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.
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Ccq1 restrains Mre11-mediated degradation to distinguish short telomeres from double-strand breaks
Telomeres protect chromosome ends and are distinguished from DNA double-strand breaks (DSBs) by means of a specialized chromatin composed of DNA repeats bound by a multiprotein complex called shelterin. We investigated the role of telomere-associated proteins in establishing end-protection by studying viable mutants lacking these proteins. Mutants were studied using a Schizosaccharomyces pombe model system that induces cutting of a ‘proto-telomere’ bearing telomere repeats to rapidly form a new stable chromosomal end, in contrast to the rapid degradation of a control DSB. Cells lacking the telomere-associated proteins Taz1, Rap1, Poz1 or Rif1 formed a chromosome end that was stable. Surprisingly, cells lacking Ccq1, or impaired for recruiting Ccq1 to the telomere, converted the cleaved proto-telomere to a rapidly degraded DSB. Ccq1 recruits telomerase, establishes heterochromatin and affects DNA damage checkpoint activation; however, these functions were separable from protection of the new telomere by Ccq1. In cells lacking Ccq1, telomere degradation was greatly reduced by eliminating the nuclease activity of Mre11 (part of the Mre11–Rad50–Nbs1/Xrs2 DSB processing complex), and higher amounts of nuclease-deficient Mre11 associated with the new telomere. These results demonstrate a novel function for S. pombe Ccq1 to effect end-protection by restraining Mre11-dependent degradation of the DNA end.
more » « less- NSF-PAR ID:
- 10489952
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Nucleic Acids Research
- Volume:
- 52
- Issue:
- 7
- ISSN:
- 0305-1048
- Format(s):
- Medium: X Size: p. 3722-3739
- Size(s):
- ["p. 3722-3739"]
- Sponsoring Org:
- National Science Foundation
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