Biogenesis of telomerase ribonucleoproteins - PubMed (original) (raw)

Review

Biogenesis of telomerase ribonucleoproteins

Emily D Egan et al. RNA. 2012 Oct.

Abstract

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.

PubMed Disclaimer

Figures

FIGURE 1.

FIGURE 1.

Diagram of TER secondary structures highlighting functional motifs. The template, pseudoknot, TBE, and STE are common to ciliate, yeast, and vertebrate TERs. The STE is distal stem–loop IV in T. thermophila, conserved region 4/5 (CR4/5) in human, and a three-way helix junction in yeasts. Species-specific RNP stability elements recruit p65 in T. thermophila, H/ACA proteins in human, and Sm proteins in yeasts (also Ku in Saccharomyces). The binding sites for holoenzyme proteins that do not affect RNA stability, namely Est1 in yeasts and the CAB box-binding protein WDR79/TCAB1 in humans, are indicated. T. thermophila TER also contains a template recognition element (TRE) that contributes to template utilization.

FIGURE 2.

FIGURE 2.

T. thermophila telomerase RNP biogenesis. T. thermophila TER is transcribed by RNA polymerase III. The binding of p65 stabilizes a kink in stem IV that optimally positions loop IV for TERT binding. The p65-TER–TERT ternary complex then interacts with a complex of p75, p50, p45, and p19 that recruits the single-stranded telomeric DNA-binding protein Teb1.

FIGURE 3.

FIGURE 3.

S. cerevisiae telomerase RNP biogenesis. S. cerevisiae TLC1 is transcribed by RNA polymerase II, and transcription is terminated by the Nrd1–Nab3–Sen1 pathway. The 3′ end is processed by the nuclear exosome and TRAMP complex, leaving a short adenosine-rich tail that is ultimately removed. Sm protein binding near the 3′ end stabilizes the RNA and recruits the cap hypermethylase Tgs1, which modifies the 5′ cap to TMG, indicated by a diamond. The TERT subunit Est2 binds directly to TLC1. The Ku heterodimer recognizes a distinct binding site on TLC1 to promote RNP accumulation and nuclear import. The regulatory subunit Est1 can bind directly to TLC1 and with Est3 stimulates telomerase function at telomeres.

FIGURE 4.

FIGURE 4.

S. pombe telomerase RNP biogenesis. S. pombe TER1 is transcribed by RNA polymerase II as a precursor that includes a downstream intron and exon. TER1 secondary structure has yet to be experimentally validated; it is illustrated here as similar to other yeast TERs. Sm proteins assemble on the precursor near the mature TER1 3′ end. Spliceosomal cleavage at the 5′ splice site releases mature TER1, which escapes ligation to the downstream exon. Sm proteins then recruit the cap hypermethylase Tgs1, which modifies the 5′ cap to TMG, indicated by a diamond. Sm proteins are then replaced by Lsm proteins that protect the 3′ end from degradation, followed by assembly of Est1 and the TERT subunit Trt1.

FIGURE 5.

FIGURE 5.

Human telomerase RNP biogenesis. The human TER, hTR, is transcribed by RNA polymerase II as a precursor that cotranscriptionally assembles with the H/ACA protein heterotrimer of dyskerin, NHP2, and NOP10 bound to the H/ACA RNP assembly chaperone NAF1. This process is aided by the dyskerin chaperone SHQ1 and a complex of NUFIP and the helicases RUVBL1 and RUVBL2. NUFIP interacts with NHP2, and the RUVBL1/RUVBL2 heterodimer interacts with dyskerin to promote H/ACA RNP assembly. The hTR precursor transcript is then processed at its 3′ end by an unknown mechanism. The G-quadruplex structure that can form near the 5′ end, represented by a wavy line, is resolved by the helicase DHX36. Then, NAF1 is exchanged for the mature H/ACA RNP protein GAR1, and sTGS1 modifies the 5′ cap to TMG, indicated by a diamond. TCAB1 and TERT bind to hTR in the active telomerase holoenzyme.

Similar articles

Cited by

References

    1. Abreu E, Aritonovska E, Reichenbach P, Cristofari G, Culp B, Terns RM, Lingner J, Terns MP 2010. TIN2-tethered TPP1 recruits human telomerase to telomeres in vivo. Mol Cell Biol 30: 2971–2982 - PMC - PubMed
    1. Akiyama BM, Loper J, Najarro K, Stone MD 2012. The C-terminal domain of Tetrahymena thermophila telomerase holoenzyme protein p65 induces multiple structural changes in telomerase RNA. RNA 18: 653–660 - PMC - PubMed
    1. Armanios M 2009. Syndromes of telomere shortening. Annu Rev Genomics Hum Genet 10: 45–61 - PMC - PubMed
    1. Autexier C, Greider CW 1995. Boundary elements of the Tetrahymena telomerase RNA template and alignment domains. Genes Dev 9: 2227–2239 - PubMed
    1. Autexier C, Lue NF 2006. The structure and function of telomerase reverse transcriptase. Annu Rev Biochem 75: 493–517 - PubMed

Publication types

MeSH terms

Substances

Grants and funding

LinkOut - more resources