Telomerase is the ribonucleoprotein complex responsible for maintaining telomeres, the long stretches of tandem repeating DNA and associated binding proteins at the ends of linear chromosomes. Telomeres protect the ends of chromosomes from being recognized by the cellular DNA repair machinery as double strand breaks and also prevent end-to-end joining. In the absence of active telomerase, telomeres shorten with each round of DNA replication. Once a critical amount of the telomere is lost, the cell enters replicative senescence and sometimes undergoes apoptosis. Telomerase expression in most somatic cells is undetectable, with the opposite true of most cancer cells-allowing them to replicate indefinitely. This difference in activity makes telomerase an attractive target for ani-proliferative drugs. The goal of this fellowship will be to acquire a cryoelectron microscopy structure of the Tetrahymena thermophila telomerase holoenzyme complex, assign each component, deduce function from structures obtained at different points in the catalytic cycle, and solve the structure of the p19 subunit by NMR and X-ray crystallography. During replication, the chromosomes of T. thermophila fragment into ~20,000 subchromosomes, each with their own telomeres. To maintain so many telomeres, T. thermophila also expresses a large amount of telomerase. This has ensured its use as a model organism.
The specific aims are to (1) purify T. thermophila telomerase holoenzyme from TAP-tagged strains for electron microscopy and obtain a negative stain structure, (2) localize individual protein and RNA subunits by antibody and affinity labeling, (3) optimize sample conditions for cryoelectron microscopy and determine a cryoelectron microscopy structure, (4) determine the structure of Tetrahymena telomerase holoenzyme with bound DNA primer and compare to the unbound structure, and (5) Use NMR and crystallography to determine the structure of the T. thermophila protein subunit p19. These experiments will reveal the complete structure of a telomerase holoenzyme as well as offer insight into what roles intermolecular contacts and subunit movement play in telomere catalysis.

Public Health Relevance

Telomerase, the RNA and protein complex responsible for maintaining telomeres, is highly active in cancer cells and inactive in most normal cells. This difference in activity is what makes telomerase a good target for anti-cancer drugs. To design these drugs, a structure of telomerase is vital, and so the aim of this fellowship is to determine the structure of the telomerase holoenzyme by electron microscopy.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Postdoctoral Individual National Research Service Award (F32)
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Special Emphasis Panel (ZRG1-F04-A (20))
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Flicker, Paula F
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University of California Los Angeles
Schools of Arts and Sciences
Los Angeles
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Jiang, Jiansen; Chan, Henry; Cash, Darian D et al. (2015) Structure of Tetrahymena telomerase reveals previously unknown subunits, functions, and interactions. Science 350:aab4070
Miracco, Edward J; Jiang, Jiansen; Cash, Darian D et al. (2014) Progress in structural studies of telomerase. Curr Opin Struct Biol 24:115-24
Hong, Kyungah; Upton, Heather; Miracco, Edward J et al. (2013) Tetrahymena telomerase holoenzyme assembly, activation, and inhibition by domains of the p50 central hub. Mol Cell Biol 33:3962-71
Jiang, Jiansen; Miracco, Edward J; Hong, Kyungah et al. (2013) The architecture of Tetrahymena telomerase holoenzyme. Nature 496:187-92