Ccq1 restrains Mre11-mediated degradation of short telomeres

Abstract

Telomeres cap chromosome ends with specialized chromatin composed of DNA repeats bound by a multiprotein complex called shelterin. Fission yeast telomeres can be formed by cleaving a “proto-telomere” bearing 48 bp of telomere repeats to form a new stable chromosomal end that prevents the rapid degradation seen at similar DNA double-strand breaks (DSBs). This end-protection was investigated in viable mutants lacking telomere-associated proteins. Telomerase, the shelterin components Taz1, Rap1, or Poz1 or the telomere-associated protein Rif1 were not required to form a stable chromosome end after cleavage of the proto-telomere. However, cells lacking the fission yeast shelterin component Ccq1 converted the cleaved telomere repeat-capped end to a rapidly degraded DSB. Degradation was greatly reduced by eliminating the nuclease activity of Mre11, a component of the Mre11-Rad50-Nbs1/Xrs2 complex that processes DSBs. These results demonstrate a novel function for Ccq1 to effect end-protection by restraining Mre11-dependent degradation.

A proto-telomere is elongated by telomerase in a shelterin-dependent manner in quiescent fission yeast cells

“…To investigate how eroded telomeres are processed in post-mitotic cells, we took advantage of the conditional telomere-repeat capped end system developed in the Runge laboratory (Figure 1A and B ) ( 40 ). This artificial end, named proto-telomere, mimics in many aspects a single short telomere in cycling cells ( 42 ). Briefly, a single I-SceI cut site was integrated ∼47 kb away from the right telomere of chromosome II (C2-R) flanked by selectable markers ( ura4 + and hph + ).…”

“…In this study, we investigated in quiescent fission yeast cells the fate of a telomere-repeat capped end. This proto-telomere mimics a native single short telomere in many aspects in cycling cells ( 40 , 42 ). Like in cycling cells, the presence of the telomeric seed (48 bp) near the break promotes de novo telomere addition despite the fact that the I-SceI 3′-overhang does not exhibit telomeric sequences.…”

“…The question arises about the mechanism by which telomerase is recruited to the proto-telomere and how it engages with the 3' single-strand end. ChIP experiments in cycling cells reveal that Taz1 is already present at the proto-telomere before the cutting and that Pot1 is recruited later, after induction of the cut ( 42 ). Thus, it is likely that Pot1 is not initially present at the ISce-I break in G0 cells and that telomerase is positioned at the tip of the proto-telomere only through its interaction with the shelterin-bridged Ccq1 protein.…”