Proteins that bind to double-stranded regions of telomeric DNA

Mouse CCDC79 (TERB1) is a meiosis-specific telomere associated protein

Background

Telomeres have crucial meiosis-specific roles in the orderly reduction of chromosome numbers and in ensuring the integrity of the genome during meiosis. One such role is the attachment of telomeres to trans-nuclear envelope protein complexes that connect telomeres to motor proteins in the cytoplasm. These trans-nuclear envelope connections between telomeres and cytoplasmic motor proteins permit the active movement of telomeres and chromosomes during the first meiotic prophase. Movements of chromosomes/telomeres facilitate the meiotic recombination process, and allow high fidelity pairing of homologous chromosomes. Pairing of homologous chromosomes is a prerequisite for their correct segregation during the first meiotic division. Although inner-nuclear envelope proteins, such as SUN1 and potentially SUN2, are known to bind and recruit meiotic telomeres, these proteins are not meiosis-specific, therefore cannot solely account for telomere-nuclear envelope attachment and/or for other meiosis-specific characteristics of telomeres in mammals.

Results

We identify CCDC79, alternatively named TERB1, as a meiosis-specific protein that localizes to telomeres from leptotene to diplotene stages of the first meiotic prophase. CCDC79 and SUN1 associate with telomeres almost concurrently at the onset of prophase, indicating a possible role for CCDC79 in telomere-nuclear envelope interactions and/or telomere movements. Consistent with this scenario, CCDC79 is missing from most telomeres that fail to connect to SUN1 protein in spermatocytes lacking the meiosis-specific cohesin SMC1B. SMC1B-deficient spermatocytes display both reduced efficiency in telomere-nuclear envelope attachment and reduced stability of telomeres specifically during meiotic prophase. Importantly, CCDC79 associates with telomeres in SUN1-deficient spermatocytes, which strongly indicates that localization of CCDC79 to telomeres does not require telomere-nuclear envelope attachment.

Conclusion

CCDC79 is a meiosis-specific telomere associated protein. Based on our findings we propose that CCDC79 plays a role in meiosis-specific telomere functions. In particular, we favour the possibility that CCDC79 is involved in telomere-nuclear envelope attachment and/or the stabilization of meiotic telomeres. These conclusions are consistent with the findings of an independently initiated study that analysed CCDC79/TERB1 functions.

The identification of telomerase subunits: catalysing telomere research

Telomerase is the ribonucleoprotein complex responsible for the addition of simple sequences onto the physical ends, or telomeres, of most eukaryotic chromosomes. The activity of this complex is essential for the maintenance of genome integrity. Recent studies have begun to dissect the mechanism of telomerase action through the identification of a reverse-transcriptase-like activity as a catalytic subunit. At the same time, the regulation of telomere length appears to be a complex and possibly species-specific process, involving factors that are likely to interact with both the telomere and telomerase.

Structural identity of telomeric complexes

A major issue in telomere research is to understand how the integrity of chromosome ends is controlled. Although several nucleoprotein complexes have been described at the telomeres of different organisms, it is still unclear how they confer a structural identity to chromosome ends in order to mask them from DNA repair and to ensure their proper replication. In this review, we describe how telomeric nucleoprotein complexes are structured, comparing different organisms and trying to link these structures to telomere biology. It emerges that telomeres are formed by a complex and specific network of interactions between DNA, RNA and proteins. The fact that these interactions and associated activities are reinforcing each other might help to guaranty the robustness of telomeric functions across the cell cycle and in the event of cellular perturbations. We propose that telomeric nucleoprotein complexes orient cell fate through dynamic transitions in their structures and their organization.

Comparative biology of telomeres: where plants stand

Telomeres are essential structures at the ends of eukaryotic chromosomes. Work on their structure and function began almost 70 years ago in plants and flies, continued through the Nobel Prize winning work on yeast and ciliates, and goes on today in many model and non-model organisms. The basic molecular mechanisms of telomeres are highly conserved throughout evolution, and our current understanding of how telomeres function is a conglomeration of insights gained from many different species. This review will compare the current knowledge of telomeres in plants with other organisms, with special focus on the functional length of telomeric DNA, the search for TRF homologs, the family of POT1 proteins, and the recent discovery of members of the CST complex.

Plasticity of telomere maintenance mechanisms in yeast

Telomeres, the nucleoprotein structures located at linear eukaryotic chromosomal termini, are essential for chromosome stability and are maintained by the special reverse transcriptase named telomerase. In the Saccharomycotina subphylum of budding yeast, telomere repeat sequences and binding factors, as well as telomerase components, are exceptionally diverse and distinct from those found in other eukaryotes. In this survey, I report a comparative analysis of the domain structures of telomere and telomerase-related factors made possible by the recent sequencing of multiple yeast genomes. This analysis revealed both conserved and variable aspects of telomere maintenance. Based on these findings, I propose a plausible series of evolutionary events in budding yeast to account for its exceptional telomere structural divergence.

