MutSβ Stimulates Holliday Junction Resolution by the SMX Complex

MutSα and MutSβ play important roles in DNA mismatch repair and are linked to inheritable cancers and degenerative disorders. Here, we show that MSH2 and MSH3, the two components of MutSβ, bind SLX4 protein, a scaffold for the assembly of the SLX1-SLX4-MUS81-EME1-XPF-ERCC1 (SMX) trinuclease complex. SMX promotes the resolution of Holliday junctions (HJs), which are intermediates in homologous recombinational repair. We find that MutSβ binds HJs and stimulates their resolution by SLX1-SLX4 or SMX in reactions dependent upon direct interactions between MutSβ and SLX4. In contrast, MutSα does not stimulate HJ resolution. MSH3-depleted cells exhibit reduced sister chromatid exchanges and elevated levels of homologous recombination ultrafine bridges (HR-UFBs) at mitosis, consistent with defects in the processing of recombination intermediates. These results demonstrate a role for MutSβ in addition to its established role in the pathogenic expansion of CAG/CTG trinucleotide repeats, which is causative of myotonic dystrophy and Huntington’s disease. [Display omitted] •MutSβ, but not MutSα, interacts directly with SLX4•MutSβ stimulates the resolution of recombination intermediates by SMX•Defects in MutSβ lead to the accumulation of unresolved anaphase bridges at mitosis•MutSβ is required for efficient Holliday junction resolution and genome stability Young et al. show that MSH2 and MSH3, two components of the MutSβ heterodimer, bind SLX4 and are required for the efficient resolution of Holliday junctions by the SMX trinuclease complex. Loss of MutSβ activity leads to an accumulation of unresolved homologous recombination ultrafine bridges at mitosis.