Eukaryotic chromosomes are linear DNA molecules that contain special structures, called telomeres, which cap the ends of chromosomes to protect against end-to-end fusion or apoptosis (1
). With a few exceptions from some insects, the telomeric DNA sequences are usually simple tandem repeats, e.g. (TTAGGG)n
as in humans. During DNA replication, telomeric DNA shortens progressively mainly due to the end-replication problem (3
). To overcome this problem, eukaryotic cells have evolved with a specialized reverse transcriptase (RT) enzyme, called telomerase, which functions to counteract this continuous degradation of telomeres by adding telomeric DNA repeats to the 3′-ends of chromosomes (5
Telomerase is a ribonucleoprotein complex composed of two essential core components, the telomerase reverse transcriptase (TERT) protein and the telomerase RNA (TR), as well as several telomerase-associated accessorial proteins. The catalytic TERT protein synthesizes telomeric DNA repeats using a short sequence in the TR component as a template (6
). The TERT gene was first identified in 1997 from the ciliate, Euplotes aediculatus
and the yeast Saccharomyces cerevisiae
). Since then, homologs of TERT have been identified in 105 species. TR sequences have currently been identified in 28 ciliates (9–11
), 14 yeasts (12
) and 43 vertebrates (14
). The size of TR varies dramatically from ~150 nt in ciliates, to 312–556 nt in vertebrates, and to over 1500 nt in yeasts. Remarkably, there is no similarity in TR sequence between these three groups of species. As a result, secondary structure models of TR have been independently established for each group of species using phylogenetic comparative analysis. The structures of TR from ciliates, yeasts and vertebrates share a similar pseudoknot structure near the template region, but vary substantially due to additional species-specific structural elements. For example, in addition to the universal pseudoknot domain, vertebrate TR contains the CR4–CR5 domain which is essential for enzymatic activity and the sno/scaRNA domain which is critical for TR biogenesis.
Due to its unusual evolutionary divergence, the telomerase holoenzyme varies significantly in its composition among different groups of species. A large number of putative telomerase-associated proteins have been identified in different groups of species through biochemical analyses such as UV cross-linking or immunoprecipitation. Interestingly, most of these proteins interact with telomerase in a species-specific manner. For example, the dyskerin protein complex appears to be associated with vertebrate telomerase (15
), but not with yeast or ciliate telomerases. Since these associated proteins are not necessary to reconstitute telomerase activity in vitro
, they likely function in the regulation and/or biogenesis of telomerase in vivo
. However, the actual roles of many of these proteins remain unknown.
Structural studies of telomerase using NMR and crystallography have been limited to small structural domains or elements of the core components, TERT and TR. For the TERT protein, only the N-terminal TEN domain of Tetrahymena
TERT has been successfully crystallized and the structure determined (16
). However, the central portion of TERT protein contains seven motifs called RT motifs (1, 2, A, B, C, D, E) that are highly conserved among all reverse transcriptases. The RT domain of TERT likely folds into a structure similar to the available crystal structures of HIV and MMLV RTs (17
). For this reason, the structure of HIV RT domain has often been used as a structural model for the TERT RT domain. For the TR component, while the secondary structure is known, the tertiary structure of the full-length RNA has not been determined. Nevertheless, NMR solution structures are available for helix II and stem-loop IV of Tetrahymena
), as well as the pseudoknot (P2b/P3), P6 and P6.1 helices of human TR (22–24
). While structural studies of small RNA and protein fragments have provided useful information, the structure of a catalytically active TR–protein complex will ultimately help reveal the mechanism of how this unique DNA polymerase functions.
Telomerase plays a vital role in the cellular immortality of stem cells. Mutations in telomerase genes affect the proliferation capacity of stem cells and have been linked to three human diseases, dyskeratosis congenita (DKC), aplastic anemia (AA) and idiopathic pulmonary fibrosis (IPF) (25–27
). The reduction of telomerase activity correlates to telomere shortening and reduced proliferative capacity in cells from patients. The three types of DKC: (i) the X-linked-recessive, (ii) autosomal dominant and (iii) autosomal recessive, have been linked to mutations in the dyskerin gene (DKC1), TERT and TR genes and Nop10 gene, respectively (28–31
). AA and IPF have been linked to mutations in both TERT and TR genes (26
). It is puzzling that mutations in the TERT or TR gene at different sites can cause an array of clinical presentations associated with DKC, AA and/or IPF. The molecular mechanism explaining the unusual phenomenon of which mutations in the same gene cause these diseases remains to be elucidated.
The unusual evolutionary divergence of telomerase and its important role in cancer, aging and human diseases have attracted a great number of researchers who are devoted to telomerase research. This database aims to facilitate and expedite the research on telomerase biology by providing a comprehensive collection of information as a useful and accessible resource.