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Multiple DNA Interactions Contribute to the Initiation of Telomerase Elongation. J Mol Biol 2017; 429:2109-2123. [PMID: 28506636 DOI: 10.1016/j.jmb.2017.04.023] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/22/2017] [Revised: 04/12/2017] [Accepted: 04/12/2017] [Indexed: 02/05/2023]
Abstract
Telomerase maintains telomere length and chromosome integrity by adding short tandem repeats of single-stranded DNA to the 3' ends, via reverse transcription of a defined template region of its RNA subunit. To further understand the telomerase elongation mechanism, we studied the primer utilization and extension activity of the telomerase from the budding yeast Naumovozyma castellii (Saccharomyces castellii), which displays a processive nucleotide and repeat addition polymerization. For the efficient initiation of canonical elongation, telomerase required 4-nt primer 3' end complementarity to the template RNA. This DNA-RNA hybrid formation was highly important for the stabilization of an initiation-competent telomerase-DNA complex. Anchor site interactions with the DNA provided additional stabilization to the complex. Our studies indicate three additional separate interactions along the length of the DNA primer, each providing different and distinct contributions to the initiation event. A sequence-independent anchor site interaction acts immediately adjacent to the base-pairing 3' end, indicating a protein anchor site positioned very close to the catalytic site. Two additional anchor regions further 5' on the DNA provide sequence-specific contributions to the initiation of elongation. Remarkably, a non-telomeric sequence in the distal 25- to 32-nt region negatively influences the initiation of telomerase elongation, suggesting an anchor site with a regulatory role in the telomerase elongation decision.
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2
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Eckert B, Collins K. Roles of telomerase reverse transcriptase N-terminal domain in assembly and activity of Tetrahymena telomerase holoenzyme. J Biol Chem 2012; 287:12805-14. [PMID: 22367200 DOI: 10.1074/jbc.m112.339853] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023] Open
Abstract
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.
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Affiliation(s)
- Barbara Eckert
- Department of Molecular and Cell Biology, University of California, Berkeley, California 94720-3200, USA
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3
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Abstract
Ribonucleoproteins (RNPs) play key roles in many cellular processes and often function as RNP enzymes. Similar to proteins, some of these RNPs exist and function as multimers, either homomeric or heteromeric. While in some cases the mechanistic function of multimerization is well understood, the functional consequences of multimerization of other RNPs remain enigmatic. In this review we will discuss the function and organization of small RNPs that exist as stable multimers, including RNPs catalyzing RNA chemical modifications, telomerase RNP, and RNPs involved in pre-mRNA splicing.
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Min B, Collins K. Multiple mechanisms for elongation processivity within the reconstituted tetrahymena telomerase holoenzyme. J Biol Chem 2010; 285:16434-43. [PMID: 20363756 DOI: 10.1074/jbc.m110.119172] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
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.
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Affiliation(s)
- Bosun Min
- Department of Molecular and Cell Biology, University of California, Berkeley, California 94720-3200, USA
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5
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Abstract
The structure and integrity of telomeres are essential for genome stability. Telomere dysregulation can lead to cell death, cell senescence, or abnormal cell proliferation. The maintenance of telomere repeats in most eukaryotic organisms requires telomerase, which consists of a reverse transcriptase (RT) and an RNA template that dictates the synthesis of the G-rich strand of telomere terminal repeats. Structurally, telomerase reverse transcriptase (TERT) contains unique and variable N- and C-terminal extensions that flank a central RT-like domain. The enzymology of telomerase includes features that are both similar to and distinct from those characteristic of other RTs. Two distinguishing features of TERT are its stable association with the telomerase RNA and its ability to repetitively reverse transcribe the template segment of RNA. Here we discuss TERT structure and function; its regulation by RNA-DNA, TERT-DNA, TERT-RNA, TERT-TERT interactions, and TERT-associated proteins; and the relationship between telomerase enzymology and telomere maintenance.
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Affiliation(s)
- Chantal Autexier
- Bloomfield Center for Research in Aging, Lady Davis Institute for Medical Research, Sir Mortimer B. Davis Jewish General Hospital, Quebec, Canada.
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Juranek SA, Lipps HJ. New Insights into the Macronuclear Development in Ciliates. INTERNATIONAL REVIEW OF CYTOLOGY 2007; 262:219-51. [PMID: 17631190 DOI: 10.1016/s0074-7696(07)62005-1] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
During macronuclear differentiation in ciliated protozoa, most amazing "DNA gymnastics" takes place, which includes DNA excision, DNA elimination, DNA reorganization, and DNA-specific amplification. Although the morphological events occurring during macronuclear development are well described, a detailed knowledge of the molecular mechanisms and the regulation of this differentiation process is still missing. However, recently several models have been proposed for the molecular regulation of macronuclear differentiation, but these models have yet to be verified experimentally. The scope of this review is to summarize recent discoveries in different ciliate species and to compare and discuss the different models proposed. Results obtained in these studies are not only relevant for our understanding of nuclear differentiation in ciliates, but also for cellular differentiation in eukaryotic organisms in general as well as for other disciplines such as bioinformatics and computational biology.
