Abstract
The identification of the human homologue of the yeast CST in 2009 posed a new challenge in our understanding of the mechanism of telomere capping in higher eukaryotes. The high-resolution structure of the human Stn1-Ten1 (hStn1-Ten1) complex presented here reveals that hStn1 consists of an OB domain and tandem C-terminal wHTH motifs, while hTen1 consists of a single OB fold. Contacts between the OB domains facilitate formation of a complex that is strikingly similar to the replication protein A (RPA) and yeast Stn1-Ten1 (Ten1) complexes. The hStn1-Ten1 complex exhibits non-specific single-stranded DNA activity that is primarily dependent on hStn1. Cells expressing hStn1 mutants defective for dimerization with hTen1 display elongated telomeres and telomere defects associated with telomere uncapping, suggesting that the telomeric function of hCST is hTen1 dependent. Taken together the data presented here show that the structure of the hStn1-Ten1 subcomplex is conserved across species. Cell based assays indicate that hTen1 is critical for the telomeric function of hCST, both in telomere protection and downregulation of telomerase function.
Highlights
Telomeres are the guanosine rich DNA repeats at the ends of linear eukaryotic chromosomes that play a crucial role in protection and replication of the genome [1,2]
We subsequently solved the structures of the hStn1N-Ten1 complex and hStn1C to 2.05 Aand 1.65 Aresolution respectively using mercury derivatives and the method of multi-wavelength anomalous dispersion (MAD) (Table 1 and 2). hStn1N adopts an OB fold, consisting of a b-barrel, composed of two three-stranded antiparallel b-sheets, sandwiched by three a-helices (Figure 1A)
The C-terminal domain of hStn1 consists of eleven a-helices and four b-strands organized into two distinct winged helix-turn-helix motifs (Figure 1C) similar to scStn1 [34,35]
Summary
Telomeres are the guanosine rich DNA repeats at the ends of linear eukaryotic chromosomes that play a crucial role in protection and replication of the genome [1,2]. In S. cerevisiae, the trimeric CST complex (scCST, composed of Cdc, scStn and scTen1) binds tightly and to the telomeric G-overhang [7] and plays a critical role in telomere maintenance [7,8,9,10]. After G-strand extension, scCST promotes telomeric C-strand fill-in by recruiting the DNA polymerase a-primase to telomeres; a process mediated by the N-terminal domain of Cdc and the C-terminal domain of Stn1 [10,18]. In addition to its role in telomere replication, scCST caps telomeres to prevent recombination and exonucleolytic degradation events that could lead to genomic instability and checkpoint-dependent cell-cycle arrest in the G2/M phase [7,19,20,21,22,23]. Loss of any scCST protein component leads to telomere phenotypes associated with uncapped telomeres [9,10,24]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
