SMX makes the cut in genome stability

The faithful duplication and preservation of our genetic material is essential for cell survival; however, DNA is susceptible to damage by extracellular and intracellular agents (e.g. ultraviolet radiation, reactive oxygen species). DNA double-strand breaks (DSBs) are thought to represent the most dangerous type of lesion, as the failure to repair a DSB can lead to loss of genetic information, chromosomal rearrangements, and cell death. Fortunately, cells contain sophisticated DNA repair pathways to counteract the deleterious effects of genotoxic agents. Mutations in DNA repair genes are linked to various diseases, including neurological defects, immunodeficiency, and cancer. Gross chromosomal rearrangements, including deletions, duplications, and translocations, are a hallmark of cancer genomes; DSB formation is an obligate step that precedes these rearrangements. DNA sequence and structure also influence chromosome breakage and repair. For example, chromosome fragile sites (CFSs), which are highly repetitive loci that form DNA secondary structures, co-localize with genomic rearrangements in cancer cells [1]. In addition, DNA sequences that give rise to nonB-form DNA structures are frequently associated with translocation and deletion breakpoints in cancer genomes [2]. Structure-selective endonucleases cleave DNA secondary structures that arise during DNA replication and repair. In eukaryotes, the SLX1-SLX4, MUS81-EME1, and XPF-ERCC1 endonucleases are essential for genome stability; these enzymes remove branched DNA structures that would otherwise impede DNA replication and/or chromosome segregation. Examples of such structures include stalled replication forks and covalently linked, four-stranded recombination intermediates called Holliday junctions (HJs). Nevertheless, DNA cleavage opens the door for indiscriminate repair that can fuel genetic rearrangements and cancer development, emphasizing the importance of regulatory mechanisms to control endonuclease activity and prevent uncontrolled DNA cleavage. Human SLX4 provides the scaffold for a tri-nuclease complex called SMX, comprised of SLX1-SLX4, MUS81EME1, and XPF-ERCC1. Human SMX is the only known example of a tri-nuclease complex. As such, predictions about assembly and activation cannot be inferred from other systems. To this end, the SMX complex has been purified to address two fundamental questions: i) what are the DNA substrates of SMX?; and ii) how does the SLX4 scaffold co-ordinate three different nucleases for DNA cleavage? SMX was found to be a promiscuous endonuclease that cleaves a broad range of DNA secondary structures in vitro3. Remarkably, SLX4 co-ordinates the SLX1 and MUS81-EME1 nucleases during HJ resolution [3, 4] (Figure 1). The involvement of two active sites from different heterodimeric enzymes leads to a non-canonical mechanism of HJ resolution. It was also shown that SLX4 activates MUS81-EME1 to cleave structures that resemble stalled replication forks [3] (Figure 1). Activation involves relaxation of MUS81-EME1’s substrate specificity, which is regulated by a helix-hairpin-helix (HhH) domain in the MUS81 N-terminus (MUS81 N-HhH). Intriguingly, MUS81 N-HhH also mediates the interaction with SLX4 via a C-terminal SAP domain (SLX4 SAP) [5]. These observations led to the proposal that the MUS81 N-HhH domain acts as a self-inhibitory gate that regulates MUS81-EME1 nuclease activity [3]. Within the context of SMX, interaction of MUS81’s N-HhH domain with the SLX4 SAP domain may activate MUS81-EME1 by Editorial