AFM data for Dynamic MutS-MuL complexes compact mismatched DNA

DNA mismatch repair (MMR) corrects errors that occur during DNA replication. In humans, mutations in the proteins MutS and MutL that initiate MMR cause Lynch Syndrome, the most common hereditary cancer. MutSα surveils the DNA, and upon recognition of a replication error, it undergoes ATP-dependent conformational changes and recruits MutLα. Subsequently PCNA activates MutL to nick the error-containing strand to allow excision and resynthesis. The structure-function properties of these obligate MutS-MutL complexes remain mostly unexplored in higher eukaryotes, and models are predominately based on studies of prokaryotic proteins. Here, we utilize atomic force microscopy (AFM) coupled with other methods to reveal time- and concentration-dependent stoichiometries and conformations of assembling human MutS-MutL-DNA complexes. We find that they assemble into multimeric complexes comprising 3-8 proteins around a mismatch on DNA. On the timescale of a few minutes, these complexes rearrange, folding and compacting the DNA. These observations contrast with dominant models of MMR initiation that envision diffusive MutS-MutL complexes that move away from the mismatch. Our results suggest MutS localizes MutL near the mismatch and promotes DNA configurations that could enhance MMR efficiency by facilitating MutL nicking the DNA at multiple sites around the mismatch. In addition, such complexes may also protect the mismatch region from nucleosome reassembly until repair occurs, and they could potentially remodel adjacent nucleosomes