Holoenzyme proteins required for the physiological assembly and activity of telomerase

Brh2 and Rad51 promote telomere maintenance in Ustilago maydis, a new model system of DNA repair proteins at telomeres

Recent studies implicate a number of DNA repair proteins in mammalian telomere maintenance. However, because several key repair proteins in mammals are missing from the well-studied budding and fission yeast, their roles at telomeres cannot be modeled in standard fungi. In this report, we explored the dimorphic fungus Ustilago maydis as an alternative model for telomere research. This fungus, which belongs to the phylum Basidiomycota, has a telomere repeat unit that is identical to the mammalian repeat, as well as a constellation of DNA repair proteins that more closely mimic the mammalian collection. We showed that the two core components of homology-directed repair (HDR) in U. maydis, namely Brh2 and Rad51, both promote telomere maintenance in telomerase positive cells, just like in mammals. In addition, we found that Brh2 is localized to telomeres in vivo, suggesting that it acts directly at chromosome ends. We surveyed a series of mutants with DNA repair defects, and found many of them to have short telomeres. Our results indicate that factors involved in DNA repair are probably also needed for optimal telomere maintenance in U. maydis, and that this fungus is a useful alternative model system for telomere research.

Telomerase RNA biosynthesis and processing

Telomerase synthesizes repetitive G-rich sequences (telomeric repeats) at the ends of eukaryotic chromosomes. This mechanism maintains the integrity of the genome, as telomere shortening leads to degradation and fusion of chromosomes. The core components of telomerase are the telomerase catalytic subunit and telomerase RNA, which possesses a small template region serving for the synthesis of a telomeric repeat. Mutations in the telomerase RNA are associated with some cases of aplastic anemia and also cause dyskeratosis congenita, myelodysplasia, and pulmonary fibrosis. Telomerase is active in 85% of cancers, and telomerase activation is one of the first steps in cell transformation. The study of telomerase and pathways where this enzyme is involved will help to understand the mechanism of the mentioned diseases and to develop new approaches for their treatment. In this review we describe the modern conception of telomerase RNA biosynthesis, processing, and functioning in the three most studied systems - yeast, vertebrates, and ciliates.

The architecture of Tetrahymena telomerase holoenzyme

Telomerase adds telomeric repeats to chromosome ends using an internal RNA template and a specialized telomerase reverse transcriptase (TERT), thereby maintaining genome integrity. Little is known about the physical relationships among protein and RNA subunits within a biologically functional holoenzyme. Here we describe the architecture of Tetrahymena thermophila telomerase holoenzyme determined by electron microscopy. Six of the seven proteins and the TERT-binding regions of telomerase RNA (TER) have been localized by affinity labelling. Fitting with high-resolution structures reveals the organization of TERT, TER and p65 in the ribonucleoprotein (RNP) catalytic core. p50 has an unanticipated role as a hub between the RNP catalytic core, p75-p19-p45 subcomplex, and the DNA-binding Teb1. A complete in vitro holoenzyme reconstitution assigns function to these interactions in processive telomeric repeat synthesis. These studies provide the first view of the extensive network of subunit associations necessary for telomerase holoenzyme assembly and physiological function.

xRRM: a new class of RRM found in the telomerase La family protein p65

Genuine La and La-related proteins group 7 (LARP7) bind to the non-coding RNAs transcribed by RNA polymerase III (RNAPIII), which end in UUU-3'OH. The La motif and RRM1 of these proteins (the La module) cooperate to bind the UUU-3'OH, protecting the RNA from degradation, while other domains may be important for RNA folding or other functions. Among the RNAPIII transcripts is ciliate telomerase RNA (TER). p65, a member of the LARP7 family, is an integral Tetrahymena thermophila telomerase holoenzyme protein required for TER biogenesis and telomerase RNP assembly. p65, together with TER and telomerase reverse transcriptase (TERT), form the Tetrahymena telomerase RNP catalytic core. p65 has an N-terminal domain followed by a La module and a C-terminal domain, which binds to the TER stem 4. We recently showed that the p65 C-terminal domain harbors a cryptic, atypical RRM, which uses a unique mode of single- and double-strand RNA binding and is required for telomerase RNP catalytic core assembly. This domain, which we named xRRM, appears to be present in and unique to genuine La and LARP7 proteins. Here we review the structure of the xRRM, discuss how this domain could recognize diverse substrates of La and LARP7 proteins and discuss the functional implications of the xRRM as an RNP chaperone.

The association of a La module with the PABP-interacting motif PAM2 is a recurrent evolutionary process that led to the neofunctionalization of La-related proteins

La-related proteins (LARPs) are largely uncharacterized factors, well conserved throughout evolution. Recent reports on the function of human LARP4 and LARP6 suggest that these proteins fulfill key functions in mRNA metabolism and/or translation. We report here a detailed evolutionary history of the LARP4 and 6 families in eukaryotes. Genes coding for LARP4 and 6 were duplicated in the common ancestor of the vertebrate lineage, but one LARP6 gene was subsequently lost in the common ancestor of the eutherian lineage. The LARP6 gene was also independently duplicated several times in the vascular plant lineage. We observed that vertebrate LARP4 and plant LARP6 duplication events were correlated with the acquisition of a PABP-interacting motif 2 (PAM2) and with a significant reorganization of their RNA-binding modules. Using isothermal titration calorimetry (ITC) and immunoprecipitation methods, we show that the two plant PAM2-containing LARP6s (LARP6b and c) can, indeed, interact with the major plant poly(A)-binding protein (PAB2), while the third plant LARP6 (LARP6a) is unable to do so. We also analyzed the RNA-binding properties and the subcellular localizations of the two types of plant LARP6 proteins and found that they display nonredundant characteristics. As a whole, our results support a model in which the acquisition by LARP4 and LARP6 of a PAM2 allowed their targeting to mRNA 3' UTRs and led to their neofunctionalization.

