Structural and functional studies of the telomere capping proteins Stn1, Ten1, and Cdc13

Telomeres must be capped to preserve chromosomal stability. One essential function of telomere end-binding proteins is protection of the genome via capping of the chromosome ends. Increasingly, data suggests telomere end-binding proteins have evolved from the DNA double-strand break repair machinery, providing insight into the potential mechanisms of the poorly understood capping process. A telomere endbinding capping complex, minimally composed of the Stn1 and Ten1 proteins, has been detected in fission yeast and budding yeast, where they associate with a third protein Cdc13. Domain mapping and structure predictions for these capping proteins provide unsubstantiated evidence for a connection to the three proteins of the single-stranded DNA binding complex, RPA. In this work, the crystal structures of the C-terminal domain of the S. cerevisiae Stn1 protein (ScStn1-C) and the full length S. pombe Ten1 (SpTen1) protein are presented. These structures reveal striking similarities to the corresponding subunits in human RPA, hRPA32 and hRPA14 respectively, signifying an evolutionary link between telomere maintenance proteins and DNA repair complexes. Relative to hRPA32, ScStn1-C contains a supplementary domain. Our in vivo analysis, using structure-directed mutagenesis, identify a required surface on this new domain for negative telomere length regulation, supporting the evolution of a telomere specific role in chromosome maintenance for ScStn1. Additionally, the structure of the SpTen1 protein and its robust similarity to hRPA14 imply Stn1 and Ten1 interact via similar protein surfaces as the corresponding RPA proteins. Taken together, our findings support a model of an evolutionarily conserved mechanism of DNA maintenance that has developed as a result of increased chromosomal structure complexity.