Human RAP1 inhibits non-homologous end joining at telomeres

Telomeres, the nucleoprotein structures at the ends of linear chromosomes, promote genome stability by distinguishing chromosome termini from DNA double-strand breaks (DSBs). Cells possess two principal pathways for DSB repair: homologous recombination and non-homologous end joining (NHEJ). Several studies have implicated TRF2 in the protection of telomeres from NHEJ, but the underlying mechanism remains poorly understood. Here, we show that TRF2 inhibits NHEJ, in part, by recruiting human RAP1 to telomeres. Heterologous targeting of hRAP1 to telomeric DNA was sufficient to bypass the need for TRF2 in protecting telomeric DNA from NHEJ in vitro. On expanding these studies in cells, we find that recruitment of hRAP1 to telomeres prevents chromosome fusions caused by the loss of TRF2/hRAP1 from chromosome ends despite activation of a DNA damage response. These results provide the first evidence that hRAP1 inhibits NHEJ at mammalian telomeres and identify hRAP1 as a mediator of genome stability.

Composition of plant telomeres

Telomeres are essential elements of eukaryotic chromosomes that differentiate native chromosome ends from deleterious DNA double-strand breaks (DSBs). This is achieved by assembling chromosome termini in elaborate high-order nucleoprotein structures that in most organisms encompass telomeric DNA, specific telomere-associated proteins as well as general chromatin and DNA repair factors. Although the individual components of telomeric chromatin are evolutionary highly conserved, cross species comparisons have revealed a remarkable flexibility in their utilization at telomeres. This review outlines the strategies used for chromosome end protection and maintenance in mammals, yeast and flies and discusses current progress in deciphering telomere structure in plants.

Subtelomeric factors antagonize telomere anchoring and Tel1-independent telomere length regulation

Yeast telomeres are anchored at the nuclear envelope (NE) through redundant pathways that require the telomere-binding factors yKu and Sir4. Significant variation is observed in the efficiency with which different telomeres are anchored, however, suggesting that other forces influence this interaction. Here, we show that subtelomeric elements and the insulator factors that bind them antagonize the association of telomeres with the NE. This is detectable when the redundancy in anchoring pathways is compromised. Remarkably, these same conditions lead to a reduction in steady-state telomere length in the absence of the ATM-kinase homologue Tel1. Both the delocalization of telomeres and reduction in telomere length can be induced by targeting of Tbf1 or Reb1, or the viral transactivator VP16, to a site 23 kb away from the TG repeat. This correlation suggests that telomere anchoring and a Tel1-independent pathway of telomere length regulation are linked, lending a functional significance to the association of yeast telomeres with the NE.

Telomere maintenance, function and evolution: the yeast paradigm

Telomeres are multifunctional genetic elements that cap chromosome ends, playing essential roles in genome stability, chromosome higher-order organization and proliferation control. The telomere field has largely benefited from the study of unicellular eukaryotic organisms such as yeasts. Easy cultivation in laboratory conditions and powerful genetics have placed mainly Saccharomyces cerevisiae, Kluveromyces lactis and Schizosaccharomyces pombe as crucial model organisms for telomere biology research. Studies in these species have made it possible to elucidate the basic mechanisms of telomere maintenance, function and evolution. Moreover, comparative genomic analyses show that telomeres have evolved rapidly among yeast species and functional plasticity emerges as one of the driving forces of this evolution. This provides a precious opportunity to further our understanding of telomere biology.

Télomère et cancer : quoi de plus à la fin ?

Telomeres are nucleoprotein complexes that cap the end of eukaryotic chromosomes. They are essential for the functions and the stability of the genomes. There is now compelling evidences that telomerase, the enzyme that adds telomeric DNA repeats to chromosome end, is an important player in oncogenesis. The absence of telomerase in somatic tissues is thought to promote genome instability at initial stages of oncogenesis, favoring the emergence of cancer-associated chromosomal abnormalities \; restablishment of telomerase activity is expected afterwards if long term cell cycling is to occur. In addition to telomerase, various factors control the structure and function of telomeres, suggesting that additional telomeric components play important roles during oncogenesis.

A telomere-binding protein (TRF2/MTBP) from mouse nuclear matrix with motives of an intermediate filament-type rod domain

In previous work, we identified a telomeric DNA-binding protein (termed telomere-membrane binding protein, MTBP) in the envelope of the frog oocyte nucleus and raised antibodies against it. Here we present immunological evidence which suggests strongly that MTBP is identical with the vertebrate telomeric DNA-binding protein TRF2 (telomere-repeat factor 2). MTBP/TRF2 possesses motif which resembles rod domain characteristic of intermediate filament (IF) proteins as shown by immunological cross-reactivity with characteristic antibodies, as well as amino acid sequence homology. Anti-MTBP antibodies recognised a protein of the same M, as TRF2 in extracts of mouse nuclei and nuclear matrix as shown by ion-exchange chromatography, gel shift assays, and Western blots. This mouse MTBP analogue forms more stable complexes with the vertebrate telomeric DNA fragment (T(2)AG(3))(135) than with the corresponding fragment from Tetrahymena (T(2)G(4))(130). Proteins in each of these complexes are recognised by anti-MTBP antibody. In situ hybridization with the vertebrate telomeric DNA sequence (T(2)AG(3))(135) and immunofluorescence with anti-MTBP antibody had shown earlier that these are co-localised in the nucleus of mouse cells, and here MTBP is shown to be associated with the residual membrane of hepatocyte nuclei using Western blotting and immunofluorescence. Some immunofluorescence signal from MTBP is localized at chromosome extremities on metaphase plates from mouse cell culture, but the main signal is seen in patches scattered around the chromosomes which were identified as remnants of the nuclear envelope by double labelling with antibodies against lamin B. These observations suggest that MTBP/TRF2 is a good candidate for the attachment of telomeres to the nuclear envelope in somatic cells.