New models of Tetrahymena telomerase RNA from experimentally derived constraints and modeling

The telomerase ribonucleoprotein complex ensures complete replication of eukaryotic chromosomes. Telomerase RNA (TER) provides the template for replicating the G-rich strand of telomeric DNA, provides an anchor site for telomerase-associated proteins, and participates in catalysis through several incompletely characterized mechanisms. A major impediment toward understanding its nontemplating roles is the absence of high content structural information for TER within the telomerase complex. Here, we used selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) to examine the structure of Tetrahymena TER free in solution and bound to tTERT in the minimal telomerase RNP. We discovered a striking difference in the two conformations and established direct evidence for base triples in the tTER pseudoknot. We then used SHAPE data, previously published FRET data, and biochemical inference to model the structure of tTER using discrete molecular dynamics simulations. The resulting tTER structure was docked with a homology model of the Tetrahymena telomerase reverse transcriptase (tTERT) to characterize the conformational changes of tTER telomerase assembly. Free in solution, tTER appears to contain four pairing regions: stems I, II, and IV, which are present in the commonly accepted structure, and stem III, a large paired region that encompasses the template and pseudoknot domains. Our interpretation of the data and subsequent modeling affords a molecular model for telomerase assemblage in which a large stem III of tTER unwinds to allow proper association of the template with the tTERT active site and formation of the pseudoknot. Additionally, analysis of our SHAPE data and previous enzymatic footprinting allow us to propose a model for stem-loop IV function in which tTERT is activated by binding stem IV in the major groove of the helix-capping loop.

Biogenesis of telomerase ribonucleoproteins

Telomerase adds simple-sequence repeats to the ends of linear chromosomes to counteract the loss of end sequence inherent in conventional DNA replication. Catalytic activity for repeat synthesis results from the cooperation of the telomerase reverse transcriptase protein (TERT) and the template-containing telomerase RNA (TER). TERs vary widely in sequence and structure but share a set of motifs required for TERT binding and catalytic activity. Species-specific TER motifs play essential roles in RNP biogenesis, stability, trafficking, and regulation. Remarkably, the biogenesis pathways that generate mature TER differ across eukaryotes. Furthermore, the cellular processes that direct the assembly of a biologically functional telomerase holoenzyme and its engagement with telomeres are evolutionarily varied and regulated. This review highlights the diversity of strategies for telomerase RNP biogenesis, RNP assembly, and telomere recruitment among ciliates, yeasts, and vertebrates and suggests common themes in these pathways and their regulation.

Structural basis for telomerase RNA recognition and RNP assembly by the holoenzyme La family protein p65

Telomerase is a ribonucleoprotein complex essential for maintenance of telomere DNA at linear chromosome ends. The catalytic core of Tetrahymena telomerase comprises a ternary complex of telomerase RNA (TER), telomerase reverse transcriptase (TERT), and the essential La family protein p65. NMR and crystal structures of p65 C-terminal domain and its complex with stem IV of TER reveal that RNA recognition is achieved by a combination of single- and double-stranded RNA binding, which induces a 105° bend in TER. The domain is a cryptic, atypical RNA recognition motif with a disordered C-terminal extension that forms an α helix in the complex necessary for hierarchical assembly of TERT with p65-TER. This work provides the first structural insight into biogenesis and assembly of TER with a telomerase-specific protein. Additionally, our studies define a structurally homologous domain (xRRM) in genuine La and LARP7 proteins and suggest a general mode of RNA binding for biogenesis of their diverse RNA targets.

The C-terminal domain of Tetrahymena thermophila telomerase holoenzyme protein p65 induces multiple structural changes in telomerase RNA

The unique cellular activity of the telomerase reverse transcriptase ribonucleoprotein (RNP) requires proper assembly of protein and RNA components into a functional complex. In the ciliate model organism Tetrahymena thermophila, the La-domain protein p65 is required for in vivo assembly of telomerase. Single-molecule and biochemical studies have shown that p65 promotes efficient RNA assembly with the telomerase reverse transcriptase (TERT) protein, in part by inducing a bend in the conserved stem IV region of telomerase RNA (TER). The domain architecture of p65 consists of an N-terminal domain, a La-RRM motif, and a C-terminal domain (CTD). Using single-molecule Förster resonance energy transfer (smFRET), we demonstrate the p65(CTD) is necessary for the RNA remodeling activity of the protein and is sufficient to induce a substantial conformational change in stem IV of TER. Moreover, nuclease protection assays directly map the site of p65(CTD) interaction to stem IV and reveal that, in addition to bending stem IV, p65 binding reorganizes nucleotides that comprise the low-affinity TERT binding site within stem-loop IV.

Roles of telomerase reverse transcriptase N-terminal domain in assembly and activity of Tetrahymena telomerase holoenzyme

Telomerase extends chromosome ends by the addition of single-stranded telomeric repeats. To support processive repeat synthesis, it has been proposed that coordination occurs between DNA interactions with the telomerase RNA template, the active site in the telomerase reverse transcriptase (TERT) core, a TERT N-terminal (TEN) domain, and additional subunits of the telomerase holoenzyme required for telomere elongation in vivo. The roles of TEN domain surface residues in primer binding and product elongation have been studied largely using assays of minimal recombinant telomerase enzymes, which lack holoenzyme subunits that properly fold and conformationally stabilize the ribonucleoprotein and/or control its association with telomere substrates in vivo. Here, we use Tetrahymena telomerase holoenzyme reconstitution in vitro to assess TEN domain sequence requirements in the physiological enzyme context. We find that TEN domain sequence substitutions in the Tetrahymena telomerase holoenzyme influence synthesis initiation and elongation rate but not processivity. Functional and direct physical interaction assays pinpoint a conserved TEN domain surface required for holoenzyme subunit association and for high repeat addition processivity. Our results add to the understanding of telomerase holoenzyme architecture and TERT domain functions with direct implications for the unique mechanism of single-stranded repeat synthesis.

Telomerase and telomere-associated proteins: structural insights into mechanism and evolution

Recent advances in our structural understanding of telomerase and telomere-associated proteins have contributed significantly to elucidating the molecular mechanisms of telomere maintenance. The structures of telomerase TERT domains have provided valuable insights into how experimentally identified conserved motifs contribute to the telomerase reverse transcriptase reaction. Additionally, structures of telomere-associated proteins in a variety of organisms have revealed that, across evolution, telomere-maintenance mechanisms employ common structural elements. For example, the single-stranded 3' overhang of telomeric DNA is specifically and tightly bound by an OB-fold in nearly all species, including ciliates (TEBP and Pot1a), fission yeast (SpPot1), budding yeast (Cdc13), and humans (hPOT1). Structures of the yeast Cdc13, Stn1, and Ten1 proteins demonstrated that telomere maintenance is regulated by a complex that bears significant similarity to the RPA heterotrimer. Similarly, proteins that specifically bind double-stranded telomeric DNA in divergent species use homeodomains to execute their functions (human TRF1 and TRF2 and budding yeast ScRap1). Likewise, the conserved protein Rap1, which is found in budding yeast, fission yeast, and humans, contains a structural motif that is known to be critical for protein-protein interaction. In addition to revealing the common underlying themes of telomere maintenance, structures have also elucidated the specific mechanisms by which many of these proteins function, including identifying a telomere-specific domain in Stn1 and how the human TRF proteins avoid heterodimerization. In this review, we summarize the high-resolution structures of telomerase and telomere-associated proteins and discuss the emergent common structural themes among these proteins. We also address how these high-resolution structures complement biochemical and cellular studies to enhance our understanding of telomere maintenance and function.

