant was unable to complement either the senescence or the telomere loss phenotype of est1 mutants. Furthermore, the mutant protein no longer coprecipitated with the Tlc1 telomerase RNA. Mutants defective in the binding of Tlc1 RNA were nevertheless capable of binding single-stranded TG-rich DNA. Our data suggest that an important role of Est1 in the telomerase complex is to bind to the Tlc1 telomerase RNA via an RRM. Since Est1 can also bind telomeric DNA, Est1 may tether telomerase to the telomere.

Inhibition of plant telomerase by telomere-binding proteins from nuclei of telomerase-negative tissues

The activity of telomerase in plant cells is precisely regulated in response to changes in cell division rate. To explore this regulatory mechanism, the effect on telomerase activity of protein extracts from nuclei of telomerase-negative tissues was examined. An inhibition of telomerase activity was found which was species-non-specific. This inhibition was due to proteins which form salt-stable, sequence-specific complexes with the G-rich telomeric strand and reduce its accessibility, as shown by gel retardation and by terminal transferase (TdT) extension of G-rich telomeric and non-telomeric (substrate) primers. A 40 kDa polypeptide was detected by SDS-PAGE after cross-linking the complex formed by extracts from tobacco leaf nuclei. Such proteins may be involved in regulation of telomerase activity in plants.

The function of DNA polymerase alpha at telomeric G tails is important for telomere homeostasis

Telomere length control is influenced by several factors, including telomerase, the components of telomeric chromatin structure, and the conventional replication machinery. Although known components of the replication machinery can influence telomere length equilibrium, little is known about why mutations in certain replication proteins cause dramatic telomere lengthening. To investigate the cause of telomere elongation in cdc17/pol1 (DNA polymerase alpha) mutants, we examined telomeric chromatin, as measured by its ability to repress transcription on telomere-proximal genes, and telomeric DNA end structures in pol1-17 mutants. pol1-17 mutants with elongated telomeres show a dramatic loss of the repression of telomere-proximal genes, or telomeric silencing. In addition, cdc17/pol1 mutants grown under telomere-elongating conditions exhibit significant increases in single-stranded character in telomeric DNA but not at internal sequences. The single strandedness is manifested as a terminal extension of the G-rich strand (G tails) that can occur independently of telomerase, suggesting that cdc17/pol1 mutants exhibit defects in telomeric lagging-strand synthesis. Interestingly, the loss of telomeric silencing and the increase in the sizes of the G tails at the telomeres temporally coincide and occur before any detectable telomere lengthening is observed. Moreover, the G tails observed in cdc17/pol1 mutants incubated at the semipermissive temperature appear only when the cells pass through S phase and are processed by the time cells reach G(1). These results suggest that lagging-strand synthesis is coordinated with telomerase-mediated telomere maintenance to ensure proper telomere length control.

MRT-2 checkpoint protein is required for germline immortality and telomere replication in C. elegans

The germ line is an immortal cell lineage that is passed indefinitely from one generation to the next. To identify the genes that are required for germline immortality, we isolated Caenorhabditis elegans mutants with mortal germ lines--worms that can reproduce for several healthy generations but eventually become sterile. One of these mortal germline (mrt) mutants, mrt-2, exhibits progressive telomere shortening and accumulates end-to-end chromosome fusions in later generations, indicating that the MRT-2 protein is required for telomere replication. In addition, the germ line of mrt-2 is hypersensitive to X-rays and to transposon activity. Therefore, mrt-2 has defects in responding both to damaged DNA and to normal double-strand breaks present at telomeres. mrt-2 encodes a homologue of a checkpoint gene that is required to sense DNA damage in yeast. These results indicate that telomeres may be identified as a type of DNA damage and then repaired by the telomere-replication enzyme telomerase.

Cisplatin (cis-Pt(NH(3))(2)Cl(2)) and cis-[Pt(NH(3))(2)(H(2)O)(2)](2+) Intrastrand Cross-Linking Reactions at the Telomere GGGT DNA Sequence Embedded in a Duplex, a Hairpin, and a Bulged Duplex: Use of Mg(2+) and Zn(2+) to Convert a Hairpin to a Bulged Duplex