Structure-specific DNA-binding proteins as the foundation for three-dimensional chromatin organization

Any functions of tandem repetitive sequences need proteins that specifically bind to them. Telomere-binding TRF2/MTBP attaches telomeres to the nuclear envelope in interphase due to its rod-domain-like motif. Interphase nuclei organized as a number of sponge-like ruffly round chromosome territories that could be rotated from outside. SAF-A/hnRNP-U and p68-helicase are proteins suitable to do that. Their location in the interchromosome territory space, ATPase domains, and the ability to be bound by satellite DNAs (satDNA) make them part of the wires used to help chromosome territory rotates. In case of active transcription p68-helicase can be involved in the formation of local "gene expression matrices" and due to its satDNA-binding specificity cause the rearrangement of the local chromosome territory. The marks of chromatin rearrangement, which have to be heritable, could be provided by SAF-A/hnRNP-U. During telophase unfolding the proper chromatin arrangement is restored according to these marks. The structural specificity of both proteins to the satDNAs provides a regulative but relatively stable mode of binding. The structural specificity of protein binding could help to find the "magic" centromeric sequence. With future investigations of proteins with the structural specificity of binding during early embryogenesis, when heterochromatin formation goes on, the molecular mechanisms of the "gene gating" hypothesis (Blobel, 1985) will be confirmed.

Telomere-Binding TRF2/MTBP Localization during Mouse Spermatogenesis and Cell Cycle of the Mouse Cells L929

Observations of the organization and distribution of telomeres (Tel) in somatic tissues still remain controversial. The Tel topography revealed by modern microscopy shows them to be associated with the nuclear envelope (NE) in a wide variety of eukaryotic cells, although not at the Rabl orientation (peripheral position at one pole of the nucleus at prophase). We used two cell types that have different nuclear architectures. The cell line L929 shows lack of any rigid Tel architecture in the nucleus. In contrast, spermatozoa have a precise architecture established during spermiogenesis. We observed Tel and membrane Tel binding protein (MTBP/TRF2) position by immunoFISH in L929 cells and by immunofluorescence and immunogold electron microscopy, using antibodies against Membrane Tel Binding Protein (MTBP/TRF2), during different stages of spermiogenesis. At all stages of the L929 cell cycle, MTBP/TRF2 is co-localized with Tel. The only Tel order found in this cell type is similar to the Rabl-orientation, probably due to fast divisions. In the mouse pachytene spermatocytes, the membrane structures abut on the synaptonemal complex (SC) attachment sites contain MTBP/TRF2. In fully formed spermatozoa and during spermiogenesis, apart from the expected MTBP/TRF2 position at the nuclear periphery, MTBP/TRF2 unexpectedly localized at the acrosomal membrane that is adjacent to the nucleus. The difference in the MTBP/TRF2 distribution in the oocyte and spermatozoa leads to the suggestion that the MTBP/TRF2 location might reflect preparation for fertilization events. The Tel distribution is not static in cultured cells throughout the cell cycle or during spermatogenesis. When the Tel are attached to the NE, as during SC formation, MTBP/TRF2 is the member of the protein complex, which appears to be responsible for this attachment.

Solution structure of a telomeric DNA complex of human TRF1

BACKGROUND

Mammalian telomeres consist of long tandem arrays of double-stranded TTAGGG sequence motif packaged by TRF1 and TRF2. In contrast to the DNA binding domain of c-Myb, which consists of three imperfect tandem repeats, DNA binding domains of both TRF1 and TRF2 contain only a single Myb repeat. In a DNA complex of c-Myb, both the second and third repeats are closely packed in the major groove of DNA and recognize a specific base sequence cooperatively.

RESULTS

The structure of the DNA binding domain of human TRF1 bound to telomeric DNA has been determined by NMR. It consists of three helices, whose architecture is very close to that of three repeats of the c-Myb DNA binding domain. Only the single Myb domain of TRF1 is sufficient for the sequence-specific recognition. The third helix of TRF1 recognizes the TAGGG part in the major groove, and the N-terminal arm interacts with the TT part in the minor groove.