The RNA accordion model for template positioning by telomerase RNA during telomeric DNA synthesis

Telomerase is a ribonucleoprotein (RNP) enzyme that maintains the ends of linear eukaryotic chromosomes and whose activation is a hallmark of 90% of all cancers. This RNP minimally contains a reverse transcriptase protein subunit (TERT) that catalyzes telomeric DNA synthesis and an RNA subunit (TER) that has templating, architectural and protein-scaffolding roles. Telomerase is unique among polymerases in that it synthesizes multiple copies of the template on the 3′ end of a primer following a single binding event, a process known as repeat addition processivity (RAP). Using biochemical assays and single-molecule Förster resonance energy transfer (smFRET) experiments on Tetrahymena thermophila telomerase, we now directly demonstrate that TER contributes to template positioning within the active site and to the template translocation required for RAP. We propose that the single-stranded RNA elements flanking the template act as a molecular accordion, undergoing reciprocal extension and compaction during telomerase translocation.

Single-molecule analysis of telomerase structure and function

The telomerase ribonucleoprotein is a specialized reverse transcriptase required to maintain protective chromosome end-capping structures called telomeres. In most cells, telomerase is not active and the natural shortening of telomeres with each round of DNA replication ultimately triggers cell growth arrest. In contrast, the presence of telomerase confers a high level of renewal capacity upon rapidly dividing cells. Telomerase is aberrantly activated in 90% of human cancers and thus represents an important target for anticancer therapeutics. However, the naturally low abundance of telomerase has hampered efforts to obtain high-resolution models for telomerase structure and function. To circumvent these challenges, single-molecule techniques have recently been employed to investigate telomerase assembly, structure, and catalysis.

Telomerase: structure, functions, and activity regulation

Telomerase is the enzyme responsible for maintenance of the length of telomeres by addition of guanine-rich repetitive sequences. Telomerase activity is exhibited in gametes and stem and tumor cells. In human somatic cells proliferation potential is strictly limited and senescence follows approximately 50-70 cell divisions. In most tumor cells, on the contrary, replication potential is unlimited. The key role in this process of the system of the telomere length maintenance with involvement of telomerase is still poorly studied. No doubt, DNA polymerase is not capable to completely copy DNA at the very ends of chromosomes; therefore, approximately 50 nucleotides are lost during each cell cycle, which results in gradual telomere length shortening. Critically short telomeres cause senescence, following crisis, and cell death. However, in tumor cells the system of telomere length maintenance is activated. Besides catalytic telomere elongation, independent telomerase functions can be also involved in cell cycle regulation. Inhibition of the telomerase catalytic function and resulting cessation of telomere length maintenance will help in restriction of tumor cell replication potential. On the other hand, formation of temporarily active enzyme via its intracellular activation or due to stimulation of expression of telomerase components will result in telomerase activation and telomere elongation that can be used for correction of degenerative changes. Data on telomerase structure and function are summarized in this review, and they are compared for evolutionarily remote organisms. Problems of telomerase activity measurement and modulation by enzyme inhibitors or activators are considered as well.

The Pot1a-associated proteins Tpt1 and Pat1 coordinate telomere protection and length regulation in Tetrahymena

We have identified two new Pot1a-associated telomere proteins, Pat1 and Tpt1, from Tetrahymena. Tpt1 is required to prevent telomere elongation and appears to be the Tetrahymena equivalent of vertebrate TPP1. Pat1 depletion causes gradual telomere shortening, indicating that it is needed for telomerase to gain access to the DNA terminus.

We have identified two new telomere proteins, Tpt1 and Pat1, from the ciliate Tetrahymena thermophila. Although Tetrahymena telomerase is well characterized, only one telomere protein had previously been identified. This was the G-overhang binding-protein Pot1a. Tpt1 and Pat1 were isolated as Pot1a binding partners and shown to localize to telomeres. As Tpt1 and Pat1 were both found to be essential, conditional cell lines were generated to explore their function. Tpt1 depletion caused a rapid growth arrest and telomere elongation in the absence of cell division. The phenotype was similar to that seen after Pot1a depletion suggesting that Tpt1 and Pot1a function together to regulate telomere length and prevent telomere deprotection. In contrast, Pat1 depletion had a modest effect on cell growth but caused progressive telomere shortening similar to that observed upon TERT depletion. Thus Pat1 appears to be needed for telomerase to maintain the chromosome terminus. Analysis of Pot1a-Tpt1-Pat1 complex formation using purified proteins indicated that Tpt1 interacts directly with Pot1a while Pat1 interacts with Tpt1. Our results indicate that Tpt1 is the Tetrahymena equivalent of mammalian TPP1, Schizosaccharomyces pombe Tpz1, and Oxytricha nova TEBPβ.

Functional importance of telomerase pseudoknot revealed by single-molecule analysis

Telomerase ribonucleoprotein (RNP) employs an RNA subunit to template the addition of telomeric repeats onto chromosome ends. Previous studies have suggested that a region of the RNA downstream of the template may be important for telomerase activity and that the region could fold into a pseudoknot. Whether the pseudoknot motif is formed in the active telomerase RNP and what its functional role is have not yet been conclusively established. Using single-molecule FRET, we show that the isolated pseudoknot sequence stably folds into a pseudoknot. However, in the context of the full-length telomerase RNA, interference by other parts of the RNA prevents the formation of the pseudoknot. The protein subunits of the telomerase holoenzyme counteract RNA-induced misfolding and allow a significant fraction of the RNPs to form the pseudoknot structure. Only those RNP complexes containing a properly folded pseudoknot are catalytically active. These results not only demonstrate the functional importance of the pseudoknot but also reveal the critical role played by telomerase proteins in pseudoknot folding.

