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DNA Distress: Just Ring 9-1-1 [9,11]. Interestingly, the Rad9 carboxyl terminus plays an important role in activation of the DNA damage The Rad9–Hus1–Rad1 checkpoint clamp (9-1-1) is a central player in the cellular checkpoint, which halts cell cycle response to DNA damage; three groups have determined the crystal structure progression to provide time for of 9-1-1, providing new insight into its loading mechanism and association with DNA repair [1]. Phosphorylation of DNA damage checkpoint and repair enzymes. this domain is required for binding and recruiting TopBP1 to sites of Michael Kemp and Aziz Sancar PCNA and 9-1-1 require the DNA damage [17] in order to activity of heteropentameric clamp activate the DNA damage response The genome is constantly exposed to loading complexes termed replication kinase ATR [18]. Unfortunately, to cellular metabolites and exogenous factor C (RFC) and Rad17-RFC, obtain recombinant 9-1-1 protein agents that induce lesions in DNA respectively, which bind to, open, suitable for crystallization, all capable of causing mutation, cancer, and clamp the complexes around three groups [9–11] used a truncated or cell death. In response to such DNA [13]. Whereas PCNA is loaded form of Rad9 lacking this region, damage, eukaryotic cells activate onto 30-primer–template junctions and hence the structures provide signaling pathways that promote DNA by the canonical RFC made up of no insight into the role of the Rad9 repair, allow bypass of lesions during RFC1–5, 9-1-1 is instead preferentially carboxyl terminus in the checkpoint DNA replication, and halt cell-cycle loaded onto 50-recessed ends by response. progression [1,2]. When damage is an alternative form of RFC containing However, by analyzing the DNA extensive or unrepairable, these the Rad17 protein in place of RFC1 binding abilities of 9-1-1 containing pathways may instead induce cell [14,15]. It is expected that the full-length or truncated Rad9, Sohn death through apoptosis. Though different loading properties of and Cho [9] provide evidence that the many proteins mediate the various PCNA and 9-1-1 may be linked Rad9 carboxy-terminal tail prevents aspects of the DNA damage to their unique roles 9-1-1 from binding to DNA containing response, a key component of this in DNA metabolism. 50-recessed primer-template junctions. network is the heterotrimeric Whereas the three identical subunits Though the mechanism of this complex Rad9–Hus1–Rad1 (9-1-1) of PCNA impart symmetry to its inhibition is not known, the authors (Figure 1). structure and provide the same suggest that the carboxy-terminal Computational modeling [3], electron interaction surface to RFC, the domain may physically block DNA from microscopy (EM) [4,5], and biochemical presence of three unique subunits in entering the central hole in 9-1-1. It analyses [4,6,7] had already revealed 9-1-1 suggests that its loading onto is therefore plausible that the clamp the 9-1-1 complex to be structurally DNA is more tightly regulated. Yeast loader Rad17-RFC or other factors similar to proliferating cell nuclear two-hybrid and pull-down assays that interact with the Rad9 carboxyl antigen (PCNA), a homotrimeric protein showing direct interactions of Rad17 terminus may alleviate its negative complex that encircles DNA and with the Rad1 and Rad9 subunits of effect during the course of 9-1-1 stimulates DNA polymerases and 9-1-1 [4,10,16] implicate these two loading onto DNA. Consistent with other enzymes involved in DNA subunits in clamp opening. Indeed, replication and repair. Though genetic through an examination of subunit and biochemical approaches show interface buried surface areas and largely separate roles for PCNA and computational analyses of interface Apoptosis DNA repair 9-1-1 in DNA metabolism, several shape complementarity, Dore´ et al. proteins functionally interact with [11] and Xu et al. [10] conclude the both complexes, including flap Rad9–Rad1 interface is weakest and endonuclease I (FEN1) and DNA therefore the most probable location Ligase I [8]. With recent reports of for ring opening. Sohn and Cho [9], 9-1-1 crystal structures [9–11],itis however, suggest that the Hus1-Rad1 9-1-1 now possible to make direct and interface may be optimal for opening detailed comparisons with PCNA because, they argue, it appears structure in order to better structurally most similar to the understand the distinct functions subunit interfaces in PCNA. DNA damage Translesion of the two clamps. Additional work is clearly required checkpoints synthesis All three independently obtained to characterize the mechanism of structures show a closed-ring 9-1-1 clamp loading onto DNA by Current Biology architecture for 9-1-1 (Figure 2), formed Rad17–RFC, and may well reveal that through a head-to-tail association of more than one loading mechanism Figure 1. The Rad9-Hus1-Rad1 (9-1-1) clamp its subunits, just as in PCNA [12]. is employed. plays a central role in the cellular response to Consistent with a previous EM An additional difference between DNA damage. analysis of the 9-1-1 complex on DNA 9-1-1 and PCNA is the presence of Through direct interactions with ligand [4], these structures show the central a carboxy-terminal w120 amino acid protein binding partners, the 9-1-1 complex functions in multiple DNA damage response hole in 9-1-1 to contain sufficient tail on Rad9 that computational pathways, including DNA damage check- space for double-stranded DNA analysis and limited proteolysis points, DNA repair, translesion synthesis, (dsDNA). To encircle DNA, both experiments predict to be unstructured and apoptosis. Current Biology Vol 19 No 17 R734

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Loading and Lineberger Comprehensive Cancer with an interdomain connection linker of the human 9-1-1 checkpoint complex onto Center, University of North Carolina School DNA by the checkpoint clamp loader of Medicine, Chapel Hill, NC 27599, USA. (IDL) that bridges the two lobes of hRad17-replication factor C complex E-mail: [email protected] each PCNA monomer. In 9-1-1, all in vitro. Proc. Natl. Acad. Sci. USA 100, 1633–1638. three IDLs are structured but have 5. Shiomi, Y., Shinozaki, A., Nakada, D., distinct conformations and charge Sugimoto, K., Usukura, J., Obuse, C., and DOI: 10.1016/j.cub.2009.07.026