CONCLUSIONS

The DNA binding domain of TRF1 can specifically and fully recognize the AGGGTT sequence. It is likely that, in the dimer of TRF1, two DNA binding domains can bind independently in tandem arrays to two binding sites of telomeric DNA that is composed of the repeated AGGGTT motif. Although TRF2 plays an important role in the t loop formation that protects the ends of telomeres, it is likely that the binding mode of TRF2 to double-stranded telomeric DNA is almost identical to that of TRF1.

c-Myc-mediated regulation of telomerase activity is disabled in immortalized cells

Myc overexpression is a hallmark of human cancer and promotes transformation by facilitating immortalization. This function has been linked to the ability of c-Myc to induce the expression of the catalytic subunit of telomerase, telomerase reverse transcriptase (TERT), as ectopic expression of TERT immortalizes some primary human cell types. c-Myc up-regulates telomerase activity in primary mouse embryonic fibroblasts (MEFs) and myeloid cells. Paradoxically, Myc overexpression also triggers the ARF-p53 apoptotic program, which is activated when MEFs undergo replicative crises following culture ex vivo. The rare immortal variants that arise from these cultures generally suffer mutations in p53 or delete Ink4a/ARF, and Myc greatly increases the frequency of these events. Alternative reading frame (ARF)- and p53-null MEFs have increased telomerase activity, as do variant immortal clones that bypass replicative crisis. Similarly, immortal murine NIH-3T3 fibroblasts and myeloid 32D.3 and FDC-P1.2 cells do not express ARF and have robust telomerase activity. However, Myc overexpression in these immortal cells results in remarkably discordant regulation of TERT and telomerase activity. Furthermore, in MEFs and 32D.3 cells TERT expression and telomerase activity are regulated independently of endogenous c-Myc. Thus, the regulation of TERT and telomerase activity is complex and is also regulated by factors other than Myc, ARF, or p53.

The fission yeast Taz1 protein protects chromosomes from Ku-dependent end-to-end fusions

A paramount role of telomeres is to prevent chromosome fusions. The fission yeast Taz1 protein regulates diverse telomere functions but is not essential for growth under stress-free conditions. Strikingly, however, taz1(-) cells exhibit lethal telomere fusions when subjected to nitrogen starvation, a treatment that induces an uncommitted G1 state. These fusions are formed by Ku-dependent nonhomologous end joining. Fusions also occur during normal growth in taz1(-) cells that lack rad22(+), a gene involved in homologous recombination. Our data suggest a model whereby taz1(-) telomeres are exposed to the prevailing mode of DNA repair, which is dictated by the cell cycle. Thus, Taz1 caps chromosome ends and provides the telomerespecific interaction that prevents Ku from treating telomeres as double-strand breaks.

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.

Nuclear envelope associated protein that binds telomeric DNAs

Rana temporaria oocytes at the 6th diplotene stage of maturation contain a special structure, the karyosphere capsule, with chromosomes covered and detached from the nuclear envelope (NE), though at the previous stage the telomeres were attached to the membrane, as characteristic of germ cells. The DNA-protein complexes from band shift assays with proteins extracted from oocyte NEs and telomeric DNA fragment (T(2)G(4))(130) were isolated and injected into a guinea pig. In the present paper the only protein of 70 kDa recognized by antibody (AB) in the NE is named the Membrane Telomere Binding Protein (MTBP). Western blots with guinea pig AB and AB against telobox peptide from TRF2 show that protein of 60 kDa (probably TRF1) belongs to the chromatin, but MTBP (TRF2 according to immunoprecipitation) belongs to the NE. In the somatic cell nuclei both proteins are present and recognized by AB against telobox peptide, but AB raised recognize only MTBP/TRF2 due to the epitope different from telobox. Combined in situ hybridization with the vertebrate telomeric DNA sequences (T(2)AG(3))(135) and immunocytochemistry with the MTBP AB showed them to be colocalized within the mouse nucleus. As it was shown by immunofluorescense of NE spread, MTBP is organized in a distinct pattern that looks like a network made of double-dots. Electron microscope immunogold staining with both ABs showed that the protein is localized on the outer surface of the oocyte NE within cup-like structures attached to the membrane. This is the first clear evidence of a protein, which could be responsible for the attachment of telomeres to the nuclear membrane.

Nutrition, oxidative damage, telomere shortening, and cellular senescence: individual or connected agents of aging?

There is substantial and long-standing literature linking the level of general nutrition to longevity. Reducing nutrition below the amount needed to sustain maximum growth increases longevity in a wide range of organisms. Oxidative damage has been shown to be a major feature of the aging process. Telomere shortening is now well established as a key process regulating cell senescence in vitro. There is some evidence that the same process may be important for aging in vivo. Very recently it has been found that oxidative damage accelerates telomere shortening. It is therefore possible for us to propose as an outline hypothesis that the level of nutrition determines oxidative damage which in turn determines telomere shortening and cell senescence and that this pathway is important in determining aging and longevity in vivo. We also propose that telomeres in addition to their well-recognized role in "counting" cell divisions are also, through their GGG sequence, important monitors of oxidative damage over the life span of a cell. This may explain the evolutionary conservations of this triplet in the repeat telomere sequence unit.