Mapping of DNA binding sites in the Tetrahymena telomerase holoenzyme proteins by UV cross-linking and mass spectrometry

The Tetrahymena telomerase holoenzyme consists of a major catalytic protein [telomerase reverse transcriptase (TERT)], an RNA subunit, and accessory proteins. We used site-specific UV cross-linking and mass spectrometry to map interactions between the holoenzyme and the telomeric DNA. In one series of experiments, an oligodeoxyribonucleotide containing a 5-iododeoxyuridine residue or 4-thio-deoxythymidine residue was cross-linked to the telomerase by irradiation with UV light-emitting diodes. The DNA was extended by the cross-linked enzyme with a radioactively labeled or unlabeled nucleotide. The complexes were subsequently resolved by SDS-PAGE. Proteins were isolated from strips in the unlabeled gels corresponding to bands observed in the radioactive gels. Mass spectrometric analysis of these proteins revealed a major cross-linking site in TERT. Serendipitous cleavage of TERT near amino acid 254 indicated that this site maps within the N-terminal cleavage product, which includes primarily the telomerase essential N-terminal (TEN) domain. Moreover, the absence of this N-terminal segment in TERT was found to cause a reduction in DNA binding by the telomerase and/or its activity to undetectable levels. In other experiments, similar unresolved cross-linked complexes were digested with trypsin, two exonucleases, and alkaline phosphatase. Tandem mass spectrometry was then used to search for peptides linked to the residual deoxyribonucleoside. Using this approach, we identified the phenylalanine residue F351 in the accessory protein p45 as a minor DNA cross-linking site. Our study constitutes the first direct mapping of DNA interaction sites in telomerase holoenzyme complexes. This mapping represents a significant contribution to the understanding of the mechanism of telomere extension by telomerase.

A modified model for translocation events of processive nucleotide and repeat additions by the recombinant telomerase

Telomerase is a unique reverse transcriptase that extends the single-stranded 3' overhangs of telomeres by copying a short template sequence within the integral RNA component of the enzyme. It shows processive nucleotide and repeat addition activities, which are realized via two types of movements: translocation of the DNA:RNA hybrid away from the active site following each nucleotide addition and translocation of the 3' end of the DNA primer relative to the RNA template after each round of repeat synthesis. Here, a model is presented to describe these two types of translocation events by the recombinant Tetrahymena telomerase, via the modification of the model that has been proposed recently. Using the present model, the dynamics of the dissociation of the DNA primer from the telomerase and the dynamics of the disruption of the DNA:RNA hybrid and then repositioning of the product 3' end to the beginning of the template are studied quantitatively. Their effects on the repeat addition processivity are theoretically studied. The theoretical results are in agreement with the available experimental data.

Oligonucleotide/oligosaccharide-binding fold proteins: a growing family of genome guardians

The maintenance of genomic stability relies on the coordinated action of a number of cellular processes, including activation of the DNA-damage checkpoint, DNA replication, DNA repair, and telomere homeostasis. Many proteins involved in these cellular processes use different types of functional modules to regulate and execute their functions. Recent studies have revealed that many DNA-damage checkpoint and DNA repair proteins in human cells possess the oligonucleotide/oligosaccharide-binding (OB) fold domains, which are known to bind single-stranded DNA in both prokaryotes and eukaryotes. Furthermore, during the DNA damage response, the OB folds of the human checkpoint and DNA repair proteins play critical roles in DNA binding, protein complex assembly, and regulating protein-protein interactions. These findings suggest that the OB fold is an evolutionarily conserved functional module that is widely used by genome guardians. In this review, we will highlight the functions of several well-characterized or newly discovered eukaryotic OB-fold proteins in the DNA damage response.

Tetrahymena telomerase protein p65 induces conformational changes throughout telomerase RNA (TER) and rescues telomerase reverse transcriptase and TER assembly mutants

The biogenesis of the Tetrahymena telomerase ribonucleoprotein particle (RNP) is enhanced by p65, a La family protein. Single-molecule and biochemical studies have uncovered a hierarchical assembly of the RNP, wherein the binding of p65 to stems I and IV of telomerase RNA (TER) causes a conformational change that facilitates the subsequent binding of telomerase reverse transcriptase (TERT) to TER. We used purified p65 and variants of TERT and TER to investigate the conformational rearrangements that occur during RNP assembly. Nuclease protection assays and mutational analysis revealed that p65 interacts with and stimulates conformational changes in regions of TER beyond stem IV. Several TER mutants exhibited telomerase activity only in the presence of p65, revealing the importance of p65 in promoting the correct RNP assembly pathway. In addition, p65 rescued TERT assembly mutants but not TERT activity mutants. Taken together, these results suggest that p65 stimulates telomerase assembly and activity in two ways. First, by sequestering stems I and IV, p65 limits the ensemble of structural conformations of TER, thereby presenting TERT with the active conformation of TER. Second, p65 acts as a molecular buttress within the assembled RNP, mutually stabilizing TER and TERT in catalytically active conformations.

Conserved and divergent features of the structure and function of La and La-related proteins (LARPs)

Genuine La proteins contain two RNA binding motifs, a La motif (LAM) followed by a RNA recognition motif (RRM), arranged in a unique way to bind RNA. These proteins interact with an extensive variety of cellular RNAs and exhibit activities in two broad categories: i) to promote the metabolism of nascent pol III transcripts, including precursor-tRNAs, by binding to their common, UUU-3'OH containing ends, and ii) to modulate the translation of certain mRNAs involving an unknown binding mechanism. Characterization of several La-RNA crystal structures as well as biochemical studies reveal insight into their unique two-motif domain architecture and how the LAM recognizes UUU-3'OH while the RRM binds other parts of a pre-tRNA. Recent studies of members of distinct families of conserved La-related proteins (LARPs) indicate that some of these harbor activity related to genuine La proteins, suggesting that their UUU-3'OH binding mode has been appropriated for the assembly and regulation of a specific snRNP (e.g., 7SK snRNP assembly by hLARP7/PIP7S). Analyses of other LARP family members suggest more diverged RNA binding modes and specialization for cytoplasmic mRNA-related functions. Thus it appears that while genuine La proteins exhibit broad general involvement in both snRNA-related and mRNA-related functions, different LARP families may have evolved specialized activities in either snRNA or mRNA-related functions. In this review, we summarize recent progress that has led to greater understanding of the structure and function of La proteins and their roles in tRNA processing and RNP assembly dynamics, as well as progress on the different LARPs.

