Dynamic human MutSα-MutLα complexes compact mismatched DNA

DNA mismatch repair (MMR) corrects errors that occur during DNA replication. In humans, mutations in the proteins MutS alpha and MutL alpha that initiate MMR cause Lynch syndrome, the most common hered-itary cancer. MutS alpha surveilles the DNA, and upon recognition of a replication error it undergoes adenosine triphosphate-dependent conformational changes and recruits MutL alpha. Subsequently, prolifer-ating cell nuclear antigen (PCNA) activates MutL alpha to nick the error-containing strand to allow excision and resynthesis. The structure-function properties of these obligate MutS alpha-MutL alpha complexes remain mostly unexplored in higher eukaryotes, and models are predominately based on studies of prokaryotic pro-teins. Here, we utilize atomic force microscopy (AFM) coupled with other methods to reveal time-and concentration-dependent stoichiom-etries and conformations of assembling human MutS alpha-MutL alpha-DNA complexes. We find that they assemble into multimeric complexes com-prising three to eight 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 mod-els of MMR initiation that envision diffusive MutS-MutL complexes that move away from the mismatch. Our results suggest MutS alpha local-izes MutL alpha near the mismatch and promotes DNA configurations that could enhance MMR efficiency by facilitating MutL alpha nicking the DNA at multiple sites around the mismatch. In addition, such complexes may also protect the mismatch region from nucleo-some reassembly until repair occurs, and they could potentially remodel adjacent nucleosomes.