Human MUS81 complexes stimulate flap endonuclease 1

The yeast heterodimeric Mus81–Mms4 complex possesses a structure-specific endonuclease activity that is critical for the restart of stalled replication forks and removal of toxic recombination intermediates. Previously, we reported that Mus81–Mms4 and Rad27 (yeast FEN1, another structure-specific endonuclease) showed mutual stimulation of nuclease activity. In this study, we investigated the interactions between human FEN1 and MUS81–EME1 or MUS81–EME2, the human homologs of the yeast Mus81–Mms4 complex. We found that both MUS81–EME1 and MUS81–EME2 increased the activity of FEN1, but FEN1 did not stimulate the activity of MUS81–EME1/EME2. The MUS81 subunit alone and its N-terminal half were able to bind to FEN1 and stimulate its endonuclease activity. A truncated FEN1 fragment lacking the C-terminal region that retained catalytic activity was not stimulated by MUS81. Michaelis–Menten kinetic analysis revealed that MUS81 increased the interaction between FEN1 and its substrates, resulting in increased turnover. We also showed that, after DNA damage in human cells, FEN1 co-localizes with MUS81. These findings indicate that the human proteins and yeast homologs act similarly, except that the human FEN1 does not stimulate the nuclease activities of MUS81–EME1 or MUS81–EME2. Thus, the mammalian MUS81 complexes and FEN1 collaborate to remove the various flap structures that arise during many DNA transactions, including Okazaki fragment processing. Structured digital abstract •  FEN1physically interacts with MUS81 and EME2 by anti tag coimmunoprecipitation (View interaction) •  FEN1physically interacts with MUS81 and EME1 by anti tag coimmunoprecipitation (View interaction) •  MUS81physically interacts with FEN1 by pull down (View interaction) •  FEN1 and MUS81colocalize by fluorescence microscopy (View interaction) •  FEN1physically interacts with MUS81 by anti tag coimmunoprecipitation (View interaction)