Identification of high affinity Tbf1p-binding sites within the budding yeast genome

The yeast TBF1 gene is essential for mitotic growth and encodes a protein that binds the human telomere repeats in vitro, although its cellular function is unknown. The sequence of the DNA-binding domain of Tbf1p is more closely related to that of the human telomeric proteins TRF1 and TRF2 than to any yeast protein sequence, yet the functional homologue of TRF1 and TRF2 is thought to be Rap1p. In this study we show that the Tbf1p DNA-binding domain can target the Gal4 transactivation domain to a (TTAGGG)(n) sequence inserted in the yeast genome, supporting the model that Tbf1p binds this sub-telomeric repeat motif in vivo. Immunofluorescence of Tbf1p shows a spotty pattern throughout the interphase nucleus and along synapsed chromosomes in meiosis, suggesting that Tbf1p binds internal chromosomal sites in addition to sub-telomeric regions. PCR-assisted binding site selection was used to define a consensus for high affinity Tbf1p-binding sites. Compilation of 50 selected oligonucleotides identified the consensus TAGGGTTGG. Five potential Tbf1p-binding sites resulting from a search of the total yeast genome were tested directly in gel shift assays and shown to bind Tbf1p efficiently in vitro, thus confirming this as a valid consensus for Tbf1p recognition.

Identification of human Rap1: implications for telomere evolution

It has been puzzling that mammalian telomeric proteins, including TRF1, TRF2, tankyrase, and TIN2 have no recognized orthologs in budding yeast. Here, we describe a human protein, hRap1, that is an ortholog of the yeast telomeric protein, scRap1p. hRap1 has three conserved sequence motifs in common with scRap1, is located at telomeres, and affects telomere length. However, while scRap1 binds telomeric DNA directly, hRap1 is recruited to telomeres by TRF2. Extending the comparison of telomeric proteins to fission yeast, we identify S. pombe Taz1 as a TRF ortholog, indicating that TRFs are conserved at eukaryotic telomeres. The data suggest that ancestral telomeres, like those of vertebrates, contained a TRF-like protein as well as Rap1. We propose that budding yeast preserved Rap1 at telomeres but lost the TRF component, possibly concomitant with a change in the telomeric repeat sequence.

Telomere maintenance and length regulation in Trypanosoma brucei

Transcription of telomere proximal variant surface glycoprotein genes is mono-allelic in bloodstream-form Trypanosoma brucei. The terminal DNA sequence at these telomeres consists of tandem T(2)AG(3) repeats, which increase in length by approximately 8 bp per cell division balanced by occasional loss of large numbers of repeats. Here we have used targeted chromosome fragmentation to investigate the sequence requirements for telomere formation in T. brucei. Telomere formation is most efficient on tandem T(2)AG(3) repeats, but can also occur on specific templates found within 'random' sequence substrates and on G-rich motifs proximal to a double-strand break. Newly formed telomeres are extended faster than other native telomeres, but as the telomere becomes longer the rate of extension declines. Telomere length regulation in T.brucei is discussed in the context of recent results from other cell types.

Telomerase: a therapeutic target for the third millennium?

Telomerase offers the potential opportunity to control cell proliferation by interfering with a totally new and unique biological process which is cell senescence. The aim of this review is to impartially present the state of the art in telomerase with the pros and the cons of the current scientific situation of this fast-growing and fascinating topic for answering the key question asked by experimental and medical oncologists: Will telomerase be a therapeutic target for the third millenium? The most convincing argument (which is a scientifically documented one) for going ahead with this target is obviously the strong correlation existing between the level and frequency of telomerase expression and the malignant properties of tumors. This has been now largely documented in established tumor cell lines and fresh tumor samples obtained from patients. Noteworthy is the very important difference of telomerase expression between malignant and normal tissues. This difference is much higher than those observed for classical enzymatic targets of chemotherapy such as thymidylate synthetase, dihydrofolate reductase and topoisomerases. If this translates to the clinical situation, telomerase inhibitors might display a good selectivity for tumor cells with a minimal toxicity for normal tissues. The most appealing criticism (which is still purely speculative) is obviously the clinical relevance of inhibiting telomerase in cancer patients. According to the paradigm currently proposed for telomeres and telomerases, it can be predicted that telomerase inhibition will not affect a tumor until its telomeres reach the critical size for entering senescence. This means that during anti-telomerase therapy, the tumor cells will continue grow undergoing 20-30 divisions until the telomeres reach a critical size leading to tumor senescence. Does this make sense, especially in patients with advanced tumors at the beginning of the therapy? Ultimately, the definitive answer to the question will not come from intellectual speculation but from the properties of telomerase inhibitors, first in tumor bearing animals, then finally in cancer patients! Several institutions are very active in the development of telomerase inhibitors. Different stategies are used: direct inhibition of telomerase, interference with telomeres (G quartets), interaction with other proteins involved in the regulation of telomerase and telomeres.

Identification and characterization of a bovine sperm protein that binds specifically to single-stranded telomeric deoxyribonucleic acid

Telomere DNA at the physical termini of chromosomes forms a single-stranded 3' overhang. In lower eukaryotes, e.g., ciliated protozoa, this DNA extension is capped by specific proteins that have been structurally and functionally characterized. Much less is known about single-stranded telomere DNA-binding proteins in vertebrates. Here we describe a new protein from bovine sperm designated bsSSTBP that specifically interacts with single-stranded (TTAGGG)(N) DNA. The bsSSTBP was extracted from nuclei by 0.6 M KCl. The native size of this protein, estimated by gel filtration, was 20-40 kDa. SDS-PAGE of the UV cross-linked complex between bsSSTBP and telomere DNA indicated that several polypeptides are involved in complex formation. Bovine sSSTB had high specificity toward nucleotide sequence, since single nucleotide substitutions in the (TTAGGG)(4) substrate suppressed binding. The minimal number of (TTAGGG) repeats required for binding of bsSSTBP was 3, and the protein recognized linear but not folded DNA structures. We propose that the bsSSTBP participates in telomere-telomere interactions and the telomere membrane localization observed in mature sperm. In mammals, somatic telomere-binding proteins are apparently substituted by sperm-specific ones that may lead to a structural reorganization of telomere domains to fulfill functions important during meiosis and fertilization.