Multiple mechanisms for elongation processivity within the reconstituted tetrahymena telomerase holoenzyme

To maintain telomeres, telomerase evolved a unique biochemical activity: the use of a single-stranded RNA template for the synthesis of single-stranded DNA repeats. High repeat addition processivity (RAP) of the Tetrahymena telomerase holoenzyme requires association of the catalytic core with the telomere adaptor subcomplex (TASC) and an RPA1-related subunit (p82 or Teb1). Here, we used DNA binding and holoenzyme reconstitution assays to investigate the mechanism by which Teb1 and TASC confer high RAP. We show that TASC association with the recombinant telomerase catalytic core increases enzyme activity. Subsequent association of the Teb1 C-terminal domain with TASC confers the capacity for high RAP even though the Teb1 C-terminal domain does not provide a high-affinity DNA interaction site. Efficient RAP also requires suppression of nascent product folding mediated by the central Teb1 DNA-binding domains (DBDs). These sequence-specific high-affinity DBDs of Teb1 can be functionally substituted by the analogous DBDs of Tetrahymena Rpa1 to suppress nascent product folding but only if the Rpa1 high-affinity DBDs are physically tethered into holoenzyme context though the Teb1 C-terminal domain. Overall, our findings reveal multiple mechanisms and multiple surfaces of protein-DNA and protein-protein interaction that give rise to elongation processivity in the synthesis of a single-stranded nucleic acid product.

Ciliate telomerase RNA loop IV nucleotides promote hierarchical RNP assembly and holoenzyme stability

Telomerase adds simple-sequence repeats to chromosome 3' ends to compensate for the loss of repeats with each round of genome replication. To accomplish this de novo DNA synthesis, telomerase uses a template within its integral RNA component. In addition to providing the template, the telomerase RNA subunit (TER) also harbors nontemplate motifs that contribute to the specialized telomerase catalytic cycle of reiterative repeat synthesis. Most nontemplate TER motifs function through linkage with the template, but in ciliate and vertebrate telomerases, a stem-loop motif binds telomerase reverse transcriptase (TERT) and reconstitutes full activity of the minimal recombinant TERT+TER RNP, even when physically separated from the template. Here, we resolve the functional requirements for this motif of ciliate TER in physiological RNP context using the Tetrahymena thermophila p65-TER-TERT core RNP reconstituted in vitro and the holoenzyme reconstituted in vivo. Contrary to expectation based on assays of the minimal recombinant RNP, we find that none of a panel of individual loop IV nucleotide substitutions impacts the profile of telomerase product synthesis when reconstituted as physiological core RNP or holoenzyme RNP. However, loop IV nucleotide substitutions do variably reduce assembly of TERT with the p65-TER complex in vitro and reduce the accumulation and stability of telomerase RNP in endogenous holoenzyme context. Our results point to a unifying model of a conformational activation role for this TER motif in the telomerase RNP enzyme.

An RPA-related sequence-specific DNA-binding subunit of telomerase holoenzyme is required for elongation processivity and telomere maintenance

Telomerase ribonucleoprotein complexes copy an internal RNA template to synthesize DNA repeats. DNA-interacting subunits other than telomerase reverse transcriptase (TERT) and telomerase RNA (TER) have been hypothesized to account for high repeat addition processivity of telomerase holoenzyme compared to the minimal catalytic RNP. Here, we present the identification of three additional subunits of Tetrahymena thermophila telomerase holoenzyme. Each of seven telomerase proteins is required for telomere maintenance and copurifies active RNP. The catalytic core (p65-TER-TERT) is assembled with a three-protein subcomplex (p75-p45-p19) and two peripheral subunits (p82 and p50). Remarkably, only a p82-enriched subset of the total holoenzyme population is capable of high repeat addition processivity, as shown by p82 immunodepletion and add-back. The RPA-like p82 subunit binds sequence specifically to multiple telomeric repeats. These discoveries establish the existence of a telomerase holoenzyme processivity subunit with sequence-specific DNA binding.

Sequence, biogenesis, and function of diverse small RNA classes bound to the Piwi family proteins of Tetrahymena thermophila

PAZ/PIWI domain (PPD) proteins carrying small RNAs (sRNAs) function in gene and genome regulation. The ciliate Tetrahymena thermophila encodes numerous PPD proteins exclusively of the Piwi clade. We show that the three Tetrahymena Piwi family proteins (Twis) preferentially expressed in growing cells differ in their genetic essentiality and subcellular localization. Affinity purification of all eight distinct Twi proteins revealed unique properties of their bound sRNAs. Deep sequencing of Twi-bound and total sRNAs in strains disrupted for various silencing machinery uncovered an unanticipated diversity of 23- to 24-nt sRNA classes in growing cells, each with distinct genetic requirements for accumulation. Altogether, Twis distinguish sRNAs derived from loci of pseudogene families, three types of DNA repeats, structured RNAs, and EST-supported loci with convergent or paralogous transcripts. Most surprisingly, Twi7 binds complementary strands of unequal length, while Twi10 binds a specific permutation of the guanosine-rich telomeric repeat. These studies greatly expand the structural and functional repertoire of endogenous sRNAs and RNPs.

A possible mechanism of processive nucleotide and repeat additions by the telomerase

Telomerase is a specialized cellular ribonucleoprotein complex that can synthesize long stretches of a DNA primer by using an intrinsic RNA template sequence. This requires that the telomerase must be able to carry out both nucleotide and repeat additions. Here, based on available structures and experimental data, a model is presented to describe these two addition activities. In the model, the forward movement of the polymerase active site along the template during the processive nucleotide addition is rectified through the incorporation of a matched base, via the Brownian ratchet mechanism. The unpairing of the DNA:RNA hybrid and then repositioning of product 3'-end after each round of repeat synthesis, which are prerequisites for the processive repeat addition, are caused by a force acting on the primer. The force results from the conformational transition of the stem III pseudoknot, which is mechanically induced by the rotation of TERT fingers together with stem IV loop towards the polymerase active site upon a nucleotide binding. Based on the model, the dynamics of processive nucleotide and repeat additions by recombinant Tetrahymena telomerase is studied analytically, which gives good quantitative explanations to the previous experimental results. Moreover, some predicted results are presented. In particular, it is shown that the repeat addition processivity is mainly determined by the difference between the free-energy change required to disrupt the DNA:RNA hybrid and that required to unfold the stem III pseudoknot. A large difference in free energy corresponds to a low repeat addition processivity while a small difference in free energy corresponds to a high repeat addition processivity.