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Affiliation(s)
- Stefan A Juranek
- Howard Hughes Medical Institute, Laboratory of RNA Molecular Biology, Rockefeller University, New York, New York 10021, USA
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7
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Yang SW, Kim SK, Kim WT. Perturbation of NgTRF1 expression induces apoptosis-like cell death in tobacco BY-2 cells and implicates NgTRF1 in the control of telomere length and stability. THE PLANT CELL 2004; 16:3370-85. [PMID: 15528297 PMCID: PMC535879 DOI: 10.1105/tpc.104.026278] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/21/2004] [Accepted: 09/06/2004] [Indexed: 05/03/2023]
Abstract
Telomeres are specialized nucleoprotein complexes that are essential for preserving chromosome integrity in eukaryotic cells. Several potential telomere binding proteins have recently been identified in higher plants, but nothing is known about their in vivo functions. We previously identified NgTRF1 as a double-stranded telomeric repeat binding factor in tobacco (Nicotiana tabacum) and here show that the binding of NgTRF1 to telomeric repeats inhibits telomerase-mediated telomere extension. To determine whether NgTRF1 is involved in telomere length regulation, we established transgenic tobacco BY-2 cell lines that overexpress or suppress NgTRF1. Pulsed-field gel electrophoresis showed that 35S::NgTRF1 cells exhibited significantly shortened telomeres (45 to 10 kb), whereas 35S::antisense-NgTRF1 cells contained longer telomeres (80 to 25 kb) compared with wild-type and 35S::GUS control cells (65 to 15 kb), indicating that telomere length inversely correlates with the amount of functional NgTRF1 in BY-2 cells. 35S::NgTRF1 cells with shorter telomeres displayed a progressive reduction in cell viability and stopped dividing after 25 to 40 successive rounds of 12-d batch subculture, in sharp contrast with control cells, which have an unlimited capacity for division. Internucleosomal DNA fragmentation, mitochondrial release of cytochrome c, and terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling positive nuclei were detected in 35S::NgTRF1 cells during prolonged subculture, indicating that enhanced cell death was attributable to an apoptosis-like mechanism. 35S::antisense-NgTRF1 cells containing low levels of NgTRF1 also exhibited a progressive decrease in cell viability and apoptotic cell death, but less so than did 35S::NgTRF1 cells, suggesting that the level of NgTRF1 is critically associated with cell viability. Taken together, these data indicate that perturbation of NgTRF1 expression results in changes in telomere length and stability, which in turn causes apoptotic cell death in transgenic BY-2 cells. These results are discussed in light of the suggestion that NgTRF1 is involved in the mechanism by which telomere length and stability are maintained. We further suggest that the structural stability of telomeres, in addition to length maintenance, is essential for their function and for the immortality of BY-2 cells.
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Affiliation(s)
- Seong Wook Yang
- Department of Biology, College of Science, Yonsei University, Seoul 120-749, Korea
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Abstract
Trypanosomatid parasite infections have a devastating impact on human health. Little is known about the requirements for parasite growth during any stage of their complex, multi-host life cycle. In most eukaryotic organisms, sustained cell proliferation requires telomerase-dependent telomere length maintenance. Here we investigate the regulation and biochemical features of telomerase from Trypanosoma cruzi, the causative agent of Chagas disease. We found that T.cruzi telomerase is active in extracts from multiple developmental stages of the parasite life cycle. Detailed characterization of the enzymatic properties of telomerase using epimatigote-stage extract revealed a unique combination of substrate specificities, consistent with the evolutionary divergence of trypanosomes from previously established model systems for telomerase biochemical characterization. Results from partial purification of T.cruzi telomerase suggest that the catalytically active enzyme is a large ribonucleoprotein complex and that the internal RNA template has an atypical, cytosine-rich permutation. These results expand our understanding of telomerase enzymology and should encourage the development of parasite-specific telomerase inhibitors as a method for disease therapy.
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Affiliation(s)
- Denise P Muñoz
- Department of Molecular and Cell Biology, 16 Barker Hall, University of California at Berkeley, Berkeley, CA 94720-3204, USA.
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Aigner S, Cech TR. The Euplotes telomerase subunit p43 stimulates enzymatic activity and processivity in vitro. RNA (NEW YORK, N.Y.) 2004; 10:1108-18. [PMID: 15208446 PMCID: PMC1370601 DOI: 10.1261/rna.7400704] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/24/2004] [Accepted: 04/29/2004] [Indexed: 05/19/2023]
Abstract
Telomerase is a reverse transcriptase that synthesizes telomeric DNA repeats at the ends of eukaryotic chromosomes. Although it is minimally composed of a conserved catalytic protein subunit (TERT) and an RNA component, additional accessory factors present in the holoenzyme play crucial roles in the biogenesis and function of the enzyme complex. Telomerase from the ciliate Tetrahymena can be reconstituted in active form in vitro. Using this system, we show that p43, a telomerase-specific La-motif protein from the ciliate Euplotes, stimulates activity and increases repeat addition processivity of telomerase. Activity enhancement by p43 requires its incorporation into a TERT.RNA.p43 ternary complex but is independent of other dissociable protein factors functioning in telomerase complex assembly. Stimulation is enhanced at elevated temperatures, supporting a role for p43 in structural stabilization of a critical region of the RNA subunit. To our knowledge, this represents the first demonstration that an authentic telomerase accessory protein can directly affect the enzymatic activity of the core enzyme in vitro.