In the past, we showed that metal species have a high affinity for the central G in the GGG sequence of the duplex d(A(1)T(2)G(3)G(4)G(5)T(6)A(7)C(8)C(9)C(10)A(11)T(12))(2) (G3-D) and that cisplatin (cis-Pt(NH(3))(2)Cl(2)) and G3-D formed an N7-Pt-N7 G(4),G(5) intrastrand cross-link preferentially over the G(3),G(4) adduct ( approximately 25:1). Thus, a putative G(4) monoadduct was postulated to cross-link in the 3'- rather than the normally more favorable 5'-direction. To evaluate this hypothesis and also to explore why the G3-D G(4),G(5) adduct had an unusual hairpin structure, we have now introduced the use of N,N'-dimethylthiourea (DMTU) as a monoadduct trap and have extended the study to a G3-D analogue with a hairpin form, d(A(1)T(2)G(3)G(4)G(5)T(6)T(7)C(8)C(9)C(10)A(11)T(12)) (G3-H). Chemical shift and 2D (1)H and (13)C NMR data indicated that the G3-H hairpin has a stem region with B-form structure and a nonhelical loop region. Zn(2+) or Mg(2+) ions transformed G3-H into a bulged duplex. Downfield shifts of G(4)H8 and G(4)C8 NMR signals indicated that Zn(2+) binds preferentially to G(4)N7. Reaction of cisplatin or cis-[Pt(NH(3))(2)(H(2)O)(2)](2+) with the bulged duplex and hairpin forms of G3-H gave a similar intrastrand cross-link ratio, G(4),G(5):G(3),G(4) = 7:3. This ratio is insensitive to DNA form or Pt leaving group. For G3-D this ratio is lower in the cis-[Pt(NH(3))(2)(H(2)O)(2)](2+) reaction ( approximately 1:1) than in the cisplatin reaction (25:1), indicating that the leaving group influences the cross-linking step for G3-D. The G(4) monoadducts of the cis-Pt(NH(3))(2)Cl(2)-G3-H and -G3-D and the cis-[Pt(NH(3))(2)(H(2)O)(2)](2+)-G3-D reactions were trapped with DMTU, but no monoadduct was trapped in the cis-[Pt(NH(3))(2)(H(2)O)(2)](2+)-G3-H reaction. The results suggest that the respective monoadducts are more long-lived for G3-D. We postulate that the G(5) in the G3-D Cl-G(4) monoadduct is placed in a favorable position to form the cross-link because of a prior conformational change induced by G(4)-A(7) stacking. This accounts for the very high selectivity for 3'-cross-linking. Nevertheless, in all other cases, regardless of the form or conformation, 3'-direction cross-linking is unusually favored at GGGT sequences, suggesting that the sequence itself contributes greatly to the 3'-cross-linking preference; since telomeres have multiple repeats of this GGGT sequence, this finding may have biological relevance.

Telomerase-mediated telomere addition in vivo requires DNA primase and DNA polymerases alpha and delta

To better understand the requirements for telomerase-mediated telomere addition in vivo, we developed an assay in S. cerevisiae that creates a chromosome end immediately adjacent to a short telomeric DNA tract. The de novo end acts as a telomere: it is protected from degradation in a CDC13-dependent manner, telomeric sequences are added efficiently, and addition occurs at a faster rate in mutant strains that have long telomeres. Telomere addition was detected in M phase arrested cells, which permitted us to determine that the essential DNA polymerases alpha and delta and DNA primase were required. This indicates that telomeric DNA synthesis by telomerase is tightly coregulated with the production of the opposite strand. Such coordination prevents telomerase from generating excessively long single-stranded tails, which may be deleterious to chromosome stability in S. cerevisiae.

Varying the number of telomere-bound proteins does not alter telomere length in tel1Delta cells

Yeast telomere DNA consists of a continuous, approximately 330-bp tract of the heterogeneous repeat TG(1-3) with irregularly spaced, high affinity sites for the protein Rap1p. Yeast monitor, or count, the number of telomeric Rap1p C termini in a negative feedback mechanism to modulate the length of the terminal TG(1-3) repeats, and synthetic telomeres that tether Rap1p molecules adjacent to the TG(1-3) tract cause wild-type cells to maintain a shorter TG(1-3) tract. To identify trans-acting proteins required to count Rap1p molecules, these same synthetic telomeres were placed in two short telomere mutants: yku70Delta (which lack the yeast Ku70 protein) and tel1Delta (which lack the yeast ortholog of ATM). Although both mutants maintain telomeres with approximately 100 bp of TG(1-3), only yku70Delta cells maintained shorter TG(1-3) repeats in response to internal Rap1p molecules. This distinct response to internal Rap1p molecules was not caused by a variation in Rap1p site density in the TG(1-3) repeats as sequencing of tel1Delta and yku70Delta telomeres showed that both strains have only five to six Rap1p sites per 100-bp telomere. In addition, the tel1Delta short telomere phenotype was epistatic to the unregulated telomere length caused by deletion of the Rap1p C-terminal domain. Thus, the length of the TG(1-3) repeats in tel1Delta cells was independent of the number of the Rap1p C termini at the telomere. These data indicate that tel1Delta cells use an alternative mechanism to regulate telomere length that is distinct from monitoring the number of telomere binding proteins.

