Dynamic human MutSα–MutLα complexes compact mismatched DNA

Abstract

DNA mismatch repair (MMR) corrects errors that occur during DNA replication. In humans, mutations in the proteins MutSα and MutLα that initiate MMR cause Lynch syndrome, the most common hereditary cancer. MutSα surveilles the DNA, and upon recognition of a replication error it undergoes adenosine triphosphate-dependent conformational changes and recruits MutLα. Subsequently, proliferating cell nuclear antigen (PCNA) activates MutLα to nick the error-containing strand to allow excision and resynthesis. The structure–function properties of these obligate MutSα–MutLα 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 MutSα–MutLα–DNA complexes. We find that they assemble into multimeric complexes comprising 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 models of MMR initiation that envision diffusive MutS–MutL complexes that move away from the mismatch. Our results suggest MutSα localizes MutLα near the mismatch and promotes DNA configurations that could enhance MMR efficiency by facilitating MutLα 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.

Linker domain function predicts pathogenic MLH1 missense variants

“…These real-time single-molecule observations support the conclusion that the human MMR proteins are only fleetingly stationary, randomly diffuse along the entire mismatched DNA both individually and together, and do not induce detectable MLH/PMS polymerization ( 5 ). Such dynamic motions significantly contrast with the static and/or collapsed properties of the MSH–MLH/PMS complexes that aggregated near the mismatch as projected by the Erie, Weninger, and Hingorani as well as the Wang and Li groups ( 17 , 18 ).…”

Modifiers of Somatic Repeat Instability in Mouse Models of Friedreich Ataxia and the Fragile X-Related Disorders: Implications for the Mechanism of Somatic Expansion in Huntington’s Disease

“…Alternatively, since typical MMR lesions are bound by two or more MutS dimers [123] and MutS␣ and MutS␤ cofractionate from human cell extracts [124], and can both bind to the same FX hairpin [67], MutS␣ may be able to contribute to the MutS␤-bound lesion when MutS␤ is limiting. The fact that loss of MSH6 has a bigger effect in the liver than in the brain in the FXD mouse model, would be consistent with this idea since liver has less MSH3 and more MSH6 than brain [35].…”

“…A simple mathematical model was developed for the competition between MutL proteins for binding to the expansion substrates. This model used the following assumptions: 1) Only a small proportion of the total cellular MutL is actually available for binding to the repeat; 2) Any one of the three MutLs can be recruited to a MutS-bound substrate; 3) Three MutLs (a MutL trimer) are required to bind productively to a substrate [123]; 4) The available MutL complexes are distributed across all the substrates in proportion to their levels/binding affinity. 5) Only those MutL trimers that contain at least one MutL␥ complex result in an expansion (indicated by a check mark); 6) Trimers that lack MutL␥ or lesions that are not bound by at least three MutL complexes do not produce an expansion (indicated by a cross).…”

The Dark Side of UV-Induced DNA Lesion Repair

“…The current model of mammalian MutSα-dependent MMR assumes that recognition of a DNA mismatch by MutSα is followed by ATP binding. This leads to conformational changes of MutSα and promotes its interaction with the MutLα complex made up of MLH1 (MutL protein homolog 1) and PMS2 (PMS1 protein homolog 2) ( Figure 4c) [137]. The MutSα-MutLα-DNA complex interacts with PCNA ( Figure 4c).…”