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Affiliation(s)
- Stefan Aigner
- Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215, USA
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10
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Witkin KL, Collins K. Holoenzyme proteins required for the physiological assembly and activity of telomerase. Genes Dev 2004; 18:1107-18. [PMID: 15131081 PMCID: PMC415636 DOI: 10.1101/gad.1201704] [Citation(s) in RCA: 109] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2004] [Accepted: 04/05/2004] [Indexed: 11/25/2022]
Abstract
Many proteins have been implicated in the physiological function of telomerase, but specific roles of telomerase-associated proteins other than telomerase reverse transcriptase (TERT) remain ambiguous. To gain a more comprehensive understanding of catalytically active enzyme composition, we performed affinity purification of epitope-tagged, endogenously assembled Tetrahymena telomerase. We identified and cloned genes encoding four telomerase proteins in addition to TERT. We demonstrate that both of the two new proteins characterized in detail, p65 and p45, have essential roles in the maintenance of telomere length as part of a ciliate telomerase holoenzyme. The p65 subunit contains an La motif characteristic of a family of direct RNA-binding proteins. We find that p65 in cell extract is associated specifically with telomerase RNA, and that genetic depletion of p65 reduces telomerase RNA accumulation in vivo. These findings demonstrate that telomerase holoenzyme proteins other than TERT play critical roles in RNP biogenesis and function.
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Affiliation(s)
- Keren L Witkin
- Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, California 94720-3204, USA
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Lin J, Blackburn EH. Nucleolar protein PinX1p regulates telomerase by sequestering its protein catalytic subunit in an inactive complex lacking telomerase RNA. Genes Dev 2004; 18:387-96. [PMID: 14977919 PMCID: PMC359393 DOI: 10.1101/gad.1171804] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
Human TRF1-binding protein PinX1 inhibits telomerase activity. Here we report that overexpression of yeast PinX1p (yPinX1p) results in shortened telomeres and decreased in vitro telomerase activity. yPinX1p coimmunoprecipitated with yeast telomerase protein Est2p even in cells lacking the telomerase RNA TLC1, or the telomerase-associated proteins Est1p and Est3p. Est2p regions required for binding to yPinX1p or TLC1 were similar. Furthermore, we found two distinct Est2p complexes exist, containing either yPinX1p or TLC1. Levels of Est2p-yPinX1p complex increased when TLC1 was deleted and decreased when TLC1 was overexpressed. Hence, we propose that yPinX1p regulates telomerase by sequestering its protein catalytic subunit in an inactive complex lacking telomerase RNA.
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Affiliation(s)
- Jue Lin
- Department of Biochemistry and Biophysics, University of California, San Francisco, California 94143-2200, USA
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12
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Lue NF, Lin YC, Mian IS. A conserved telomerase motif within the catalytic domain of telomerase reverse transcriptase is specifically required for repeat addition processivity. Mol Cell Biol 2003; 23:8440-9. [PMID: 14612390 PMCID: PMC262686 DOI: 10.1128/mcb.23.23.8440-8449.2003] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
Telomerase is a ribonucleoprotein reverse transcriptase responsible for the maintenance of one strand of the telomere terminal repeats. The catalytic protein subunit of the telomerase complex, known as TERT, possesses a reverse transcriptase (RT) domain that mediates nucleotide addition. The RT domain of TERT is distinguishable from retroviral and retrotransposon RTs in having a sizable insertion between conserved motifs A and B', within the so-called fingers domain. Sequence analysis revealed the existence of conserved residues in this region, named IFD (insertion in fingers domain). Mutations of some of the conserved residues in Saccharomyces cerevisiae TERT (Est2p) abolished telomerase function in vivo, testifying to their importance. Significant effects of the mutations on telomerase activity in vitro were observed, with most of the mutants exhibiting a uniform reduction in activity regardless of primer sequence. Remarkably, one mutant manifested a primer-specific defect, being selectively impaired in extending primers that form short hybrids with telomerase RNA. This mutant also accumulated products that correspond to one complete round of repeat synthesis, implying an inability to effect the repositioning of the DNA product relative to the RNA template that is necessary for multiple repeat addition. Our results suggest that the ability to stabilize short RNA-DNA hybrids is crucial for telomerase function in vivo and that this ability is mediated in part by a more elaborate fingers domain structure.
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Affiliation(s)
- Neal F Lue
- Department of Microbiology & Immunology, W. R. Hearst Microbiology Research Center, Weill Medical College of Cornell University, New York, New York 10021, USA.