Single-Molecule Fluorescence Studies of RNA: A Decade's Progress

Over the past decade, single-molecule fluorescence studies have elucidated the structure-function relationship of RNA molecules. The real-time observation of individual RNAs by single-molecule fluorescence has unveiled the dynamic behavior of complex RNA systems in unprecedented detail, revealing the presence of transient intermediate states and their kinetic pathways. This review provides an overview of how single-molecule fluorescence has been used to explore the dynamics of RNA folding and catalysis.

A comprehensive analysis of the La-motif protein superfamily

The extremely well-conserved La motif (LAM), in synergy with the immediately following RNA recognition motif (RRM), allows direct binding of the (genuine) La autoantigen to RNA polymerase III primary transcripts. This motif is not only found on La homologs, but also on La-related proteins (LARPs) of unrelated function. LARPs are widely found amongst eukaryotes and, although poorly characterized, appear to be RNA-binding proteins fulfilling crucial cellular functions. We searched the fully sequenced genomes of 83 eukaryotic species scattered along the tree of life for the presence of LAM-containing proteins. We observed that these proteins are absent from archaea and present in all eukaryotes (except protists from the Plasmodium genus), strongly suggesting that the LAM is an ancestral motif that emerged early after the archaea-eukarya radiation. A complete evolutionary and structural analysis of these proteins resulted in their classification into five families: the genuine La homologs and four LARP families. Unexpectedly, in each family a conserved domain representing either a classical RRM or an RRM-like motif immediately follows the LAM of most proteins. An evolutionary analysis of the LAM-RRM/RRM-L regions shows that these motifs co-evolved and should be used as a single entity to define the functional region of interaction of LARPs with their substrates. We also found two extremely well conserved motifs, named LSA and DM15, shared by LARP6 and LARP1 family members, respectively. We suggest that members of the same family are functional homologs and/or share a common molecular mode of action on different RNA baits.

Disease-associated human telomerase RNA variants show loss of function for telomere synthesis without dominant-negative interference

Telomerase adds simple-sequence repeats to chromosome ends to offset the terminal sequence loss inherent in each cycle of genome replication. Inherited mutations in genes encoding subunits of the human telomerase holoenzyme give rise to disease phenotypes including hematopoietic failure and pulmonary fibrosis. Disease-associated variants of the human telomerase RNA are expressed in heterozygous combination with wild-type telomerase RNA. Here, we exploit a sensitized human primary cell assay system to investigate the biological function of disease-linked telomerase RNA variants and their impact on the function of coexpressed wild-type telomerase RNA. We find that telomerase RNA variants discovered in patients with dyskeratosis congenita or aplastic anemia show loss of function without any indication of dominant-negative impact on telomere maintenance by the coexpressed wild-type RNA. To reconcile this result with contradictory findings from reconstitution assays in vitro, we demonstrate that the lack of dominant-negative impact on telomere maintenance correlates with physiological assembly of active human telomerase holoenzyme ribonucleoproteins harboring monomers rather than higher-order multimers of telomerase RNA and telomerase reverse transcriptase. These findings support loss of function of telomerase RNA as a general mechanism of human disease.

C. elegans La-related protein, LARP-1, localizes to germline P bodies and attenuates Ras-MAPK signaling during oogenesis

RNA regulators are critical for animal development, especially in the germ line where gene expression is often modulated by changes in mRNA stability, translation, and localization. In this paper, we focus on Caenorhabditis elegans LARP-1, a representative of one La-related protein (Larp) family found broadly among eukaryotes. LARP-1 possesses a signature La motif, which is an ancient RNA-binding domain, plus a second conserved motif, typical of LARP-1 homologs and therefore dubbed the LARP1 domain. LARP-1 appears to bind RNA in vitro via both the La motif and the LARP1 domain. larp-1 null mutants have an oogenesis defect reminiscent of hyperactive Ras-MAPK signaling; this defect is suppressed or enhanced by down- or up-regulating the Ras-MAPK pathway, respectively. Consistent with a role in down-regulating the Ras-MAPK pathway, larp-1 null mutants have higher than normal levels of selected pathway mRNAs and proteins. LARP-1 protein colocalizes with P bodies, which function in RNA degradation. We suggest that LARP-1 functions in P bodies to attenuate the abundance of conserved Ras-MAPK mRNAs. We also propose that the cluster of LARP-1 homologs may function generally to control the expression of key developmental regulators.

Identification of ATPases pontin and reptin as telomerase components essential for holoenzyme assembly

Telomerase is a multisubunit ribonucleoprotein (RNP) complex that adds telomere repeats to the ends of chromosomes. Three essential telomerase components have been identified thus far: the telomerase reverse transcriptase (TERT), the telomerase RNA component (TERC), and the TERC-binding protein dyskerin. Few other proteins are known to be required for human telomerase function, limiting our understanding of both telomerase regulation and mechanisms of telomerase action. Here, we identify the ATPases pontin and reptin as telomerase components through affinity purification of TERT from human cells. Pontin interacts directly with both TERT and dyskerin, and the amount of TERT bound to pontin and reptin peaks in S phase, evidence for cell-cycle-dependent regulation of TERT. Depletion of pontin and reptin markedly impairs telomerase RNP accumulation, indicating an essential role in telomerase assembly. These findings reveal an unanticipated requirement for additional enzymes in telomerase biogenesis and suggest alternative approaches for inhibiting telomerase in cancer.

Structure of the RNA-binding domain of telomerase: implications for RNA recognition and binding

Telomerase, a ribonucleoprotein complex, replicates the linear ends of eukaryotic chromosomes, thus taking care of the "end of replication problem." TERT contains an essential and universally conserved domain (TRBD) that makes extensive contacts with the RNA (TER) component of the holoenzyme, and this interaction is thought to facilitate TERT/TER assembly and repeat-addition processivity. Here, we present a high-resolution structure of TRBD from Tetrahymena thermophila. The nearly all-helical structure comprises a nucleic acid-binding fold suitable for TER binding. An extended pocket on the surface of the protein, formed by two conserved motifs (CP and T motifs) comprises TRBD's RNA-binding pocket. The width and the chemical nature of this pocket suggest that it binds both single- and double-stranded RNA, possibly stem I, and the template boundary element (TBE). Moreover, the structure provides clues into the role of this domain in TERT/TER stabilization and telomerase repeat-addition processivity.