Sequence-specific binding of Taz1p dimers to fission yeast telomeric DNA

The fission yeast (Schizosaccharomyces pombe) taz1 gene encodes a telomere-associated protein. It contains a single copy of a Myb-like motif termed the telobox that is also found in the human telomere binding proteins TRF1 and TRF2, and Tbf1p, a protein that binds to sequences found within the sub-telomeric regions of budding yeast (Saccharomyces cerevisiae) chromosomes. Taz1p was synthesised in vitro and shown to bind to a fission yeast telomeric DNA fragment in a sequence specific manner that required the telobox motif. Like the mammalian TRF proteins, Taz1p bound to DNA as a preformed homodimer. The isolated Myb-like domain was also capable of sequence specific DNA binding, although with less specificity than the full-length dimer. Surprisingly, a protein extract produced from a taz1- fission yeast strain still contained the major telomere binding activity (complex I) we have characterised previously, suggesting that there could be other abundant telomere binding proteins in fission yeast. One candidate, SpX, was also synthesised in vitro, but despite the presence of two telobox domains, no sequence specific binding to telomeric DNA was detected.

Cloning and characterisation of the chicken gene encoding the telomeric protein TRF2

The telomere-associated protein TRF2 binds as a homodimer to double-stranded (TTAGGG)n arrays in vitro and localises to chromosome ends in vivo. Inhibition of TRF2 in human cell lines and recent electron microscopy analyses suggest that TRF2 plays a crucial role in the maintenance of telomere integrity. To study the role of TRF2 in vertebrate telomere biology using an alternative model system, we report the isolation and characterisation of the chicken TRF2 locus. The TRF2 protein is highly conserved between mammals and birds, particularly within the dimerisation and myb-type DNA binding domains. However, the chicken ORF predicts an additional protein domain consisting of 15 copies of a degenerate 13 amino acid repeat. Indirect immunofluorescence reveals the localisation of a FLAG-tagged version of the chicken TRF2 protein at chromosome ends in both chicken and human cells suggesting that the protein is functionally conserved.

Dependence of the regulation of telomere length on the type of subtelomeric repeat in the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, chromosomes terminate with approximately 400 bp of a simple repeat poly(TG(1-3)). Based on the arrangement of subtelomeric X and Y' repeats, two types of yeast telomeres exist, those with both X and Y' (Y' telomeres) and those with only X (X telomeres). Mutations that result in abnormally short or abnormally long poly(TG(1-3)) tracts have been previously identified. In this study, we investigated telomere length in strains with two classes of mutations, one that resulted in short poly(TG(1-3)) tracts (tel1) and one that resulted in elongated tracts (pif1, rap1-17, rif1, or rif2). In the tel1 pif1 strain, Y' telomeres had about the same length as those in tel1 strains and X telomeres had lengths intermediate between those in tel1 and pif1 strains. Strains with either the tel1 rap1-17 or tel1 rif2 genotypes had short tracts for all chromosome ends examined, demonstrating that the telomere elongation characteristic of rap1-17 and rif2 strains is Tel1p-dependent. In strains of the tel1 rif1 or tel1 rif1 rif2 genotypes, telomeres with Y' repeats had short terminal tracts, whereas most of the X telomeres had long terminal tracts. These results demonstrate that the regulation of telomere length is different for X and Y' telomeres.

Progressive cis-inhibition of telomerase upon telomere elongation

In yeast, the constant length of telomeric DNA results from a negative regulation of telomerase by the telomere itself. Here we follow the return to equilibrium of an abnormally shortened telomere. We observe that telomere elongation is restricted to a few base pairs per generation and that its rate decreases progressively with increasing telomere length. In contrast, in the absence of telomerase or in the presence of an over-elongated telomere, the degradation rate linked to the succession of generations appears to be constant, i.e. independent of telomere length. Together, these results indicate that telomerase is gradually inhibited at its site of action by the elongating telomere. The implications of this finding for the dynamics of telomere length regulation are discussed in this study.

Limitations of silencing at native yeast telomeres

Silencing at native yeast telomeres, in which the subtelomeric elements are intact, is different from silencing at terminal truncations. The repression of URA3 inserted in different subtelomeric positions at several chromosome ends was investigated. Many ends exhibit very little silencing close to the telomere, while others exhibit substantial repression in limited domains. Silencing at native ends is discontinuous, with maximal repression found adjacent to the ARS consensus sequence in the subtelomeric core X element. The level of repression declines precipitously towards the centromere. Mutation of the ARS sequence or an adjacent Abf1p-binding site significantly reduces silencing. The subtelomeric Y' elements are resistant to silencing along their whole length, yet silencing can be re-established at the proximal X element. Deletion of PPR1, the transactivator of URA3, and SIR3 overexpression do not increase repression or extend spreading of silencing to the same extent as with terminally truncated ends. sir1Delta causes partial derepression at X-ACS, in contrast to the lack of effect seen at terminal truncations. orc2-1 and orc5-1 have no effect on natural silencing yet cause derepression at truncated ends. X-ACS silencing requires the proximity of the telomere and is dependent on SIR2, SIR3, SIR4 and HDF1. The structures found at native yeast telomeres appear to limit the potential of repressive chromatin.