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Huard S, Moriarty TJ, Autexier C. The C terminus of the human telomerase reverse transcriptase is a determinant of enzyme processivity. Nucleic Acids Res 2003; 31:4059-70. [PMID: 12853623 PMCID: PMC165952 DOI: 10.1093/nar/gkg437] [Citation(s) in RCA: 82] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
The catalytic subunit of telomerase (TERT) contains conserved reverse transcriptase-like motifs but N- and C-terminal regions unique to telomerases. Despite weak sequence conservation, the C terminus of TERTs from various organisms has been implicated in telomerase-specific functions, including telomerase activity, functional multimerization with other TERT molecules, enzyme processivity and telomere length maintenance. We studied hTERT proteins containing small C-terminal deletions or substitutions to identify and characterize hTERT domains mediating telomerase activity, hTERT multimerization and processivity. Using sequence alignment of five vertebrate TERTs and Arabidopsis thaliana TERT, we identified blocks of highly conserved amino acids that were required for human telomerase activity and functional hTERT complementation. We adapted the non-PCR-based telomerase elongation assay to characterize telomerase expressed and reconstituted in the in vitro transcription/translation rabbit reticulocyte lysate system. Using this assay, we found that the hTERT C terminus, like the C terminus of Saccharomyces cerevisiae TERT, contributes to successive nucleotide addition within a single 6-base telomeric repeat (type I processivity). Certain mutations in the hTERT C terminus also reduced the repetitive addition of multiple telomeric repeats (type II processivity). Our results suggest a functionally conserved role for the TERT C terminus in telomerase enzyme processivity.
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Affiliation(s)
- Sylvain Huard
- Department of Anatomy and Cell Biology, McGill University, Montréal, Québec, Canada, H3A 2B4
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14
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Abstract
Telomerase-mediated healing of broken chromosomes gives rise to terminal deletions and is repressed in most organisms. In ciliated protozoa, however, chromosome fragmentation and de novo telomere addition are part of the developmental program. Work by in this issue of Cell indicates that in Euplotes crassus, this is mediated through switching between different telomerase reverse transcriptase isoforms.
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Affiliation(s)
- Gaël Cristofari
- Swiss Institute for Experimental Cancer Research (ISREC), CH-1066 Epalinges, Switzerland
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Karamysheva Z, Wang L, Shrode T, Bednenko J, Hurley LA, Shippen DE. Developmentally programmed gene elimination in Euplotes crassus facilitates a switch in the telomerase catalytic subunit. Cell 2003; 113:565-76. [PMID: 12787498 DOI: 10.1016/s0092-8674(03)00363-5] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The primary function of telomerase is to maintain preexisting telomere tracts. In the ciliate Euplotes crassus, however, telomerase RNP structure and substrate recognition are altered during macronuclear development to facilitate de novo telomere addition. We found that E. crassus harbors three TERT genes encoding the telomerase catalytic subunit that not only vary in their nucleotide and predicted protein sequences, but also in their expression profiles. Expression of EcTERT-1 and -3 correlates with the requirement for telomere maintenance, while that of EcTERT-2 correlates with de novo telomere synthesis. All three genes appear to require ribosomal frameshifting for expression of catalytically active protein. The transcriptionally active form of EcTERT-2 exists only transiently in mated cells and is absent from the vegetative macronucleus. Thus, telomerase expression in Euplotes is controlled by unique regulatory mechanisms that culminate in a developmental switch to a different catalytic subunit with properties suited to de novo telomere addition.
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Affiliation(s)
- Zemfira Karamysheva
- Department of Biochemistry and Biophysics, Texas A&M University, 2128 TAMU, College Station, TX 77843, USA
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Leeper T, Leulliot N, Varani G. The solution structure of an essential stem-loop of human telomerase RNA. Nucleic Acids Res 2003; 31:2614-21. [PMID: 12736311 PMCID: PMC156033 DOI: 10.1093/nar/gkg351] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
The ribonucleoprotein enzyme telomerase maintains chromosome ends in most eukaryotes and is critical for a cell's genetic stability and its proliferative viability. All telomerases contain a catalytic protein component homologous to viral reverse transcriptases (TERT) and an RNA (TR) that provides the template sequence as well as a scaffold for ribonucleoprotein assembly. Vertebrate telomerase RNAs have three essential domains: the template, activation and stability domains. Here we report the NMR structure of an essential RNA element derived from the human telomerase RNA activation domain. The sequence forms a stem-loop structure stabilized by a GU wobble pair formed by two of the five unpaired residues capping a short double helical region. The remaining three loop residues are in a well-defined conformation and form phosphate-base stacking interactions reminiscent of other RNA loop structures. Mutations of these unpaired nucleotides abolish enzymatic activity. The structure rationalizes a number of biochemical observations, and allows us to propose how the loop may function in the telomerase catalytic cycle. The pre-formed structure of the loop exposes the bases of these three essential nucleotides and positions them to interact with other RNA sequences within TR, with the reverse transcriptase or with the newly synthesized telomeric DNA strand. The functional role of this stem-loop appears to be conserved in even distantly related organisms such as yeast and ciliates.
