Elsevier

DNA Repair

Volume 3, Issues 8–9, August–September 2004, Pages 1009-1014
DNA Repair

Review
Dial 9–1–1 for DNA damage: the Rad9–Hus1–Rad1 (9–1–1) clamp complex

https://doi.org/10.1016/j.dnarep.2004.03.032Get rights and content

Abstract

Genotoxic stress activates checkpoint signaling pathways that block cell cycle progression, trigger apoptosis, and regulate DNA repair. Studies in yeast and humans have shown that Rad9, Hus1, Rad1, and Rad17 play key roles in checkpoint activation. Three of these proteins–Rad9, Hus1, and Rad1–interact in a heterotrimeric complex (dubbed the 9-1-1 complex), which resembles a PCNA-like sliding clamp, whereas Rad17 is part of a clamp-loading complex that is related to the PCNA clamp loader, replication factor-C (RFC). In response to genotoxic damage, the 9-1-1 complex is loaded around DNA by the Rad17-containing clamp loader. The DNA-bound 9-1-1 complex then facilitates ATR-mediated phosphorylation and activation of Chk1, a protein kinase that regulates S-phase progression, G2/M arrest, and replication fork stabilization. In addition to its role in checkpoint activation, accumulating evidence suggests that the 9-1-1 complex also participates in DNA repair. Taken together, these findings suggest that the 9-1-1 clamp is a multifunctional complex that is loaded onto DNA at sites of damage, where it coordinates checkpoint activation and DNA repair.

Introduction

To ensure the stability of their genomes, cells activate complex signaling networks in response to DNA damage and replication stress [1], [2]. These pathways, termed checkpoint signaling pathways, first detect the damage and then transduce the signal to downstream effectors that block cell cycle progression, activate apoptosis, and influence DNA repair. The Rad9, Hus1, and Rad1 (using Schizosaccharomyces pombe nomenclature) orthologs, which form the Rad9–Hus1–Rad1 (9–1–1) complex, are conserved participants in checkpoint activation that play critical roles in cellular responses to DNA damage. For clarity, this review will use the S. pombe nomenclature to identify proteins in vertebrate systems. To refer to the Saccharomyces cerevisiae orthologs, we will use the S. pombe name followed by the S. cerevisiae name in parentheses (e.g., Rad9 (scDdcl)).

Section snippets

Biochemical function of the Rad9–Hus1–Rad1 (9–1–1) complex

The Rad9, Hus1, and Rad1 orthologs were initially found in genetic screens designed to identify genes that affected sensitivity to genotoxins. Although required for activation of checkpoint signaling pathways, the initial cloning of these genes did not provide clues to the functions of the encoded proteins, and it was not immediately obvious how they participated in checkpoint activation. Insight into the roles of these proteins came from studies using molecular modeling techniques, which

9–1–1’s role in checkpoint activation

The 9–1–1 complex is loaded onto chromatin in response to many different genotoxic stresses, including alkylation, ultraviolet light and ionizing radiation, and replication inhibitors, suggesting that the complex plays a role in the cellular responses activated by many types of DNA damage. Broadly speaking, these types of DNA damage activate two checkpoint signaling pathways, which regulate genotoxin-induced G1 arrest, S-phase slowing, G2/M arrest, apoptosis, and influence DNA repair. For

The role of the 9–1–1 complex in DNA repair

The role of the 9–1–1 complex in DNA repair has not been evaluated in mammalian cells; nonetheless, the heterotrimeric clamp’s role in DNA repair in yeast is now coming into focus. Thus, this review will examine the role of the 9–1–1 complex in DNA repair in yeast.

Summary

The identification of the biochemical function of the 9–1–1 subunits and their roles in checkpoint activation has proceeded at a rapid pace. These studies have given rise to a model in which genotoxins create DNA structures that lead to the loading of the 9–1–1 complex via the Rad17–RFC clamp-loading complex. Once bound to DNA, the 9–1–1 complex serves as a sliding clamp that functions as a central regulator of checkpoint activation and DNA repair by tethering specific proteins to the sites of

References (59)

  • J. Walter et al.

    Initiation of eukaryotic DNA replication: origin unwinding and sequential chromatin association of Cdc45, RPA, and DNA polymerase alpha

    Mol. Cell

    (2000)
  • R.P. St. Onge et al.

    A role for the phosphorylation of Rad9 in checkpoint signaling

    J. Biol. Chem.

    (2003)
  • M.J. Chen et al.

    ATM-dependent phosphorylation of human Rad9 is required for ionizing radiation-induced checkpoint activation

    J. Biol. Chem.

    (2001)
  • P. Roos-Mattjus et al.

    Genotoxin-induced Rad9–Hus1–Rad1 (9–1–1) chromatin association is an early checkpoint signaling event

    J. Biol. Chem.

    (2002)
  • S. Broomfield et al.

    DNA postreplication repair and mutagenesis in Saccharomyces cerevisiae

    Mutat. Res.

    (2001)
  • E.C. Friedberg et al.

    Error-prone DNA polymerases: novel structures and the benefits of infidelity

    Cell

    (2001)
  • T. Bessho et al.

    Human DNA damage checkpoint protein hRAD9 is a 3′ to 5′ exonuclease

    J. Biol. Chem.

    (2000)
  • V.M. Dufault et al.

    Identification and characterization of RAD9B, a paralog of the RAD9 checkpoint gene

    Genomics

    (2003)
  • H. Hang et al.

    Identification and characterization of a paralog of human cell cycle checkpoint gene HUS1

    Genomics

    (2002)
  • R.T. Abraham

    Cell cycle checkpoint signaling through the ATM and ATR kinases

    Genes Dev.

    (2001)
  • Y. Shiloh

    ATM and related protein kinases: safeguarding genome integrity

    Nat. Rev. Cancer

    (2003)
  • T. Caspari et al.

    Characterization of Schizosaccharomyces pombe Hus1: a PCNA-related protein that associates with Rad1 and Rad9

    Mol. Cell. Biol.

    (2000)
  • C. Venclovas et al.

    Structure-based predictions of Rad1, Rad9, Hus1 and Rad17 participation in sliding clamp and clamp-loading complexes

    Nucleic Acids Res.

    (2000)
  • R.P.St. Onge et al.

    The human G2 checkpoint control protein hRAD9 is a nuclear phosphoprotein that forms complexes with hRAD1 and hHUS1

    Mol. Biol. Cell

    (1999)
  • R. Kaur et al.

    Structure–function analysis of fission yeast Hus1–Rad1–Rad9 checkpoint complex

    Mol. Biol. Cell

    (2001)
  • L.A. Lindsey-Boltz et al.

    Purification and characterization of human DNA damage checkpoint Rad complexes

    Proc. Natl. Acad. Sci. U.S.A.

    (2001)
  • Y. Shiomi et al.

    Clamp and clamp loader structures of the human checkpoint protein complexes, Rad9–1–1 and Rad17–RFC

    Genes Cells

    (2002)
  • S.P. Bell et al.

    DNA replication in eukaryotic cells

    Annu. Rev. Biochem.

    (2002)
  • U. Hubscher et al.

    Eukaryotic DNA polymerases

    Annu. Rev. Biochem.

    (2002)
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