Chromatin decondensation in S-phase involves recruitment of Cdk2 by Cdc45 and histone H1 phosphorylation.

Department of Biochemistry and Molecular Genetics, University of Virginia School of Medicine, Charlottesville, Virginia 22908, USA.
Cdc45 is required for initiation of DNA replication and fork progression, but its function in these processes remains unknown. We show that targeting Cdc45 to specific chromosomal sites in mammalian cells results in large-scale chromatin decondensation that strongly correlates with histone H1 phosphorylation. Cdk2 is recruited to sites of Cdc45 decondensation, and Cdk2 inhibitors reduce the level of decondensation. Targeting wild-type Cdk2, but not kinase-defective Cdk2, to chromatin is also effective at inducing decondensation involving phospho-H1. Cdc45, Cdk2, Cyclin A, and phospho-H1 associate with chromatin during S-phase, and Cdc45, Cdk2, and an active H1 kinase physically interact. Replicating DNA and phospho-H1 foci colocalize in vivo, and S-phase progression and H1 phosphorylation are directly related and Cdk2 dependent. Because Cdk2 colocalizes with replication foci and H1 regulates higher-order chromatin, we suggest a model in which Cdc45 recruits Cdk2 to replication foci, resulting in H1 phosphorylation, chromatin decondensation, and facilitation of fork progression.
Mesh Terms:
3T3 Cells, Animals, CDC2-CDC28 Kinases, CHO Cells, Cell Cycle Proteins, Chromatin, Cricetinae, Cricetulus, Cyclin A, Cyclin-Dependent Kinase 2, DNA Replication, Fibroblasts, HeLa Cells, Histones, Humans, Immunoblotting, Immunohistochemistry, Mice, Phosphorylation, Precipitin Tests, S Phase
J. Cell Biol. Mar. 14, 2005; 168(6);875-86 [PUBMED:15753125]
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