Purification of human telomerase complexes identifies factors involved in telomerase biogenesis and telomere length regulation

The identities and roles of proteins associated with human telomerase remain poorly defined. To gain insight, we undertook an affinity purification of endogenously assembled human telomerase complexes. We show that specific subsets of H/ACA, Sm, and hnRNP proteins associate with active and inactive telomerase RNPs, while two NTPase proteins associate preferentially with active enzyme. All three core H/ACA-motif binding proteins are telomerase holoenzyme components essential for RNP accumulation. On the other hand, telomerase RNPs lacking interaction with Sm proteins or hnRNP C remain fully functional for telomere elongation. Curiously, overexpression of either associated hnRNP protein (hnRNP C and hnRNP U) or either NTPase protein (NAT10 and GNL3L) induced telomere shortening. Our findings suggest that endogenous human telomerase complexes are more heterogeneous than those of single-celled eukaryotes, have predominantly shared rather than telomerase-specific proteins, and make numerous regulatory interactions.

Expression and suppression of human telomerase RNA

Telomeres are maintained by the ribonucleoprotein (RNP) enzyme telomerase, which replenishes telomeres through its unique mechanism of internal RNA-templated addition of telomeric DNA. Telomerase is active in most human cancers, typically because its core protein subunit, TERT, is up-regulated. Although the major known function of telomerase in cancer is to replenish telomeric DNA and maintain cell immortality, the regulation of the RNA component of telomerase is not well understood. In the course of investigations that have implicated telomerase RNA in key aspects of cancer progression, including metastasis, we explored some of the cis-acting elements affecting telomerase RNA expression and knockdown. The expression efficiency and subsequent RNA processing to produce the mature hTER differed considerably among various promoters. Together with other results, these findings establish that the crucial elements of the hTER gene affecting RNA-processing efficiency to produce the mature hTER RNA are the promoter and internal telomerase RNA-coding sequences.

Stepwise protein-mediated RNA folding directs assembly of telomerase ribonucleoprotein

Telomerase is an essential cellular ribonucleoprotein (RNP) that solves the end replication problem and maintains chromosome stability by adding telomeric DNA to the termini of linear chromosomes. Genetic mutations that abrogate the normal assembly of telomerase RNP cause human disease. It is therefore of fundamental and medical importance to decipher cellular strategies for telomerase biogenesis, which will require new insights into how specific interactions occur in a precise order along the RNP assembly pathway. Here we use a single-molecule approach to dissect the individual assembly steps of telomerase. Direct observation of complex formation in real time revealed two sequential steps of protein-induced RNA folding, establishing a hierarchical RNP assembly mechanism: interaction with the telomerase holoenzyme protein p65 induces structural rearrangement of telomerase RNA, which in turn directs the binding of the telomerase reverse transcriptase to form the functional ternary complex. This hierarchical assembly process is facilitated by an evolutionarily conserved structural motif within the RNA. These results identify the RNA folding pathway during telomerase biogenesis and define the mechanism of action for an essential telomerase holoenzyme protein.

The unmasking of telomerase

Telomerase is a ribonucleoprotein complex that reverse transcribes a portion of its RNA subunit during the synthesis of G-rich DNA at the 3' end of each chromosome in most eukaryotes. This activity compensates for the inability of the normal DNA replication machinery to fully replicate chromosome termini. The roles of telomerase in cellular immortality and tumor biology have catalyzed a significant interest in this unusual polymerase. Recently the first structures of two domains, the CR4/CR5 and pseudoknot, of human telomerase RNA (hTR) were reported, offering a structural basis for interpreting biochemical studies and possible roles of hTR mutations in human diseases. Structures of the stem II and stem IV domains of Tetrahymena thermophila TR as well as the N-terminal domain of the T. thermophila telomerase reverse transcriptase have also been determined. These studies complement previous biochemical studies, providing rich insight into the structural basis for telomerase activity.

Positive and negative regulation of Tetrahymena telomerase holoenzyme

Telomerase replenishes the telomeric repeats that cap eukaryotic chromosome ends. To perform DNA synthesis, the active site of telomerase reverse transcriptase (TERT) copies a template within the integral telomerase RNA (TER). In vivo, TERT and TER and additional subunits form a telomerase holoenzyme capable of telomere elongation. We previously purified epitope-tagged Tetrahymena thermophila TERT and characterized two of the associated proteins. Here we characterize the remaining two proteins that were enriched by TERT purification. The primary sequence of the p75 polypeptide lacks evident homology with other proteins, whereas the p20 polypeptide is the Tetrahymena ortholog of a conserved multifunctional protein, Skp1. Genetic depletion of p75 induced telomere shortening without affecting the accumulation of TER or TERT, suggesting that p75 promotes telomerase function at the telomere. Affinity purification of p75 coenriched telomerase activity and each other known telomerase holoenzyme protein. On the other hand, genetic depletion of Skp1p induced telomere elongation, suggesting that this protein plays a negative regulatory role in the maintenance of telomere length homeostasis. Affinity purification of Skp1p did not detectably enrich active telomerase but did copurify ubiquitin ligase machinery. These studies reveal additional complexity in the positive and negative regulation of Tetrahymena telomerase function.

The biogenesis and regulation of telomerase holoenzymes

Chromosome stability requires a dynamic balance of DNA loss and gain in each terminal tract of telomeric repeats. Repeat addition by a specialized reverse transcriptase, telomerase, has an important role in maintaining this equilibrium. Insights that have been gained into the cellular pathways for biogenesis and regulation of telomerase ribonucleoproteins raise new questions, particularly concerning the dynamic nature of this unique polymerase.

Structural study of elements of Tetrahymena telomerase RNA stem-loop IV domain important for function

Tetrahymena telomerase RNA (TER) contains several regions in addition to the template that are important for function. Central among these is the stem-loop IV domain, which is involved in both catalysis and RNP assembly, and includes binding sites for both the holoenzyme assembly protein p65 and telomerase reverse transcriptase (TERT). Stem-loop IV contains two regions with high evolutionary sequence conservation: a central GA bulge between helices, and a terminal loop. We solved the solution structure of loop IV and modeled the structure of the helical region containing the GA bulge, using NMR and residual dipolar couplings. The central GA bulge with flanking C-G base pairs induces a approximately 50 degrees semi-rigid bend in the helix. Loop IV is highly structured, and contains a conserved C-U base pair at the top of the helical stem. Analysis of new and previous biochemical data in light of the structure provides a rationale for some of the sequence conservation in this region of TER. The results suggest that during holoenzyme assembly the protein p65 recognizes a bend in stem IV, and this binding to central stem IV helps to position the structured loop IV for interaction with TERT and other region(s) of TER.