Cohabitation of insulators and silencing elements in yeast subtelomeric regions

In budding yeast, the telomeric DNA is flanked by a combination of two subtelomeric repetitive sequences, the X and Y' elements. We have investigated the influence of these sequences on telomeric silencing. The telomere-proximal portion of either X or Y' dampened silencing when located between the telomere and the reporter gene. These elements were named STARs, for subtelomeric anti-silencing regions. STARs can also counteract silencer-driven repression at the mating-type HML locus. When two STARs bracket a reporter gene, its expression is no longer influenced by surrounding silencing elements, although these are still active on a second reporter gene. In addition, an intervening STAR uncouples the silencing of neighboring genes. STARs thus display the hallmarks of insulators. Protection from silencing is recapitulated by multimerized oligonucleotides representing Tbf1p- and Reb1p-binding sites, as found in STARs. In contrast, sequences located more centromere proximal in X and Y' elements reinforce silencing. They can promote silencing downstream of an insulated expressed domain. Overall, our results suggest that the silencing emanating from telomeres can be propagated in a discontinuous manner via a series of subtelomeric relay elements.

Telomere length in fibroblasts and blood cells from healthy centenarians

Several lines of evidence indicate that telomere shortening during in vitro aging of human somatic cells plays a causal role in cellular senescence. A critical telomere length seems to be associated with the replicative block characterizing senescent cells. In this paper we analyzed the mean length of the terminal restriction fragments (TRF) in fibroblast strains from 4 healthy centenarians, that is, in cells aged in vivo, and from 11 individuals of different ages. No correlation between mean TRF length and donor age was found. As expected, telomere shortening was detected during in vitro propagation of centenarian fibroblasts, suggesting that in fibroblasts aged in vivo telomeres can be far from reaching a critical length. Accordingly, chromosome analysis did not show the presence of telomeric associations in early passage centenarian fibroblasts. In blood cells from various individuals, the expected inverse correlation between mean TRF length and donor age was found. In particular, a substantial difference (about 2 kb) between telomere length in the two cell types was observed in the same centenarian. Expression analysis of three senescence-induced genes, i.e., fibronectin, apolipoprotein J, and p21, revealed for only the fibronectin expression levels a clear positive correlation with donor age. Our results suggest that (1) telomere shortening could play a different role in the aging of different cell types and (2) the characteristics of fibroblasts aged in vitro might not be representative of what occurs in vivo.

ROUGH SHEATH2: a Myb protein that represses knox homeobox genes in maize lateral organ primordia

The regulation of members of the knotted1-like homeobox (knox) gene family is required for the normal initiation and development of lateral organs. The maize rough sheath2 (rs2) gene, which encodes a Myb-domain protein, is expressed in lateral organ primordia and their initials. Mutations in the rs2 gene permit ectopic expression of knox genes in leaf and floral primordia, causing a variety of developmental defects. Ectopic KNOX protein accumulation in rs2 mutants occurs in a subset of the normal rs2-expressing cells. This variegated accumulation of KNOX proteins in rs2 mutants suggests that rs2 represses knox expression through epigenetic means.

Telomerases

Telomerases are RNA-dependent polymerases that catalyse the synthesis of the telomeric DNA at the tips of eukaryotic chromosomes. The recent identification of the catalytic subunit of telomerases from several different species suggests that the core of the telomerase is conserved. The proposed sequence and structural homology between the telomerase catalytic subunit and reverse transcriptases, together with a wealth of genetic and biochemical information, has led to significant advances in our understanding of the mechanism by which telomerases synthesise telomeric DNA.

The yeast telomere length counting machinery is sensitive to sequences at the telomere-nontelomere junction

Saccharomyces cerevisiae telomeres consist of a continuous 325 +/- 75-bp tract of the heterogeneous repeat TG1-3 which contains irregularly spaced, high-affinity sites for the protein Rap1p. Yeast cells monitor or count the number of telomeric Rap1p molecules in a negative feedback mechanism which modulates telomere length. To investigate the mechanism by which Rap1p molecules are counted, the continuous telomeric TG1-3 sequences were divided into internal TG1-3 sequences and a terminal tract separated by nontelomeric spacers of different lengths. While all of the internal sequences were counted as part of the terminal tract across a 38-bp spacer, a 138-bp disruption completely prevented the internal TG1-3 sequences from being considered part of the telomere and defined the terminal tract as a discrete entity separate from the subtelomeric sequences. We also used regularly spaced arrays of six Rap1p sites internal to the terminal TG1-3 repeats to show that each Rap1p molecule was counted as about 19 bp of TG1-3 in vivo and that cells could count Rap1p molecules with different spacings between tandem sites. As previous in vitro experiments had shown that telomeric Rap1p sites occur about once every 18 bp, all Rap1p molecules at the junction of telomeric and nontelomeric chromatin (the telomere-nontelomere junction) must participate in telomere length measurement. The conserved arrangement of these six Rap1p molecules at the telomere-nontelomere junction in independent transformants also caused the elongated TG1-3 tracts to be maintained at nearly identical lengths, showing that sequences at the telomere-nontelomere junction had an effect on length regulation. These results can be explained by a model in which telomeres beyond a threshold length form a folded structure that links the chromosome terminus to the telomere-nontelomere junction and prevents telomere elongation.

