Kubota T, Hiraga SI, Yamada K, Lamond AI, Donaldson AD

Yeast cells lacking Ctf18, the major subunit of an alternative Replication Factor C complex, have multiple problems with genome stability. To understand the in vivo function of the Ctf18 complex, we analyzed chromatin composition in a ctf18Δ mutant using the quantitative proteomic technique of SILAC (Stable Isotope Labeling by Amino ...

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acids in Cell culture). 307 of the 491 reported chromosomal proteins were quantitated. The most marked abnormalities occurred when cells were challenged with the replication inhibitor hydroxyurea. Compared to wild-type, hydroxyurea-treated ctf18Δ cells exhibited increased chromatin association of replisome progression complex components including Cdc45, Ctf4 and GINS complex subunits, the polymerase processivity clamp PCNA and the single-stranded DNA-binding complex RPA. Chromatin composition abnormalities observed in ctf18Δ cells were very similar to those of an mrc1Δ mutant, which is defective in the activating the Rad53 checkpoint kinase in response to DNA replication stress. We found that ctf18Δ cells are also defective in Rad53 activation, revealing that the Ctf18 complex is required for engagement of the DNA replication checkpoint. Inappropriate initiation of replication at late origins, due to loss of the checkpoint, probably causes the elevated level of chromatin-bound replisome proteins in the ctf18Δ mutant. The role of Ctf18 in checkpoint activation is not shared by all Replication Factor C-like complexes, since proteomic analysis revealed that cells lacking Elg1 (the major subunit of a different Replication Factor C-like complex) display a different spectrum of chromatin abnormalities. Identification of Ctf18 as a checkpoint protein highlights the usefulness of chromatin proteomic analysis for understanding the in vivo function of proteins that mediate chromatin transactions.

Date: Apr. 19, 2011

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