Tying up the Ends: Plasticity in the Recognition of Single-Stranded DNA at Telomeres

Abstract

Telomeres terminate nearly exclusively in ssDNA overhangs comprised of the G-rich 3′ end. This overhang varies widely in length from species to species, ranging from just a few bases to several hundred nucleotides. These overhangs are not merely a remnant of DNA replication, but rather are the result of complex further processing. Proper management of the telomeric overhang is required both to deter the action of the DNA-damage machinery and to present the ends properly to the replicative enzyme telomerase. This Current Topic review addresses the biochemical and structural features used by the proteins that manage these variable telomeric overhangs. The Pot1 protein tightly binds the single-stranded overhang, preventing DNA damage sensors from binding. Pot1 also orchestrates the access of telomerase to that same substrate. The remarkable plasticity of the binding interface exhibited by the S. pombe Pot1 provides mechanistic insight into how these roles may be accomplished, and disease-associated mutations clustered around the DNA-binding interface in the hPOT1 highlight the importance of this function. The budding yeast Cdc13-Stn1-Ten1, a telomeric RPA complex closely associated with telomere function, also interacts with ssDNA in a fashion that allows degenerate sequences to be recognized. A related human complex composed of hCTC1-hSTN1-hTEN1 has recently emerged with links to both telomere maintenance and general DNA replication and also exhibits mutations associated with telomere pathologies. Overall, these sequence-specific ssDNA binders exhibit a range of recognition properties that allow them to perform their unique biological functions.

Shared Subunits of Tetrahymena Telomerase Holoenzyme and Replication Protein A Have Different Functions in Different Cellular Complexes

“…Telomerase binds a chromosome 3Ј overhang in competition, and also coordination, with other single-stranded DNA (ssDNA)-binding proteins (17,18). Throughout most of the cell cycle, the telomere 3Ј overhang is sequestered by DNA binding and telomere remodeling activities of the ssDNA-binding protein Pot1 (18,19).…”

“…Throughout most of the cell cycle, the telomere 3Ј overhang is sequestered by DNA binding and telomere remodeling activities of the ssDNA-binding protein Pot1 (18,19). The Pot1 N-terminal pair of oligonucleotide/ oligosaccharide-binding fold (OB-fold) domains interacts sequence-specifically with the telomeric repeat G-strand, whereas the Pot1 C-terminal region interacts with vertebrate TPP1/fission yeast Tpz1/Tetrahymena Tpt1 (19,20).…”

“…CST has evolutionarily variable ssDNA binding properties and variable biological roles linked to a high degree of large subunit divergence (18,27). Vertebrate CST contributes to DNA replication at sites throughout the genome and, with distinct structural requirements, to telomere-specific processes, such as the post-replication cytidine-rich strand (C-strand) fill-in by polymerase ␣-primase (28 -30).…”

“…Vertebrate CST also has been proposed to inhibit telomerase access to chromosome ends, although this role is not uniformly evident across different studies (30,31). Budding yeast CST function is telomere-specific; it stimulates C-strand fill-in and contributes to chromosome end-capping, and, when disassembled in S-phase, its Cdc13 subunit recruits telomerase holoenzyme (18). A single OB-fold domain within budding yeast Cdc13 is necessary and sufficient for sequence-specific recognition of G-strand ssDNA (32), whereas detectable binding of vertebrate or Tetrahymena CST to DNA requires all three subunits (28,31,33).…”

Progress in Human andTetrahymenaTelomerase Structure Determination

“…For this reason and also space limitations, this review focuses on human and ciliate Tetrahymena telomerase. We refer the interested reader to excellent reviews on yeast and plant telomerase (28, 77, 89, 94, 98, 120, 172, 173). Advances in single molecule and fluorescence resonance energy transfer studies of telomerase are covered in this volume by Parks & Stone.…”