Yeast Ku protein plays a direct role in telomeric silencing and counteracts inhibition by Rif proteins

Abstract

Yku70p/Yku80p, the yeast Ku protein homologue, is a DNA end-binding heterodimer involved in non-homologous end joining. It also binds to telomeres, where it plays an important role in the maintenance of telomeric DNA structure [1] [2] [3] [4] [5]. Ku protein, together with Rap1p, a telomeric DNA (TG(1-3) repeat)-binding protein, is also required to initiate transcriptional silencing, or telomere-position effect (TPE). Here, we provide evidence for a direct role of Ku in TPE, which is most likely to be in either the recruitment or activation of Sir4 protein at the telomere. Surprisingly, however, the essential role of Ku in TPE is to overcome the inhibitory effect of two Rap1p-interacting proteins, Rif1p and Rif2p, both of which also play an important role in telomere length regulation [6] [7]. Previous studies showed that Rif and Sir proteins compete for binding to the carboxyl terminus of Rap1p [7] [8] [9]. In the absence of this competition, for example, when RIF genes are mutated, Ku is no longer necessary for TPE, whereas the Rap1p carboxyl terminus is still absolutely required. We show that Rif1p is localized to telomeres, indicating that its inhibitory effect on TPE is direct. Our data implicate a role for Ku in the competition between Sir and Rif proteins for access to the telomeric array of Rap1p molecules, which results in a balance between telomeric silencing and telomere length control.

ELG1 , a regulator of genome stability, has a role in telomere length regulation and in silencing

“…Genes involved in telomere length regulation often influence the structure of the telomeric heterochromatin, and these changes are sometimes associated with an increase or decrease in the transcription of genes placed adjacent to the telomere (13,33,37).…”

“…The yKu70p͞yKu80p heterodimer influences telomere length by protecting telomere ends from degradation. It also has additional functions in telomeric silencing (through recruiting Sir proteins to the telomeres), telomeric nuclear localization, and telomere replication timing (12)(13)(14)(15). Cdc13p binds the telomeric single strand, recruits telomerase to the telomere, and prevents degradation (16,17).…”

Telomeric Protein Distributions and Remodeling Through the Cell Cycle inSaccharomyces cerevisiae

“…Previous cytological evidence has shown that Rap1p colocalizes with YЈ elements at the telomeres in relatively intact nuclei (Gotta et al, 1996) and that Rif1p and Rap1p generally colocalize in vivo (Mishra and Shore, 1999). In the chromatin spread method, spheroplasts are gently disrupted and their nuclear contents are spread locally on the slide.…”

“…All strains containing the tlc1-476A mutation used in this study were heteroallelic for the TLC1 locus and also contained a wild-type (WT) copy of the TLC1 gene. The W303-1a and RIF1-9xMYC strains used in this study were obtained from David Shore (Mishra and Shore, 1999). Mating, sporulation, and tetrad dissection were done according to standard methods (Fink, 1991).…”

The Sir2 protein family: A novel deacetylase for gene silencing and more

“…Silencers recruit SIR proteins through the combined action of different DNA-binding factors: the origin recognition complex (ORC) and either or both of two multifunctional regulators, repressor͞activator protein 1 (Rap1p) and ARS binding factor 1 (Abf1p). At telomeres, the TG 1-3 repeat binding protein Rap1 is essential for Sir protein recruitment, but recent studies show that the Ku heterodimer also plays an important role in promoting SIR recruitment, which is counteracted by other factors (5). The subsequent assembly of Sir2͞3͞4 complexes along the adjacent nucleosomes is thought to be driven by a network of interactions between the SIR proteins (6) and a set of critical contacts between both Sir3p and Sir4p and the N-terminal tails of the core histones H3 and H4 (7).…”