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Affiliation(s)
- Thomas Leeper
- Department of Biochemistry, University of Washington, Seattle WA 98195-1700, USA
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Wang L, Dean SR, Shippen DE. Oligomerization of the telomerase reverse transcriptase from Euplotes crassus. Nucleic Acids Res 2002; 30:4032-9. [PMID: 12235387 PMCID: PMC137098 DOI: 10.1093/nar/gkf513] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
The telomerase ribonucleoprotein reverse transcriptase uses its RNA subunit as a template to synthesize telomeric repeats and maintain telomere tracts on chromosome ends. In the ciliate Euplotes crassus, the core telomerase ribonucleoprotein particle undergoes a developmentally programmed assembly into three higher order complexes after mating. Here, we provide evidence using oligonucleotide-directed affinity purification that all of the E.crassus telomerase complexes contain at least two enzyme active sites. Furthermore, we show using co-immunoprecipitation experiments that EcTERT, the telomerase catalytic subunit, undergoes multimerization in vitro. Two independent interaction domains were identified in EcTERT, one at the N-terminus that spans amino acids 186-354 and one at the C-terminus that spans amino acids 755-857. Unexpectedly, we found that TERT can form head-to-head, tail-to-tail and head-to-tail oligomers in vitro, implying that E.crassus telomerase has the potential to assume different conformations in vivo. Together, these data indicate that oligomerization is a conserved feature of telomerase and that the minimal functional unit of the enzyme is a dimer.
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Affiliation(s)
- Libin Wang
- Department of Biochemistry and Biophysics, 2128 TAMU, Texas A&M University, College Station, TX 77843-2128, USA
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18
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Williams KR, Doak TG, Herrick G. Telomere formation on macronuclear chromosomes of Oxytricha trifallax and O. fallax: alternatively processed regions have multiple telomere addition sites. BMC Genet 2002; 3:16. [PMID: 12199911 PMCID: PMC128808 DOI: 10.1186/1471-2156-3-16] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/28/2002] [Accepted: 08/28/2002] [Indexed: 11/13/2022] Open
Abstract
BACKGROUND Ciliates employ massive chromatid breakage and de novo telomere formation during generation of the somatic macronucleus. Positions flanking the 81-MAC locus are reproducibly cut. But those flanking the Common Region are proposed to often escape cutting, generating three nested macronuclear chromosomes, two retaining "arms" still appended to the Common Region. Arm-distal positions must differ (in cis) from the Common Region flanks. RESULTS The Common-Region-flanking positions also differ from the arm-distal positions in that they are "multi-TAS" regions: anchored PCR shows heterogeneous patterns of telomere addition sites, but arm-distal sites do not. The multi-TAS patterns are reproducible, but are sensitive to the sequence of the allele being processed. Thus, random degradation following chromatid cutting does not create this heterogeneity; these telomere addition sites also must be dictated by cis-acting sequences. CONCLUSIONS Most ciliates show such micro-heterogeneity in the precise positions of telomere addition sites. Telomerase is believed to be tightly associated with, and act in concert with, the chromatid-cutting nuclease: heterogeneity must be the result of intervening erosion activity. Our "weak-sites" hypothesis explains the correlation between alternative chromatid cutting at the Common Region boundaries and their multi-TAS character: when the chromatid-breakage machine encounters either a weak binding site or a weak cut site at these regions, then telomerase dissociates prematurely, leaving the new end subject to erosion by an exonuclease, which pauses at cis-acting sequences; telomerase eventually heals these resected termini. Finally, we observe TAS positioning influenced by trans-allelic interactions, reminiscent of transvection.
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Affiliation(s)
- Kevin R Williams
- Department of Pathology, University of Utah School of Medicine, Salt Lake City UT 84132-2501, USA
| | - Thomas G Doak
- Department of Pathology, University of Utah School of Medicine, Salt Lake City UT 84132-2501, USA
| | - Glenn Herrick
- Department of Pathology, University of Utah School of Medicine, Salt Lake City UT 84132-2501, USA
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Ray S, Karamysheva Z, Wang L, Shippen DE, Price CM. Interactions between telomerase and primase physically link the telomere and chromosome replication machinery. Mol Cell Biol 2002; 22:5859-68. [PMID: 12138196 PMCID: PMC133977 DOI: 10.1128/mcb.22.16.5859-5868.2002] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
In the ciliate Euplotes crassus, millions of new telomeres are synthesized by telomerase and polymerase alpha-primase during macronuclear development in mated cells. Concomitant with de novo telomere formation, telomerase assembles into higher-order complexes of 550 kDa, 1,600 kDa, and 5 MDa. We show here that telomerase is physically associated with the lagging-strand replication machinery in these complexes. Antibodies against DNA primase precipitated telomerase activity from all three complexes from mated cells but not the 280-kDa telomerase complex from vegetatively growing cells. Moreover, when telomerase was affinity purified, primase copurified with enzyme from mated cells but not with the 280-kDa vegetative complex. Thus, the association of telomerase and primase is developmentally regulated. Intriguingly, PCNA (proliferating cell nuclear antigen) was also found in the 5-MDa complex from mated cells. We therefore speculate that this complex is a complete telomere synthesis machine, while the smaller complexes are assembly intermediates. The physical association of telomerase and primase explains the coordinate regulation of telomeric G- and C-strand synthesis and the efficiency of telomere addition in E. crassus.
