Elucidation of multiple high-resolution states of human MutSβ by cryo-EM reveals interplay between ATP/ADP binding and heteroduplex DNA recognition

Human and mouse genetic studies have demonstrated a role for DNA mismatch repair (MMR) molecular machines in modulating the rate of somatic expansion of the huntingtin (HTT) CAG repeats, and onset and progression of Huntington's Disease (HD). MutSβ, a key component of the MMR pathway, is a heterodimeric protein of MSH2 and MSH3 that recognizes and initiates the repair of extrahelical DNA extrusions. Loss-of-function of mouse Msh3 and reduced-expression alleles of human MSH3 lead to slower rates of somatic expansion and delayed disease onset in humans, signifying MSH3 as a promising therapeutic target for HD. Here we report biochemical and cryo-electron microscopy analyses of human MutSβ, demonstrating MutSβ undergoes conformational changes induced by nucleotide and DNA binding. We present multiple conformations of MutSβ including the DNA-free MutSβ compatible with precisely complementary base-paired homoduplex DNA binding, two distinct structures of MutSβ bound to (CAG)2 DNA, a sliding clamp form and a DNA-unbound, ATP-bound conformation. Along with evidence for novel conformational states adopted by MutSβ to initiate the MMR cascade, these structures provide a foundation for structure-guided drug discovery.