After genotoxic insult, the decision to repair or undergo cell death is pivotal for undamaged cell survival, and requires a highly controlled coordination of both pathways. Disruption of this regulation results in tumorigenesis and failure of cancer therapy. Mismatch repair (MMR) proteins have a unique role by contributing to both pathways, though direct evidence for their function in the DNA damage response is ambiguous. We report separation of function mutants in the ATPase domains of yeast MutS homologous (MSH) proteins that uncouple MMR-dependent DNA repair from damage response to cisplatin. While mutations in the ATPase domain have devastating effects on the mutation rate of the cell, ATPase processing is mostly dispensable for the cell death phenotype; only limited processing by the MSH6 subunit is required in DNA damage response. Different DNA binding patterns and nucleotide sensitivity of Msh2/Msh6-DNA adduct and protein-mismatch complexes, respectively, suggest that the presence of different DNA lesions influences the requirement for ATP. Limited proteolysis of purified protein gives first indications for differences in nucleotide-induced conformational changes in the presence of platinated DNA. Structural modeling of bacterial MutS proteins reinforces nucleotide-dependent differences in structures that contribute to the distinction between DNA damage response and repair. Our results demonstrate the uncoupling of MMR-dependent damage response from repair and present first indications for the involvement of distinct conformational changes in MSH proteins in this process. These data present evidence for a mechanism of MMR-dependent damage response that differs from MMR; these results have strong implications for the chemotherapeutic treatment of MMR-defective tumors.