A Degrading Job

Abstract

Checkpoint responses lead to cell cycle arrest following DNA damage, which allows time for DNA repair. In Saccharomyces cerevisiae , Rad53 kinase is critical to this arrest and also for deoxyribonucleotide triphosphate production during S phase. Most of Rad53's functions depend on its activation by the upstream checkpoint kinase Mec1; however, rad53 deletion mutants show a slow-growth phenotype, whereas mec1 deletion mutants do not. During S phase, histone synthesis is coordinated with DNA replication, and newly synthesized histones are deposited behind the replication fork by chaperones (see Quivy and Almouzni). Gunjan and Verreault investigated the role of Rad3, which can interact with the histone chaperone Asf1, in histone metabolism in wild-type and mutant strains of S. cerevisiae . Yeast lacking--or expressing kinase-deficient--Rad3 were hypersensitive to histone overexpression (which is toxic), whereas yeast lacking Mec1 were not. Western analysis of tagged overexpressed histones or of fractionated or chaperone-bound endogenous histones indicated that degradation of excess histones that were not packaged into chromatin was impaired in rad53 mutants. Therefore, rad53 mutant cells accumulated excess histones. Histone overexpression increased sensitivity to DNA damage, without directly damaging DNA, and reducing histone gene dosage suppressed the rad53 mutant phenotype of sensitivity to DNA damage, slow growth, and chromosome loss. Rad53 formed a complex with histones that did not include Asf1 or any other known histone chaperone proteins; complex formation was affected by Rad53 kinase activity. Thus, Rad53 appears to have a novel function of monitoring and maintaining histone levels that is independent of Mec1 and its activation by DNA damage. A. Gunjan, A. Verreault, A Rad53 kinase-dependent surveillance mechanism that regulates histone protein levels in S. cerevisiae . Cell 115 , 537-549 (2003). [Online Journal] J.-P. Quivy, G. Almouzni, Rad53: A controller ensuring the fine-tuning of histone levels. Cell 115 , 508-510 (2003). [Online Journal]