The principal role of Ku in telomere length maintenance is promotion of Est1 association with telomeres.
Bottom Line: The promotion of TLC1 nuclear localization and Est2 recruitment have been proposed to be the principal role of Ku in telomere length maintenance, but neither model has been directly tested.Moreover, restoration of TLC1 nuclear localization, even when combined with Est2 recruitment, does not bypass the role of Ku.Together, our results unexpectedly demonstrate that the principal role of Ku in telomere length maintenance is to promote the association of Est1 with telomeres, which may in turn allow for efficient recruitment and activation of the telomerase holoenzyme.
Affiliation: Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, Texas 77030.Show MeSH
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Mentions: In the context of a fusion, Cdc13-Est1 and Cdc13-Est2 might differentially promote TLC1 nuclear localization in the absence of Ku–TLC1 interaction and, thereby, result in differences in telomere elongation. To examine this possibility, TLC1 localization was determined by FISH in WT, yku80-135i, and yku80∆ strains alone or expressing a Cdc13–Est1 or Cdc13–Est2 fusion. As previously reported, TLC1 was mainly cytoplasmic in yku80-135i and yku80∆ strains not expressing a fusion protein (Figure 4A). Interestingly, both the Cdc13-Est1 and the Cdc13-Est2 fusion restored TLC1 nuclear localization to about the same extent in yku80-135i and yku80∆ strains with ∼40–60% of cells having mainly nuclear TLC1 (Figure 4A). Expression of a Cdc13–Est1 fusion had a slightly greater effect on TLC1 nuclear retention in yku80-135i and yku80∆ strains compared to expression of a Cdc13–Est2 fusion. However, the yku80-135i/CDC13-EST2 and yku80∆/CDC13-EST1 strains had a comparable number of cells with TLC1 mainly in the nucleus (51 and 49.5%, respectively), yet telomere lengths in these strains were vastly different (Figure 4B). Therefore, differences in the ability of the Cdc13–Est1 and Cdc13–Est2 fusion to retain TLC1 in the nucleus could not fully account for differences in their ability to promote telomere elongation in Ku mutant strains. These findings suggest that, although TLC1 nuclear localization is important for telomere length maintenance, Ku makes additional contributions to telomere elongation.
Affiliation: Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, Texas 77030.