TIN2, a new regulator of telomere length in human cells

Telomeres are DNA-protein structures that cap linear chromosomes and are essential for maintaining genomic stability and cell phenotype. We identified a novel human telomere-associated protein, TIN2, by interaction cloning using the telomeric DNA-binding-protein TRF1 as a bait. TIN2 interacted with TRF1 in vitro and in cells, and co-localized with TRF1 in nuclei and metaphase chromosomes. A mutant TIN2 that lacks amino-terminal sequences effects elongated human telomeres in a telomerase-dependent manner. Our findings suggest that TRF1 is insufficient for control of telomere length in human cells, and that TIN2 is an essential mediator of TRF1 function.

Telomere-telomere recombination is an efficient bypass pathway for telomere maintenance in Saccharomyces cerevisiae

Many Saccharomyces telomeres bear one or more copies of the repetitive Y' element followed by approximately 350 bp of telomerase-generated C(1-3)A/TG(1-3) repeats. Although most cells lacking a gene required for the telomerase pathway die after 50 to 100 cell divisions, survivors arise spontaneously in such cultures. These survivors have one of two distinct patterns of telomeric DNA (V. Lundblad and E. H. Blackburn, Cell 73:347-360, 1993). The more common of the two patterns, seen in type I survivors, is tandem amplification of Y' followed by very short tracts of C(1-3)A/TG(1-3) DNA. By determining the structure of singly tagged telomeres, chromosomes in type II survivors were shown to end in very long and heterogeneous-length tracts of C(1-3)A/TG(1-3) DNA, with some telomeres having 12 kb or more of C(1-3)A/TG(1-3) repeats. Maintenance of these long telomeres required the continuous presence of Rad52p. Whereas type I survivors often converted to the type II structure of telomeric DNA, the type II pattern was maintained for at least 250 cell divisions. However, during outgrowth, the structure of type II telomeres was dynamic, displaying gradual shortening as well as other structural changes that could be explained by continuous gene conversion events with other telomeres. Although most type II survivors had a growth rate similar to that of telomerase-proficient cells, their telomeres slowly returned to wild-type lengths when telomerase was reintroduced. The very long and heterogeneous-length telomeres characteristic of type II survivors in Saccharomyces are reminiscent of the telomeres in immortal human cell lines and tumors that maintain telomeric DNA in the absence of telomerase.

Essential functions of amino-terminal domains in the yeast telomerase catalytic subunit revealed by selection for viable mutants

Telomerase is a ribonucleoprotein complex that adds telomeric DNA repeats to the ends of most eukaryotic chromosomes. The reverse transcriptase subunit of telomerase (TERT) differs from retroviral reverse transcriptases in having a long basic amino-terminal extension. We made a large library containing random mutations in the amino terminus of the EST2 gene, which encodes the Saccharomyces cerevisiae TERT, and selected functional alleles by their ability to rescue senescence of telomerase-negative cells. Through analysis of 265 mutations, the amino terminus of Est2p was found to contain at least four essential regions. This domain structure was verified by a combination of deletion and alanine-block mutations. Mutations within two essential domains of the protein reduced RNA binding, suggesting that the amino terminus of Est2p makes important contacts with the intrinsic RNA component of telomerase. A mutant close to the amino terminus retained RNA binding and in vitro enzymatic activity but was defective in vivo, suggesting a role in interaction with other macromolecular components of telomerase.

Telomere instability in a human cancer cell line

Telomere maintenance is essential in immortal cancer cells to compensate for DNA lost from the ends of chromosomes, to prevent chromosome fusion, and to facilitate chromosome segregation. However, the high rate of fusion of chromosomes near telomeres, termed telomere association, in many cancer cell lines has led to the proposal that some cancer cells may not efficiently perform telomere maintenance. Deficient telomere maintenance could play an important role in cancer because telomere associations and nondisjunction have been demonstrated to be mechanisms for genomic instability. To investigate this possibility, we have analyzed the telomeres of the human squamous cell carcinoma cell line SQ-9G, which has telomere associations in approximately 75% of the cells in the population. The absence of detectable telomeric repeat sequences at the sites of these telomere associations suggests that they result from telomere loss. The analysis of telomere length by quantitative in situ hybridization demonstrated that, compared to the human squamous cell carcinoma cell line SCC-61 which has few telomere associations, SQ-9G has more extensive heterogeneity in telomere length and more telomeres without detectable telomeric repeat sequences. The dynamics of the changes in telomere length also demonstrated a higher rate of fluctuation in telomere length, both on individual telomeres and coordinately on all telomeres. These results demonstrate that telomere maintenance can play a role in the genomic instability seen in cancer cells.