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Reference: Grandin N, et al. (2005) Activation of Mrc1, a mediator of the replication checkpoint, by telomere erosion. Biol Cell 97(10):799-814

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Abstract


Background information: In budding yeast, the loss of either telomere sequences (in telomerase-negative cells) or of telomere capping (in mutants of two telomere end protection proteins, Cdc13 and Yku) lead, by distinct pathways, to telomeric senescence. Following DNA damage, activation of Rad53, which together with Chk1 represents a protein kinase central to all checkpoint pathways, normally requires Rad9, a checkpoint adaptor. Results: We report that in telomerase-negative (tlc1Delta) cells activation of Rad53, although diminished, could still take place in the absence of Rad9. In contrast, Rad9 was essential for Rad53 activation in cells that entered senescence in the presence of functional telomerase, namely in senescent cells bearing mutations in telomere end protection proteins (cdc13-1 yku70Delta). In telomerase-negative cells deleted for RAD9, Mrc1, another checkpoint adaptor previously implicated in the DNA replication checkpoint, mediated Rad53 activation. Rad9 and Rad53, as well as other DNA damage checkpoint proteins (Mec1, Mec3, Chk1 and Dun1), were required for complete DNA damage-induced cell cycle arrest following loss of telomerase function. However, unexpectedly given the formation of an active Rad53-Mrc1 complex in tlc1Delta rad9Delta cells, Mrc1 did not mediate the cell cycle arrest elicited by telomerase loss. Finally, we report that Rad9, Mrc1, Dun1 and Chk1 are activated by phosphorylation following telomerase inactivation. Conclusions: These data indicate that loss of telomere capping or of telomere sequences, which both provoke telomeric senescence, are perceived as two distinct types of damages. In contrast with the Rad53-Rad9-mediated cell cycle arrest that functions in a similar way in both types of telomeric senescence, activation of Rad53-Mrc1 might represent a specific response to telomerase inactivation and/or telomere shortening, the functional significance of which has yet to be uncovered.

Reference Type
Journal Article
Authors
Grandin N, Bailly A, Charbonneau M
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