Distinct Functions of POT1 at Telomeres

Abstract

<p>Telomeres are nucleoprotein complexes that constitute the ends of eukaryotic chromosomes. Telomeres differentiate the end of the chromosome from sites of DNA damage and control cellular replicative potential. The loss of function of telomeres results in several biological consequences. First, dysfunctional telomeres elicit DNA damage responses and repair activities, which frequently induce cytogenetic abnormalities and genomic instability that are characteristic of human cancer. Second, cellular immortalization resulting from inappropriate elongation of telomeres is a critical component of tumorigenesis. Alternatively, as telomere shortening limits replicative potential, abnormally short telomeres can result in premature cellular senescence that is associated with human pathology ranging from anemia to atherosclerosis. </p><p> Telomeric DNA is composed of tandem repeats of G-rich double-stranded (ds)DNA that terminates in a G-rich 3' single-stranded (ss)DNA overhang. Telomeres are thought to assume a lariat structure termed the t-loop, which is decorated by an assortment of telomere-associated proteins. The most unique and least well characterized of these proteins is POT1. POT1 binds telomeric ssDNA via a pair of N-terminal OB-folds. Through its C-terminal protein-interaction domain, POT1 directly binds the telomeric dsDNA-binding protein TRF2 and participates in heterodimeric complex with the protein TPP1. Inhibition of POT1 induces a robust DNA damage response at telomeres and deregulation of telomere length homeostasis, indicating that POT1 is important in maintaining telomere stability and in regulating telomere length. The goal of my thesis work was to determine which of the three major functions of POT1 - telomeric ssDNA-, TPP1-, or TRF2-binding - were required to properly localize POT1 to telomeres and to prevent the telomere instability and length deregulation that occur in the absence of POT1. Using separation-of-function mutants of POT1 deficient in at least one of these activities, I found that POT1 depends on its heterodimeric partner TPP1 in cis with telomeric ssDNA-binding to preserve telomere stability, while POT1 depends on its protein interaction with TRF2 to localize to telomeres and its TRF2- and telomeric ssDNA-binding activities in cis to regulate telomere length.</p>