Preliminary studies carried out with recombinant MUS81-EME1 and MUS81-EME2 purified from E scherichia coli showed that MUS81-EME2 exhibits 10-fold greater nucleolytic activity than MUS81-EME1 on a 3′-flap substrate ( 33). No yeast orthologue of EME2 has been identified, indicating that MUS81-EME2 promotes reactions that are specific to higher eukaryotes. The ERCC4 domains of EME1 and EME2, however, have diverged in amino acid sequence, thus making these subunits catalytically inactive. MUS81, EME1 and EME2 are all members of the XPF/MUS81 family of proteins and contain an ERCC4 endonuclease domain and a helix-hairpin-helix (HhH) 2 domain ( 32). EME2 was identified by its sequence similarity with EME1, with the highest homology observed in the C-terminal domain ( 26). ![]() In addition to its interaction with EME1, human MUS81 can also interact with the EME2 protein. cerevisiae Mms4 and human EME1 appear to be the regulatory subunits of the Mus81-Mms4 and MUS81-EME1 endonucleases, respectively. The formation of a MUS81-EME1-SLX1-SLX4 complex appears to be important for Holliday junction resolution, especially in the absence of BLM, and is critical for proper chromosome segregation ( 9–11). In contrast to yeast, however, phosphorylation does not directly activate the nuclease activity of the enzyme but promotes an interaction between MUS81-EME1 and a second structure-selective nuclease SLX1-SLX4 ( 9). Similarly, in human cells, phosphorylation of EME1 by CDK, and to a lesser extent by PLK1, correlates with increased MUS81-EME1 nuclease activity at prometaphase ( 9, 28). cerevisiae Mms4 is phosphorylated in a cell cycle-dependent manner by the cyclin-dependent kinase Cdk and the Polo-like kinase Cdc5 leading to a stimulation of Mus81-Mms4 activity ( 28–31). Recombinant human MUS81-EME1 exhibits similar substrate specificities ( 9, 26, 27). In contrast, intact HJs are cleaved with a relatively low efficiency. Purified recombinant Mus81-Eme1 and Mus81-Mms4 proteins are active on a range of DNA substrates including 3′-flaps, replication forks and nicked Holliday junctions (HJs), which they cleave by the introduction of a nick close to the branch point ( 22–25). Similarly, Mus81-deficient mice exhibit defects in the repair of meiotic double strand breaks and reduced numbers of mature epididymal sperm ( 21). In addition to the mitotic functions, MUS81 is important for meiosis: for example, in yeast, Mus81-Eme1 ( Schizosaccharomyces pombe) and Mus81-Mms4 ( S acch a romyces cerevisiae) are required for the resolution of meiotic recombination intermediates ( 15–20). MUS81 is also required for telomere maintenance in cells that use an Alternative Lengthening of Telomeres (ALT) telomerase-independent mechanism for telomere maintenance ( 14). In both yeast and mammalian cells, loss of MUS81 activity leads to a hypersensitivity to replication fork-stalling agents such as cisplatin, camptothecin or hydroxyurea ( 2, 3, 12, 13). MUS81 is the catalytic subunit of the MUS81-EME1 structure-selective endonuclease that plays important roles in DNA repair, including (i) the repair of interstrand cross-links ( 3), (ii) the repair and restart of stalled replication forks ( 4–7) and (iii) the resolution of recombination intermediates ( 8–11). The MUS81 protein is required for the maintenance of genomic stability, and its loss has been associated with cancer development ( 1, 2). ![]() These studies suggest that MUS81-EME1 and MUS81-EME2 exhibit similar and yet distinct DNA structure selectivity, indicating that the two MUS81 complexes may promote different nucleolytic cleavage reactions in vivo. Additionally, MUS81-EME2 acts on 5′-flap structures to cleave off a duplex arm, in reactions that cannot be promoted by MUS81-EME1. In contrast to MUS81-EME1, however, MUS81-EME2 cuts D-loop recombination intermediates and in so doing disengages the D-loop structure by cleaving the 3′-invading strand. Like MUS81-EME1, MUS81-EME2 cleaves 3′-flaps, replication forks and nicked Holliday junctions, and exhibits limited endonuclease activity with intact Holliday junctions. We find that MUS81-EME2 is a more active endonuclease than MUS81-EME1 and exhibits broader substrate specificity. Here, we have purified MUS81-EME2 and compared its activities with MUS81-EME1. Little is presently known about the activities of MUS81-EME2. Although the actions of MUS81-EME1 have been extensively investigated, MUS81 is the catalytic subunit of two human structure-selective endonucleases, MUS81-EME1 and MUS81-EME2. ![]() MUS81 plays important cellular roles in the restart of stalled replication forks, the resolution of recombination intermediates and in telomere length maintenance.
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