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Affiliation(s)
- Saugata Ray
- Department of Pediatrics, University of California, San Diego School of Medicine, La Jolla, California 92093-0627, USA
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20
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Ford LP, Wright WE, Shay JW. A model for heterogeneous nuclear ribonucleoproteins in telomere and telomerase regulation. Oncogene 2002; 21:580-3. [PMID: 11850782 DOI: 10.1038/sj.onc.1205086] [Citation(s) in RCA: 120] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
The heterogeneous nuclear ribonucleoproteins (hnRNPs) are a large family of nucleic acid binding proteins that are often found in, but not restricted to, the 40S-ribonucleoprotein particle. Subsets of hnRNPs are strictly nuclear while others shuttle between the nucleus and cytoplasm. Members of the hnRNP family have been implicated to have roles in many aspects of mRNA maturation/turnover and in telomere and telomerase regulation. Telomeres are repetitive DNA elements mainly found at the ends of human chromosomes. In most normal cells, telomeres shorten with each cell division. Telomere shortening can be compensated for by a ribonucleoprotein complex, called telomerase. Telomerase, consisting of an integral RNA and catalytic protein component as well as several auxiliary factors, extends the 3'-G-rich strand of the ends of the telomeres. Here we present new data and describe a model that implicates the telomerase bound hnRNPs in promoting telomere access by interacting with telomeres. Telomere bound hnRNPs include hnRNP A1, A2-B1, D and E and telomerase bound hnRNPs including hnRNPA1 C1/C2 and D. The telomere and telomerase bound hnRNPs may prove to be good targets for regulating telomere length.
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Affiliation(s)
- Lance P Ford
- Ambion, Inc., 2130 Woodward Street, Austin, Texas, TX 78744-1832, USA
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21
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Mason DX, Autexier C, Greider CW. Tetrahymena proteins p80 and p95 are not core telomerase components. Proc Natl Acad Sci U S A 2001; 98:12368-73. [PMID: 11592988 PMCID: PMC60060 DOI: 10.1073/pnas.221456398] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2001] [Accepted: 08/29/2001] [Indexed: 11/18/2022] Open
Abstract
Telomeres provide stability to eukaryotic chromosomes and consist of tandem DNA repeat sequences. Telomeric repeats are synthesized and maintained by a specialized reverse transcriptase, termed telomerase. Tetrahymena thermophila telomerase contains two essential components: Tetrahymena telomerase reverse transcriptase (tTERT), the catalytic protein component, and telomerase RNA that provides the template for telomere repeat synthesis. In addition to these two components, two proteins, p80 and p95, were previously found to copurify with telomerase activity and to interact with tTERT and telomerase RNA. To investigate the role of p80 and p95 in the telomerase enzyme, we tested the interaction of p80, p95, and tTERT in several different recombinant expression systems and in Tetrahymena extracts. Immunoprecipitation of recombinant proteins showed that although p80 and p95 associated with each other, they did not associate with tTERT. In in vitro transcription and translation lysates, tTERT was associated with telomerase activity, but p80 and p95 were not. p80 bound telomerase RNA as well as several other unrelated RNAs, suggesting p80 has a general affinity for RNA. Immunoprecipitations from Tetrahymena extracts also showed no evidence for an interaction between the core tTERT/telomerase RNA complex and the p80 and p95 proteins. These data suggest that p80 and p95 are not associated with the bulk of active telomerase in Tetrahymena.
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Affiliation(s)
- D X Mason
- Department of Molecular Biology and Genetics, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
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22
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Hardy CD, Schultz CS, Collins K. Requirements for the dGTP-dependent repeat addition processivity of recombinant Tetrahymena telomerase. J Biol Chem 2001; 276:4863-71. [PMID: 11096070 DOI: 10.1074/jbc.m005158200] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Telomerase is a reverse transcriptase responsible for adding simple sequence repeats to chromosome 3'-ends. The template for telomeric repeat synthesis is carried within the RNA component of the telomerase ribonucleoprotein complex. Telomerases can copy their internal templates with repeat addition processivity, reusing the same template multiple times in the extension of a single primer. For some telomerases, optimal repeat addition processivity requires high micromolar dGTP concentrations, a much higher dGTP concentration than required for processive nucleotide addition within a repeat. We have investigated the requirements for dGTP-dependent repeat addition processivity using recombinant Tetrahymena telomerase. By altering the template sequence, we show that repeat addition processivity retains the same dGTP-dependence even if dGTP is not the first nucleotide incorporated in the second repeat. Furthermore, no dNTP other than dGTP can stimulate repeat addition processivity, even if it is the first nucleotide incorporated in the second repeat. Using structural variants of dGTP, we demonstrate that the stimulation of repeat addition processivity is specific for dGTP base and sugar constituents but requires only a single phosphate group. However, all nucleotides that stimulate repeat addition processivity also inhibit or compete with dGTP incorporation into product DNA. By assaying telomerase complexes reconstituted with a variety of altered templates, we find that repeat addition processivity has an unanticipated template or product sequence specificity. Finally, we show that a novel, nascent product DNA binding site establishes dGTP-dependent repeat addition processivity.