A novel RNA binding domain in tetrahymena telomerase p65 initiates hierarchical assembly of telomerase holoenzyme

Telomerase reverse transcriptase (TERT) and telomerase RNA (TER) assemble as part of a holoenzyme that synthesizes telomeric repeats at chromosome ends. Genetic approaches have identified proteins that are required for in vivo association of TERT and TER, including the Tetrahymena telomerase holoenzyme protein p65. Here, we use quantitative assays to define the mechanisms underlying p65 function in holoenzyme biogenesis. We demonstrate that four modules of p65 contribute affinity for TER, including a C-terminal domain that recognizes the conserved dinucleotide bulge of central stem IV. This C-terminal domain is necessary and sufficient for p65's function in enhancing the recruitment of TERT to TER. Finally, we show that p65 and TERT assemble on TER with hierarchical rather than cooperative binding. These findings elucidate an extensive network of p65-TER recognition specificity and define a novel p65 RNA binding domain that initiates telomerase holoenyzme biogenesis.

Structure of stem-loop IV of Tetrahymena telomerase RNA

Conserved domains within the RNA component of telomerase provide the template for reverse transcription, recruit protein components to the holoenzyme and are required for enzymatic activity. Among the functionally essential domains in ciliate telomerase RNA is stem-loop IV, which strongly stimulates telomerase activity and processivity even when provided in trans. The NMR structure of Tetrahymena thermophila stem-loop IV shows a highly structured distal stem-loop linked to a conformationally flexible template-proximal region by a bulge that severely kinks the entire RNA. Through extensive structure-function studies, we identify residues that contribute to both these structural features and to enzymatic activity, with no apparent effect on the binding of TERT protein. We propose that the bending induced by the GA bulge and the flexibility of the template-proximal region allow positioning of the prestructured apical loop during the catalytic cycle.

hnRNP A1 associates with telomere ends and stimulates telomerase activity

Telomerase is a ribonucleoprotein enzyme complex that reverse-transcribes an integral RNA template to add short DNA repeats to the 3'-ends of telomeres. G-quadruplex structure in a DNA substrate can block its extension by telomerase. We have found that hnRNP A1--which was previously implicated in telomere length regulation--binds to both single-stranded and structured human telomeric repeats, and in the latter case, it disrupts their higher-order structure. Using an in vitro telomerase assay, we observed that depletion of hnRNP A/B proteins from 293 human embryonic kidney cell extracts dramatically reduced telomerase activity, which was fully recovered upon addition of purified recombinant hnRNP A1. This finding suggests that hnRNP A1 functions as an auxiliary, if not essential, factor of telomerase holoenzyme. We further show, using chromatin immunoprecipitation, that hnRNP A1 associates with human telomeres in vivo. We propose that hnRNP A1 stimulates telomere elongation through unwinding of a G-quadruplex or G-G hairpin structure formed at each translocation step.

Structure and function of telomerase RNA

Maintenance of telomeres by the enzyme telomerase is essential for genomic stability and cell viability in ciliates, vertebrates and yeast. The minimal components of telomerase required for catalytic activity are the telomerase reverse transcriptase (TERT) protein and the template-containing telomerase RNA (TER). Recent studies have afforded significant advances in the biophysical characterization of telomerase RNAs from various species. The first TER structures have been reported, for regions of the catalytically essential pseudoknot and CR4/CR5 domains of human TER, and provide a structural basis for interpretation of mutational and biochemical data. The domains and interactions of the Tetrahymena thermophila telomerase holoenzyme RNA and protein components have been further characterized biochemically, and structures of the TER template boundary element and the N-terminal domain of T. thermophila TERT have been determined. Phylogenetic and biochemical analyses of yeast TERs have revealed core structural elements in common with ciliates and vertebrates, and the minimal domains required for function in vivo.

Structure of the Tetrahymena thermophila telomerase RNA helix II template boundary element

Telomere addition by telomerase requires an internal templating sequence located in the RNA subunit of telomerase. The correct boundary definition of this template sequence is essential for the proper addition of the nucleotide repeats. Incorporation of incorrect telomeric repeats onto the ends of chromosomes has been shown to induce chromosomal instability in ciliate, yeast and human cells. A 5′ template boundary defining element (TBE) has been identified in human, yeast and ciliate telomerase RNAs. Here, we report the solution structure of the TBE element (helix II) from Tetrahymena thermophila telomerase RNA. Our results indicate that helix II and its capping pentaloop form a well-defined structure including unpaired, stacked adenine nucleotides in the stem and an unusual syn adenine nucleotide in the loop. A comparison of the T.thermophila helix II pentaloop with a pentaloop of the same sequence found in the 23S rRNA of the Haloarcula marismortui ribosome suggests possible RNA and/or protein interactions for the helix II loop within the Tetrahymena telomerase holoenzyme.

Starvation-induced cleavage of the tRNA anticodon loop in Tetrahymena thermophila

Amino acid deprivation triggers dramatic physiological responses in all organisms, altering both the synthesis and destruction of RNA and protein. Here we describe, using the ciliate Tetrahymena thermophila, a previously unidentified response to amino acid deprivation in which mature transfer RNA (tRNA) is cleaved in the anticodon loop. We observed that anticodon loop cleavage affects a small fraction of most or all tRNA sequences. Accumulation of cleaved tRNA is temporally coordinated with the morphological and metabolic changes of adaptation to starvation. The starvation-induced endonucleolytic cleavage activity targets tRNAs that have undergone maturation by 5' and 3' end processing and base modification. Curiously, the majority of cleaved tRNAs lack the 3' terminal CCA nucleotides required for aminoacylation. Starvation-induced tRNA cleavage is inhibited in the presence of essential amino acids, independent of the persistence of other starvation-induced responses. Our findings suggest that anticodon loop cleavage may reduce the accumulation of uncharged tRNAs as part of a specific response induced by amino acid starvation.

Telomerase: regulation, function and transformation

Work from several laboratories over the past decade indicates that the acquisition of constitutive telomerase expression is a critical step during the malignant transformation of human cells. Normal human cells transiently express low levels of telomerase, the ribonucleoprotein responsible for extending and maintaining telomeres, and exhibit telomere shortening after extended passage, whereas most cancers exhibit constitutive telomerase expression and maintain telomeres at stable lengths. These observations establish a direct connection between immortalization and stabilization of telomere structure. However, recent work suggests that telomerase also contributes to cancer development beyond its role in maintaining stable telomere lengths. In this review, we summarize recent observations that support the concept that telomerase plays multiple roles in facilitating human cell transformation.