Two modes of survival of fission yeast without telomerase

Deletion of the telomerase catalytic subunit gene trt1+ in Schizosaccharomyces pombe results in death for the majority of cells, but a subpopulation survives. Here it is shown that most survivors have circularized all of their chromosomes, whereas a smaller number maintain their telomeres presumably through recombination. When the telomeric DNA-binding gene taz1+ is also deleted, trt1- taz1- survivors use the recombinational mode more frequently. Moreover, the massive elongation of telomeres in taz1- cells is absent in the double mutant. Thus, Taz1p appears to regulate telomeric recombination as well as telomerase activity in fission yeast.

Modulation of telomerase activity by telomere DNA-binding proteins in Oxytricha

Telomere proteins protect the chromosomal terminus from nucleolytic degradation and end-to-end fusion, and they may contribute to telomere length control and the regulation of telomerase. The current studies investigate the effect of Oxytricha single-stranded telomere DNA-binding protein subunits alpha and beta on telomerase elongation of telomeric DNA. A native agarose gel system was used to evaluate telomere DNA-binding protein complex composition, and the ability of telomerase to use these complexes as substrates was characterized. Efficient elongation occurred in the presence of the alpha subunit. Moreover, the alpha-DNA cross-linked complex was a substrate for telomerase. At higher alpha concentrations, two alpha subunits bound to the 16-nucleotide single-stranded DNA substrate and rendered it inaccessible to telomerase. The formation of this alpha . DNA . alpha complex may contribute to regulation of telomere length. The alpha . beta . DNA ternary complex was not a substrate for telomerase. Even when telomerase was prebound to telomeric DNA, the addition of alpha and beta inhibited elongation, suggesting that these telomere protein subunits have a greater affinity for the DNA and are able to displace telomerase. In addition, the ternary complex was not a substrate for terminal deoxynucleotidyltransferase. We conclude that the telomere protein inhibits telomerase by rendering the telomeric DNA inaccessible, thereby helping to maintain telomere length.

Telomerase and chromosome end maintenance

The past year has seen significant advances in our understanding of telomerase and other factors involved in chromosome end maintenance. The protein subunit of telomerase that provides the active site for telomeric DNA synthesis was identified in ciliates, yeast and mammals. It is structurally related to reverse transcriptase and thus represents the first member of this protein family with an essential cellular function. Telomere DNA-binding proteins that may mediate the interaction of telomerase with telomeres have been identified and further characterized in diverse eukaryotes. A further elucidation of telomeric DNA structure has influenced our view of how telomeres replicate.

The yeast telomere length regulator TEL2 encodes a protein that binds to telomeric DNA

TEL2 is required for telomere length regulation and viability in Saccharomyces cerevisiae. To investigate the mechanism by which Tel2p regulates telomere length, the majority (65%) of the TEL2 ORF was fused to the 3'-end of the gene for maltose binding protein, expressed in bacteria and the purified protein used in DNA binding studies. Rap1p, the major yeast telomere binding protein, recognizes a 13 bp duplex site 5'-GGTGTGTGGGTGT-3' in yeast telomeric DNA with high affinity. Gel shift experiments revealed that the MBP-Tel2p fusion binds the double-stranded yeast telomeric Rap1p site in a sequence-specific manner. Analysis of mutated sites showed that MBP-Tel2p could bind 5'-GTGTGTGG-3' within this 13 bp site. Methylation interference analysis revealed that Tel2p contacts the 5'-terminal guanine in the major groove. MBP-Tel2p did not bind duplex telomeric DNA repeats from vertebrates, Tetrahymena or Oxytricha. These results suggest that Tel2p is a DNA binding protein that recognizes yeast telomeric DNA.

Telomeric localization of TRF2, a novel human telobox protein

Natural chromosomal ends are stabilized by proteins that bind duplex telomeric DNA repeats. In human cells, the TTAGGG Repeat Factor 1 (TRF1) was identified by two independent studies, one screening for factors that bind duplex telomeric DNA and the other screening for proteins containing a particular Myb motif called the telobox, which is required for telomeric repeat recognition (Fig. 1a; refs 3-5). A second human open reading frame, orf2, contains a telobox sequence and encodes a polypeptide that specifically recognizes mammalian telomeric repeat DNA in vitro. We show that two proteins of 65 and 69 kD, expressed in HeLa cells, contain the orf2 telobox sequence. These proteins are collectively termed TRF2. Affinity-purified antibodies specific for anti-TRF2 label the telomeres of intact human chromosomes, strengthening the correlation between occurrence of telobox and telomere-repeat recognition in vivo.