Telomere Cap Components Influence the Rate of Senescence in Telomerase-Deficient Yeast Cells

Abstract

Cells lacking telomerase undergo senescence, a progressive reduction in cell division that involves a cell cycle delay and culminates in “crisis,” a period when most cells become inviable. In telomerase-deficient Saccharomyces cerevisiae cells lacking components of the nonsense-mediated mRNA decay (NMD) pathway (Upf1,Upf2, or Upf3 proteins), senescence is delayed, with crisis occurring ∼10 to 25 population doublings later than in Upf⁺ cells. Delayed senescence is seen in upfΔ cells lacking the telomerase holoenzyme components Est2p and TLC1 RNA, as well as in cells lacking the telomerase regulators Est1p and Est3p. The delay of senescence in upfΔ cells is not due to an increased rate of survivor formation. Rather, it is caused by alterations in the telomere cap, composed of Cdc13p, Stn1p, and Ten1p. In upfΔ mutants, STN1 and TEN1 levels are increased. Increasing the levels of Stn1p and Ten1p in Upf⁺ cells is sufficient to delay senescence. In addition, cdc13-2 mutants exhibit delayed senescence rates similar to those of upfΔ cells. Thus, changes in the telomere cap structure are sufficient to affect the rate of senescence in the absence of telomerase. Furthermore, the NMD pathway affects the rate of senescence in telomerase-deficient cells by altering the stoichiometry of telomere cap components.

We thank M. Carbonneau, B. Futcher, A. Jacobson, P. Kaufman, and V. Lundblad for yeast strains and plasmids; Mark McClellan for technical assistence with TEN1 promoter regulation; and Jeff Dahlseid, Michael Culbertson, David Kirkpatrick, Neal Lue, Munira Basrai, and members of the Berman laboratory for many helpful discussions.

This work was supported by National Institutes of Health grants GM 38626 (J.B.) and F32 GM 63352 (L.G.).