Structural and biochemical characterization of telomere protection and length regulation by the yeast CST complex

Cdc13 is an essential budding yeast protein with high affinity and specificity for single stranded telomeric DNA. Cdc13 has two main functions: 1) it localizes to single stranded telomere overhangs and protects them from damage and 2) recruits polymerases involved in telomere replication. It accomplishes these tasks by acting as a central hub for various protein co-factors. For example, Cdc13 binds the Stn1/Ten1 heterodimer, forming the CST complex, which acts as a protective cap against genomic instability, such as chromosome end-to-end fusions and exonucleolytic degradation. Furthermore, Cdc13 recruits telomerase - the enzyme responsible for telomere replication - to the telomere ends. These events are responsible for telomere homeostasis. Deregulation of telomere homeostasis can lead to cellular senescence, catastrophic genomic damage and/or continuous cellular division (as seen in human cancers). Therefore, Cdc13 and its putative human homologs, Ctc1 and Pot1 are essential for biological processes related to aging and carcinogenesis. Despite the important role of Cdc13 in genome maintenance, the precise mechanism of telomere maintenance by this protein is not fully understood. To further elucidate the function of Cdc13, we solved x-ray crystal structures of two domains of the protein; Cdc13N and OB2. These domains adopt a similar oligosaccharide/oligonucleotide (OB) binding fold; however, they have distinct structural features which give rise to different functions. Briefly, we found that both domains, as well as full-length Cdc13 form stable homodimers. In the case of Cdc13N, homodimerization is important for binding long single stranded telomeric DNA and telomerase regulation. OB2 homodimerization was essential for proper CST complex assembly and telomere capping. These results give further insight into the mechanism of CST mediated telomere maintenance. The research presented here provides further illumination into the mechanism of Cdc13 mediated telomere capping and telomerase recruitment. Additionally, due to the recent discovery of Ctc1, a human homologue of Cdc13, this work provides some starting points for structural and biochemical studies probing the role of Ctc1 in aging related diseases and carcinogenesis