SLX4, a coordinator of multiple DNA structure-specific endonucleases, is important for several DNA repair pathways. Non-covalent interactions of SLX4 with ubiquitin are required for localizing SLX4 to DNA-interstrand crosslinks (ICLs), yet how SLX4 is targeted to other functional contexts remains unclear. Here, we show that SLX4 binds SUMO-2/3 chains via SUMO-interacting motifs (SIMs). The SIMs of SLX4 are dispensable for ICL repair, but important for processing CPT-induced replication intermediates, suppressing fragile site instability, and localizing SLX4 to ALT telomeres. The localization of SLX4 to laser-induced DNA damage also requires the SIMs, as well as DNA-end resection, UBC9 and MDC1. Furthermore, the SUMO binding of SLX4 enhances its interaction with specific DNA-damage sensors or telomere-binding proteins, including RPA, MRE11-RAD50-NBS1 and TRF2. Thus, the interactions of SLX4 with SUMO and ubiquitin increase its affinity for factors recognizing different DNA lesions or telomeres, helping to direct the SLX4 complex in distinct functional contexts.

The ATR (ATM [ataxia telangiectasia-mutated]-and Rad3-related) checkpoint is a crucial DNA damage signaling pathway. While the ATR pathway is known to transmit DNA damage signals through the ATR-Chk1 kinase cascade, whether post-translational modifications other than phosphorylation are important for this pathway remains largely unknown. Here, we show that protein SUMOylation plays a key role in the ATR pathway. ATRIP, the regulatory partner of ATR, is modified by SUMO2/3 at K234 and K289. An ATRIP mutant lacking the SUMOylation sites fails to localize to DNA damage and support ATR activation efficiently. Surprisingly, the ATRIP SUMOylation mutant is compromised in the interaction with a protein group, rather than a single protein, in the ATR pathway. Multiple ATRIP-interacting proteins, including ATR, RPA70, TopBP1, and the MRE11-RAD50-NBS1 complex, exhibit reduced binding to the ATRIP SUMOylation mutant in cells and display affinity for SUMO2 chains in vitro, suggesting that they bind not only ATRIP but also SUMO. Fusion of a SUMO2 chain to the ATRIP SUMOylation mutant enhances its interaction with the protein group and partially suppresses its localization and functional defects, revealing that ATRIP SUMOylation promotes ATR activation by providing a unique type of protein glue that boosts multiple protein interactions along the ATR pathway.[Keywords: ATR; ATRIP; checkpoint; SUMOylation] Supplemental material is available for this article.Received January 18, 2014; revised version accepted June 2, 2014.The maintenance of genomic stability requires not only DNA repair machineries but also signal transduction pathways that regulate and coordinate the DNA damage response (DDR) (Ciccia and Elledge 2010). In human cells, DNA damage signaling is primarily initiated by the ataxia telangiectasia-mutated (ATM) and the ATMand Rad3-related (ATR) kinases. Whereas ATM is activated by DNA double-stranded breaks (DSBs), ATR is elicited by a much broader spectrum of DNA damage and replication stress (Cimprich and Cortez 2008;Marechal and Zou 2013;Shiloh and Ziv 2013). Once activated, ATM and ATR phosphorylate and activate their effector kinases, Chk2 and Chk1, respectively. Together, the ATMChk2 and ATR-Chk1 kinase cascades phosphorylate a number of substrates involved in DNA repair, DNA replication, and cell cycle transitions, coordinating these processes to suppress genomic instability. In addition to the phosphorylation events mediated by the ATM and ATR pathways, several other types of post-translational modifications (PTMs), such as ubiquitylation, SUMOylation, methylation, acetylation, and poly-ADP ribosylation, are also implicated in the DDR (Bekker-Jensen and Mailand 2010;Huen et al. 2010;Lukas et al. 2011;Jackson and Durocher 2013). We recently found that the efficient activation of ATR relies on a ubiquitylation circuitry mediated by RPA-ssDNA (RPA-coated ssDNA) and PRP19 (Marechal et al. 2014). This new finding raises a question as to whether other PTMs also participate in DNA damage signaling through the ATR path...