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Affiliation(s)
- C D Hardy
- Division of Biochemistry and Molecular Biology, Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, CA 94720-3204, USA
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23
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Bryan TM, Goodrich KJ, Cech TR. A mutant of Tetrahymena telomerase reverse transcriptase with increased processivity. J Biol Chem 2000; 275:24199-207. [PMID: 10807925 DOI: 10.1074/jbc.m003246200] [Citation(s) in RCA: 78] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The protein catalytic subunit of telomerase (TERT) is a reverse transcriptase (RT) that utilizes an internal RNA molecule as a template for the extension of chromosomal DNA ends. In all retroviral RTs there is a conserved tyrosine two amino acids preceding the catalytic aspartic acids in motif C, a motif that is critical for catalysis. In TERTs, however, this position is a leucine, valine, or phenylalanine. We developed and characterized a robust in vitro reconstitution system for Tetrahymena telomerase and tested the effects of amino acid substitutions on activity. Substitution of the retroviral-like tyrosine in motif C did not change overall enzymatic activity but increased processivity. This increase in processivity correlated with an increased affinity for telomeric DNA primer. Substitution of an alanine did not increase processivity, while substitution of a phenylalanine had an intermediate effect. The data suggest that this amino acid is involved in interactions with the primer in telomerase as in other RTs, and show that mutating an amino acid to that conserved in retroviral RTs makes telomerase more closely resemble these other RTs.
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Affiliation(s)
- T M Bryan
- Howard Hughes Medical Institute, Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, USA
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24
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Abstract
Telomerase is a cellular reverse transcriptase specialized for use of a template carried within the RNA component of the enzyme ribonucleoprotein complex. Substrates for telomerase are single-stranded oligonucleotides in vitro and chromosome ends in vivo. In vitro, a bound substrate is extended by an initial round of DNA synthesis on the internal RNA template and in some cases by multiple rounds of template copying before product dissociation. In vivo, de novo synthesis of one strand of a telomeric repeat sequence by telomerase balances the sequence loss resulting from incomplete replication of linear chromosome ends by RNA primer-requiring DNA polymerases. Telomerase biochemistry has been studied extensively by using partially purified cell extracts. Telomerase components are being identified and beginning to be produced in recombinant form. This review focuses on the enzyme mechanism of telomerases from ciliate species, thus far the most intensively studied systems.
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Affiliation(s)
- K Collins
- Department of Molecular and Cell Biology, University of California at Berkeley 94720-3204, USA.
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25
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Abstract
Telomerase, a specialized RNA-directed DNA polymerase that extends telomeres of eukaryotic chromosomes, is repressed in normal human somatic cells but is activated during development and upon neoplasia. Whereas activation is involved in immortalization of neoplastic cells, repression of telomerase permits consecutive shortening of telomeres in a chromosome replication-dependent fashion. This cell cycle-dependent, unidirectional catabolism of telomeres constitutes a mechanism for cells to record the extent of DNA loss and cell division number; when telomeres become critically short, the cells terminate chromosome replication and enter cellular senescence. Although neither the telomere signaling mechanisms nor the mechanisms whereby telomerase is repressed in normal cells and activated in neoplastic cells have been established, inhibition of telomerase has been shown to compromise the growth of cancer cells in culture; conversely, forced expression of the enzyme in senescent human cells extends their life span to one typical of young cells. Thus, to switch telomerase on and off has potentially important implications in anti-aging and anti-cancer therapy. There is abundant evidence that the regulation of telomerase is multifactorial in mammalian cells, involving telomerase gene expression, post-translational protein-protein interactions, and protein phosphorylation. Several proto-oncogenes and tumor suppressor genes have been implicated in the regulation of telomerase activity, both directly and indirectly; these include c-Myc, Bcl-2, p21(WAF1), Rb, p53, PKC, Akt/PKB, and protein phosphatase 2A. These findings are evidence for the complexity of telomerase control mechanisms and constitute a point of departure for piecing together an integrated picture of telomerase structure, function, and regulation in aging and tumor development-Liu, J.-P. Studies of the molecular mechanisms in the regulation of telomerase activity.
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Affiliation(s)
- J P Liu
- Molecular Signaling Laboratory, Baker Medical Research Institute, Prahran, Victoria, Australia.
