The Rap1p-Telomere Complex Does Not Determine the Replicative Capacity of Telomerase-Deficient Yeast

Abstract

Telomeres are nucleoprotein structures that cap the ends of chromosomes and thereby protect their stability and integrity. In the presence of telomerase, the enzyme that synthesizes telomeric repeats, telomere length is controlled primarily by Rap1p, the budding yeast telomeric DNA binding protein which, through its C-terminal domain, nucleates a protein complex that limits telomere lengthening. In the absence of telomerase, telomeres shorten with every cell division, and eventually, cells enter replicative senescence. We have set out to identify the telomeric property that determines the replicative capacity of telomerase-deficient budding yeast. We show that in cells deficient for both telomerase and homologous recombination, replicative capacity is dependent on telomere length but not on the binding of Rap1p to the telomeric repeats. Strikingly, inhibition of Rap1p binding or truncation of the C-terminal tail of Rap1p in Kluyveromyces lactis and deletion of the Rap1p-recruited complex in Saccharomyces cerevisiae lead to a dramatic increase in replicative capacity. The study of the role of telomere binding proteins and telomere length on replicative capacity in yeast may have significant implications for our understanding of cellular senescence in higher organisms.

Saccharomyces cerevisiae as a Model to Study Replicative Senescence Triggered by Telomere Shortening

“…Although they were not dividing, these cells were metabolically viable, as assessed with a vital staining (Ijpma and Greider, 2003). Furthermore, strains with longer initial telomere lengths can sustain additional population doublings in the absence of telomerase, further supporting the idea that telomere length determines proliferation potential (Smolikov and Krauskopf, 2003; Lebel et al, 2009). …”

“…Strikingly, when telomerase was removed in these cells, senescence was not accelerated. This suggests that telomeric Rap1 is not essential for the growth of telomerase-deficient cells and consequently, that the loss of viability likely does not stem from telomere fusions (Smolikov and Krauskopf, 2003). In contrast to these results, in an independent work, deletion of DNL4 was found to suppress growth defects of est2 Δ cells, indicating that growth defects of senescing cells are due in part to fusion events between telomeres (Meyer and Bailis, 2008).…”

“…Also, the original telomere length greatly influences these rates, as longer telomeres favor the emergence of survivors, and, in Kluyveromyces lactis , a related budding yeast, the presence of a single long telomere allows the senescence to be bypassed (Topcu et al, 2005; Lebel et al, 2009; Chang et al, 2011b). So longer telomeres not only increase the global proliferative capacity of cells deficient in telomerase activity (Smolikov and Krauskopf, 2003; Lebel et al, 2009) but also potentially improve the fitness of cells in which an accidental repair such as BIR has occurred between a long and a critically short telomere. It is important to note that a survivor character is a recessive trait, as telomerase reintroduction in such strains immediately restores the telomerase-mediated mode of telomere maintenance (Makovets et al, 2008; Becerra et al, 2011).…”