Research Focus Repairing subtelomeric DSBs at the nuclear periphery Angela Taddei 1,2 and Susan M. Gasser 1 1 Friedrich Miescher Institute for Biomedical Research, Maulbeerstrasse 66, CH-4058 Basel, Switzerland 2 UMR 218, Centre National de la Recherche Scientifique/ Institut Curie-Section de Recherche, 26 rue d’Ulm, 75231 Paris Cedex 05, France Nuclear organization creates microenvironments favor- ing distinct nuclear functions. In budding yeast, silent chromatin regions such as telomeres are clustered at the nuclear periphery, creating zones of transcriptional repression. Recently, in the Journal of Cell Biology, Therizols et al. report that ‘telomere tethering at the nuclear periphery is essential for DNA double strand break repair in subtelomeric regions’. Here, we discuss these results and their functional implications. Introduction In yeast, flies and humans, the protective ends of chromosomes, known as telomeres, nucleate the formation of an altered chromatin structure that represses tran- scription of adjacent RNA pol II genes in a heritable fashion [termed telomere position effect (TPE)] [1]. In budding yeast, the 32 telomeres cluster at the nuclear periphery in four to five groups, forming discrete focal compartments that sequester the silencing factors Sir2, Sir3 and Sir4 [1]. These histone-binding proteins mediate repression and are recruited to telomeres by protein– protein interactions. Notably, the yKu heterodimer, which has a conserved function in non homologous-end joining (NHEJ) as well as protecting native chromosome ends [2], cooperates with the telomere-repeat binding protein Rap1 to nucleate Sir complex binding. The Sir complex can then spread along the chromatin fiber through interactions with histone tails, leading to the variegated and heritable repression of telomere proximal genes [1]. Although we know many molecular details about TPE, the mechanisms that anchor telomeres are not yet completely elucidated. Sir4 and the yKu heterodimer, which bind chromatin and DNA, respectively [3], are necessary for telomere anchoring (Figure 1). But because yeast has no nuclear lamina homologue and because neither yKu nor the Sir proteins have membrane spanning domains, nuclear envelope (NE) components must also be implicated in yeast telomere tethering. One of these is the enhancer of silent chromatin 1 (Esc1), which is found on the nucleoplasmic surface of the inner bilayer of the NE, primarily between pores [3]. Esc1 interacts directly with Sir4. Many other mutations have been identified in yeast that result in telomere delocalization from the NE, although these often act indirectly by influencing the binding or abundance of Sir4 or yKu. Telomere anchoring and TPE are sensitive to mutations in the Nup84 nuclear pore subcomplex Earlier work suggested that components of the nuclear pore complex (NPC) are involved in telomere anchoring, although these results were not generally reproducible [4]. Therizols et al. [5] now investigated the role of the Nup84 nuclear pore subcomplex (Box 1) in telomere anchoring. The authors show that by deleting one of the components of the Nup84 complex, telomere XIL is clearly delocalized from the nuclear periphery and the silencing of a reporter gene inserted on the XIL subtelomeric region is reduced (Table 1). Both the anchoring and TPE defects could stem from an impaired recruitment of the Sir proteins, because this telomere is completely delocalized in a sir4 mutant. Accordingly, they find that Sir3–GFP is partially deloca- lized from telomeric foci in the nup145C mutant, a truncation that eliminates an essential component of the Nup84 complex [5]. These results suggest that some telomeres could interact directly with pores, although several lines of evidence suggest that pores are not universally required for telomere anchoring. Fluorescence imaging can clearly distinguish nuclear pores from telomeric foci in wild-type cells [3] and telomeres remain evenly distributed along the NE in a nup133D mutant, despite a clustering of nuclear pores [4]. Nup84 complex is required for efficient DSB repair in subtelomeric regions The efficiency of double strand break (DSB) repair in haploid yeast correlates with the distance of the damaged site from the chromosome end [6] (Box 2). To test whether this could be related to telomere position, Therizols et al. [5] analyzed survival rates after induction of a DSB in cells with mutant Nup84 complexes, in which telomeres are delocalized. DSBs were generated at either a subtelomeric or an internal position along the left arm of chromosome XI (Figure 2). As previously reported, in a wild-type haploid strain most of the cells could not form colonies when the DSB was induced at internal sites, but survival rates increased 20-fold when the break was targeted to a subtelomeric zone [6]. This increase is due to repair events that involve either telomere addition or nonreci- procal recombination, events that cannot be tolerated at internal positions because they lead to the loss of essential genes (Figure 2). Strikingly, mutants of the Nup84 complex specifically compromised survival when the breaks were subtelo- meric, but not when they were located at a more Corresponding author: Taddei, A. (angela.taddei@fmi.ch). Available online 18 April 2006 Update TRENDS in Cell Biology Vol.16 No.5 May 2006 www.sciencedirect.com 0962-8924/$ - see front matter Q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tcb.2006.03.005