The SLX4/BTBD12 complex

Repair of double strand breaks via formation of Holliday junctions requires resolution by DNA unwinding and/or cleavage. The identification of endonucleases or so-called Holliday junction resolvases that can repair these damage intermediates in mammalian cells continues to be a major question in the field. In other model organisms, the yeast endonuclease complex Slx1-Slx4 has activity towards mobile Holliday junctions in vitro but products are not ligatable. The Drosophila ortholog MUS312 has a role in generation of crossovers during meiosis. Here we identify a versatile complex containing the BTB protein BTBD12 and we show that this protein is the human ortholog of yeast Slx4 and Drosophila MUS312. Human SLX4 interacts with several structure specific endonucleases (ERCC4-ERCC1, MUS81-EME1, SLX1), telomere binding proteins (TRF2-TERF2IP), mismatch repair proteins (MSH2-MSH3), the protein kinase PLK1 and a previously uncharacterized open reading frame, C20orf94. SLX4 is required for cellular resistance to DNA interstrand crosslinks and DNA-protein adducts generated by mitomycin C and camptothecin, respectively. In vitro , SLX4 complexes cleave 5' flap, 3' flap, replication fork and Holliday junction substrates. The MUS81-EME1 module of this complex cleaves 3' flaps, replication forks and nicked Holliday junctions generated by SLX1-SLX4. SLX1-SLX1 cleaves Holliday junctions in a symmetrical manor and functions as a classical Holliday junction resolvase. SLX4 also has a role in homologous recombination in mitotic cells that isn't attributed to ERCC4 or MUS81. The first nuclear Holliday junction resolvases were recently described as human GEN1 and S. cerevisiae Yen1. However, to date no role in vivo has been described for these resolvases and Yen1 is not conserved in S. pombe . In this work we have determined that the SLX4 complex is a novel classical Holliday junction resolvase in mammalian cells and is required for several aspects of DNA repair in vivo .