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26
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Hargrove BW, Bhattacharyya A, Domitrovich AM, Kapler GM, Kirk K, Shippen DE, Kunkel GR. Identification of an essential proximal sequence element in the promoter of the telomerase RNA gene of Tetrahymena thermophila. Nucleic Acids Res 1999; 27:4269-75. [PMID: 10518620 PMCID: PMC148703 DOI: 10.1093/nar/27.21.4269] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Telomerase is a ribonucleoprotein reverse transcriptase that synthesizes and maintains telomeric DNA. Studies of telomeres and telomerase are facilitated by the large number of linear DNA molecules found in ciliated protozoa, such as Tetrahymena thermophila. To examine the expression of telomerase, we investigated the transcription of the RNA polymerase III-directed gene encoding the RNA subunit (TER1) of this enzyme. A chimeric gene containing the Glaucoma chattoni TER1 transcribed region flanked by 5' and 3' Tetrahymena regions was used to identify promoter elements following transformation of Tetrahymena cells. Disruption of a conserved proximal sequence element (PSE) located at -55 in the Tetrahymena TER1 5' flanking region eliminated expression of the chimeric gene. In addition, mutation of an A/T-rich element at -25 decreased expression markedly. A gel mobility shift assay and protein-DNA cross-linking identified a PSE-binding polypeptide of 50-60 kDa in Tetrahymena extracts. Gel filtration analysis revealed a native molecular mass of approximately 160 kDa for this binding activity. Our results point to a similar architecture between ciliate telomerase RNA and metazoan U6 small nuclear RNA promoters.
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MESH Headings
- Animals
- Base Sequence
- Cell Line
- Conserved Sequence/genetics
- DNA, Protozoan/chemistry
- DNA, Protozoan/genetics
- DNA, Protozoan/metabolism
- DNA, Recombinant/genetics
- DNA-Binding Proteins/chemistry
- DNA-Binding Proteins/metabolism
- Gene Dosage
- Gene Expression Regulation
- Genes, Protozoan/genetics
- Molecular Weight
- Mutation/genetics
- Promoter Regions, Genetic/genetics
- RNA Polymerase III/metabolism
- RNA, Protozoan/analysis
- RNA, Protozoan/genetics
- RNA, Small Nuclear/genetics
- Response Elements/genetics
- Telomerase/genetics
- Telomerase/metabolism
- Templates, Genetic
- Tetrahymena thermophila/cytology
- Tetrahymena thermophila/enzymology
- Tetrahymena thermophila/genetics
- Tetrahymenina/enzymology
- Tetrahymenina/genetics
- Transcription Factors/chemistry
- Transcription Factors/metabolism
- Transcription, Genetic/genetics
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Affiliation(s)
- B W Hargrove
- Department of Biochemistry and Biophysics, Texas A&M University, College Station, TX 77843-2128, USA
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27
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Abstract
Recent studies on the telomerase reverse transcriptase have benefited from the identification of the catalytic core subunits. Cellular factors that participate in the assembly of the core enzyme have been identified and regulatory mechanisms that control telomerase activity are beginning to be elucidated.
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Affiliation(s)
- R G Weilbaecher
- Department of Molecular and Human Genetics, Baylor College of Medicine,One Baylor Plaza, Houston, TX 77030, USA.
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28
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Abstract
Telomerase is a ribonucleoprotein reverse transcriptase specialized for use of a sequence within its integral RNA component as the template for DNA synthesis. Telomerase adds telomeric simple sequence repeats to single-stranded primers in vitro or chromosome ends in vivo. We have investigated the sequences and structures of recombinant Tetrahymena thermophila telomerase RNA necessary for physical association and activity with the catalytic protein subunit expressed in rabbit reticulocyte lysate. In contrast with previous results using another reconstitution method, we find that phylogenetically conserved primary sequences and a phylogenetically nonconserved secondary structure are essential for telomerase RNA function. Telomerase RNA binding to the catalytic protein subunit requires sequences 5' of the template and is highly sequence specific. Other telomerase RNA sequences are required for enzyme activity and proper template use but not for protein interaction affinity. In addition, we demonstrate that the production of active recombinant telomerase requires a factor in rabbit reticulocyte lysate that promotes ribonucleoprotein assembly. These studies demonstrate multiple functions for the telomerase RNA and indicate that recombinant telomerase activity requires more than the catalytic protein and RNA components of the enzyme that have been identified to date.
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Affiliation(s)
- J D Licht
- Division of Biochemistry and Molecular Biology, Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, California 94720-3204, USA
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29
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Abstract
During the past year, major advances have been made in understanding the link between telomerase expression and cell immortality. Studies of yeast telomeres have revealed an unexpected role for the non-homologous end-joining machinery in telomere maintenance and have provided the first definitive evidence that telomeres play a critical role in meiosis. Identification of new telomere proteins has led to a better understanding of vertebrate telomere structure and function.
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Affiliation(s)
- C M Price
- Department of Chemistry and Biochemistry, N146 Beadle, University of Nebraska, Lincoln, Nebraska 68588, USA.
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30
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Abstract
Immortal cell populations are able to proliferate indefinitely. Immortalization is associated with activation of processes that compensate for the telomeric shortening that accompanies cell division in normal somatic cells. In many immortal cell lines, telomere maintenance is provided by the action of the ribonucleoprotein enzyme complex, telomerase. Some immortal cell lines have undetectable or very low levels of telomerase activity and there is evidence that these cells maintain their telomeres by an alternative mechanism.
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Affiliation(s)
- L M Colgin
- Children's Medical Research Institute, 214 Hawkesbury Road, Westmead, NSW 2145, Australia
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31
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Abstract
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.
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Affiliation(s)
- M O'Reilly
- Medical Research Council Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK
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