| Telomeres are specialized protein-DNA complexes that compose the natural linear chromosomes termini. Telomeres prevent chromosome ends from deleterious degradation and fusions. In most eukaryotes, telomerase replicates telomeres to ensure complete genome replication. In this work, I studied the structures and functions of three telomere-associated protein complexes.;POT1 binds the single-stranded telomere overhangs and protects chromosome ends. TPP1 binds POT1 to form part of shelterin complex at telomeres. The crystal structure of TPP1 reveals a structural similarity to TEBPbeta of a ciliated protozoan, suggesting that TPP1 is the missing beta-subunit of human POT1. Besides, structural and bioinformatic analyses suggest that TPP1 is evolutionarily conserved with yeast Est3. Telomere end-binding proteins have generally been found to inhibit telomerase activity. However, we find that POT1-TPP1 complex increases the activity and processivity of human telomerase. We propose that the POT1--TPP1 complex is a telomerase processivity factor.;Taz1, the fission yeast double-stranded telomeric DNA binding protein, has been considered as the structural homologue of mammalian TRF1/TRF2. TRF1/TRF2 contains a central TRFH domain, which is crucial for their dimerization and telomere localization. However, because of low sequence identity between Taz1 and TRF1/TRF2, a definitive answer to their structural similarity is unknown. My crystal structure shows that the helical Taz1TRFH is not a structural homologue of human TRF1TRFH/TRF2TRFH. Notably, Taz1 TRFH is a monomer, and Taz1 dimerization domain is outside of TRFH. The structure of the Taz1D reveals that Taz1 employs a different architectural principle for homodimerization. Moreover, we determined the NMR structure of Taz1-Rap1 complex. Strikingly, with low sequence similarity, the Taz1-Rap1 closely resembles human TRF2-RAP1.;The BLM-TOPOIIIalpha-RMI1/RMI2 (BTR) complex is a DNA structure-specific "dissolvasome". BTR is essential for genome integrity and potentially functions in telomere maintenance in telomerase-negative cancer cells. However, little is known about the molecular architecture of BTR and how it dissolves homologues recombination intermediates. My structural data demonstrate that both RMI1 and RMI2 are OB-fold containing proteins. RMI2 and RMI1-C form an RPA-like complex, which is essential for BTR stability. The crystal structure of RMI1-N reveals a non-canonical OB-fold with two extra structural elements that are essential for BTR's dissolution activity. |
