Highly conserved MutS and MutL homologs operate as protein dimers in mismatch repair (MMR). MutS recognizes mismatched nucleotides forming ATP-bound sliding clamps, which subsequently load MutL sliding clamps that coordinate MMR excision. Several MMR models envision static MutS-MutL complexes bound to mismatched DNA via a positively charged cleft (PCC) located on the MutL N-terminal domains (NTD). We show MutL-DNA binding is undetectable in physiological conditions. Instead, MutS sliding clamps exploit the PCC to position a MutL NTD on the DNA backbone, likely enabling diffusion-mediated wrapping of the remaining MutL domains around the DNA. The resulting MutL sliding clamp enhances MutH endonuclease and UvrD helicase activities on the DNA, which also engage the PCC during strand-specific incision/excision. These MutS clamp-loader progressions are significantly different from the replication clamp-loaders that attach the polymerase processivity factors β-clamp/PCNA to DNA, highlighting the breadth of mechanisms for stably linking crucial genome maintenance proteins onto DNA.

DNA mismatch repair (MMR) is accomplished by highly conserved MutS and MutL homologs. MutS proteins recognize mismatch nucleotides and in the presence of ATP form a stable sliding clamp on the DNA. The MutS sliding clamp then promotes the cascade assembly of a MutL sliding clamp, which ultimately coordinates downstream mismatch excision. The MutS clamp-loader mechanics are unknown. Here we have examined a conserved positively charged cleft (PCC) located on the MutL N-terminal domain (NTD) proposed to mediate stable DNA binding events in several MMR models. We show that MutL does not bind DNA in physiological ionic conditions. Instead, the MutS sliding clamps and DNA together exploit the PCC to position the MutL NTD for clamp loading. Once in a sliding clamp form, the MutL PCC aids in UvrD helicase capture but not interactions with MutH during mismatch excision. The MutS-DNA clamp-loader progressions are significantly different from the replication clamp-loaders that attach polymerase processivity factors such as beta-clamp and PCNA to the DNA. These studies underlining the breadth of mechanisms for stably linking crucial genome maintenance proteins to the DNA.

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