Telomerase and Telomere-Associated Proteins: Structural Insights into Mechanism and Evolution

Abstract

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, 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.

Synergism of the Two Myb Domains of Tay1 Protein Results in High Affinity Binding to Telomeres

“…Telomere binding factors associated with either ss-or dsDNA regions of the chromosomal ends represent an evolutionarily highly divergent group of proteins (7,8). Their only common feature is the presence of a conserved sequence and/or structural domain (OB-fold or Myb domain) mediating specific binding to telomeric repeats.…”

“…These efforts are made easier by the fact that despite their dissimilarities at the level of amino acid sequences, telomere-associated proteins share common structural elements (8). All known ds telomeric DNA-binding proteins contain at least one conserved Myb/homeodomain, although the rest of the proteins have very different sequence and domain topology (1,8). Thus, Myb domains of known telomeric proteins can be used effectively as queries for searches within whole genomic sequences of organisms distantly related to established models for telomere biology (9 -11).…”

Replication of Telomeres and the Regulation of Telomerase

“…For the discussion of telomere protection and the three-dimensional structures of telomere components, the reader is referred to excellent recent reviews (de Lange 2009;Jain and Cooper 2010;Lewis and Wuttke 2012).…”

“…It contains canonical reverse transcriptase motifs also found in retroelements (Lingner et al 1997;Nakamura and Cech 1998). In addition, Tert contains a telomerase essential amino-terminal domain (TEN), a telomerase RNA-binding domain (TRBD), and a carboxy-terminal extension (reviewed by Lewis and Wuttke 2012). Other telomerase subunits are less well conserved.…”

The Regulation of Telomerase by Alternative Splicing of TERT

“…The telomerase catalytic cycle has two phases: nucleotide addition to the 3' end of the telomeric DNA primer and the template translocation for the synthesis of additional repeats. The translocation involves realignment of the RNA template to the new DNA 3' end (Lewis & Wuttke, 2012;Podlevsky & Chen, 2012). Vertebrate telomerases add six nucleotides (5'-GGTTAG-3') at the end of the DNA primer.…”