Fission Yeast Tel1ATM and Rad3ATR Promote Telomere Protection and Telomerase Recruitment
Nakamura TM (2009) Fission Yeast Tel1ATM and Rad3ATR Promote Telomere Protection and Telomerase
Recruitment. PLoS Genet 5(8): e1000622. doi:10.1371/journal.pgen.1000622
ATM ATR Fission Yeast Tel1 and Rad3 Promote Telomere Protection and Telomerase Recruitment
Bettina A. Moser 0
Lakxmi Subramanian 0
Lyne Khair 0
Ya-Ting Chang 0
Toru M. Nakamura 0
Nancy Maizels, University of Washington, United States of America
0 Department of Biochemistry and Molecular Genetics, University of Illinois at Chicago , Chicago, Illinois , United States of America
The checkpoint kinases ATM and ATR are redundantly required for maintenance of stable telomeres in diverse organisms, including budding and fission yeasts, Arabidopsis, Drosophila, and mammals. However, the molecular basis for telomere instability in cells lacking ATM and ATR has not yet been elucidated fully in organisms that utilize both the telomere protection complex shelterin and telomerase to maintain telomeres, such as fission yeast and humans. Here, we demonstrate by quantitative chromatin immunoprecipitation (ChIP) assays that simultaneous loss of Tel1ATM and Rad3ATR kinases leads to a defect in recruitment of telomerase to telomeres, reduced binding of the shelterin complex subunits Ccq1 and Tpz1, and increased binding of RPA and homologous recombination repair factors to telomeres. Moreover, we show that interaction between Tpz1-Ccq1 and telomerase, thought to be important for telomerase recruitment to telomeres, is disrupted in tel1D rad3D cells. Thus, Tel1ATM and Rad3ATR are redundantly required for both protection of telomeres against recombination and promotion of telomerase recruitment. Based on our current findings, we propose the existence of a regulatory loop between Tel1ATM/Rad3ATR kinases and Tpz1-Ccq1 to ensure proper protection and maintenance of telomeres in fission yeast.
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Funding: This work was supported by UIC start-up funds, the Sidney Kimmel Scholar Program, and NIH grant GM078253 to TMN. LS is supported by a
predoctoral fellowship from the American Heart Association. The funders had no role in study design, data collection and analysis, decision to publish, or preparation
of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
Telomeres, the nucleoprotein protective structures at ends of
eukaryotic chromosomes, are essential for stable maintenance of
eukaryotic genomes [1]. In most eukaryotic species, telomeric
DNA is made up of short repetitive G-rich sequences that can be
extended by the specialized reverse transcriptase telomerase, to
overcome the inability of semi-conservative DNA replication
machineries to fully replicate ends of linear DNA [2]. While most
of the telomeric G-rich repeats are composed of double-stranded
DNA (dsDNA), telomeres end with G-rich 39 single-stranded DNA
(ssDNA), known as G-tail. Both dsDNA and ssDNA portions are
important for maintaining functional telomeres as they provide
binding sites for telomeric repeat sequence-specific binding
proteins, as well as various DNA repair and checkpoint proteins,
that are critical for proper maintenance of telomeres.
In mammalian cells, the shelterin complex, composed of
TRF1, TRF2, TIN2, RAP1, TPP1 and POT1, plays critical
roles in the stable maintenance of telomeres [1]. TRF1 and
TRF2 bind specifically to telomeric dsDNA G-rich repeats via
their C-terminal myb-like DNA binding domain, while POT1
binds to the telomeric G-tail via its N-terminal OB-fold domains
[1]. On the other hand, RAP1, despite the fact that it is
evolutionarily related to the budding yeast dsDNA telomeric
repeat-binding protein Rap1, cannot directly bind to DNA, and
it is recruited to telomeres via its interaction with TRF2 [1].
Likewise, TIN2 is recruited to telomeres by its ability to interact
with both TRF1 and TRF2 [3]. TIN2 plays a central role in the
formation of the shelterin complex through its ability to interact
with the POT1 binding partner TPP1. Previous studies have
shown that TRF2 is essential for preventing fusion of telomeres
by non-homologous end-joining (NHEJ) and for attenuating
ATM-dependent checkpoint signaling [4]. On the other hand,
POT1 is critical for protection of telomeres against nucleolytic
processing and for attenuating ATR-dependent checkpoint
signaling [4]. The POT1-TPP1 sub-complex was also found to
interact with the telomerase complex and to increase processivity
of telomerase [5,6].
Fission yeast Schizosaccharomyces pombe is an attractive model
system for understanding how the shelterin complex contributes to
telomere function since this organism utilizes proteins that show a
high degree of conservation to the mammalian shelterin subunits
[7]. In contrast, the more extensively studied budding yeast
Saccharomyces cerevisiae, while providing unparalleled detailed
molecular understanding on how telomere maintenance is
regulated, cannot provide much insight into how the shelterin
components might contribute to telomere (...truncated)