Interaction between the Msh2 and Msh6 nucleotide-binding sites in the Saccharomyces cerevisiae Msh2-Msh6 complex.

Indirect evidence has suggested that the Msh2-Msh6 mispair-binding complex undergoes conformational changes upon binding of ATP and mispairs, resulting in the formation of Msh2-Msh6 sliding clamps and licensing the formation of Msh2-Msh6-Mlh1-Pms1 ternary complexes. Here, we have studied eight mutant Msh2-Msh6 complexes with defective responses to nucleotide binding and/or mispair binding and used them to study the conformational changes required for sliding clamp formation and ternary complex assembly. ATP binding to the Msh6 nucleotide-binding site results in a conformational change that allows binding of ATP to the Msh2 nucleotide-binding site, although ATP binding to the two nucleotide-binding sites appears to be uncoupled in some mutant complexes. The formation of Msh2-Msh6-Mlh1-Pms1 ternary complexes requires ATP binding to only the Msh6 nucleotide-binding site, whereas the formation of Msh2-Msh6 sliding clamps requires ATP binding to both the Msh2 and Msh6 nucleotide-binding sites. In addition, the properties of the different mutant complexes suggest that distinct conformational states mediated by communication between the Msh2 and Msh6 nucleotide-binding sites are required for the formation of ternary complexes and sliding clamps.

Biochemical basis for dominant mutations in the Saccharomyces cerevisiae MSH6 gene.

Here, the ATP-binding, ATP hydrolysis, mispair-binding, sliding clamp formation, and Mlh1-Pms1 complex interaction properties of dominant mutant Msh2-Msh6 complexes have been characterized. The results demonstrate two mechanisms for dominance. In one, seen with the Msh6-S1036P and Msh6-G1067D mutant complexes, the mutant complex binds mispaired bases, is defective for ATP-induced sliding clamp formation and assembly of ternary complexes with Mlh1-Pms1, and occludes mispaired bases from other mismatch repair pathways. In the second, seen with the Msh6-G1142D complex, the mutant complex binds mispaired bases and is defective for ATP-induced sliding clamp formation but assembles ternary complexes with Mlh1-Pms1 that either occlude the mispaired base or prevent Mlh1-Pms1 from acting in alternate mismatch repair pathways.

The mismatch repair complex hMutS alpha recognizes 5-fluorouracil-modified DNA: implications for chemosensitivity and resistance.

BACKGROUND & AIMS: Recent evidence suggests that patients with advanced microsatellite unstable (MSI) colorectal cancers lack a survival benefit with 5-fluorouracil (5-FU)-based chemotherapy. Additionally, tumor cells with MSI (caused by defective DNA mismatch repair) are more resistant to 5-FU in culture compared with microsatellite stable cells, despite similar amounts of 5-FU incorporation into the cell's DNA. We examined whether the component of the DNA mismatch repair (MMR) system that normally recognizes single base pair mismatches could specifically recognize 5-FU incorporated into DNA as a potential mechanism for chemosensitivity. METHODS: We synthesized oligonucleotides with and without incorporated 5-FU and created oligonucleotides with a single base pair mismatch (as a positive control) to perform electromobility gel shift assays (EMSA) with a purified, baculovirus-synthesized hMutS alpha MMR complex. We also utilized surface plasmon resonance to measure relative binding differences between the oligonucleotides and hMutS alpha in real time. RESULTS: Using EMSA, we demonstrate that hMutS alpha recognizes and binds 5-FU-modified DNA. The reaction is specific as added ATP dissociates the hMutS alpha complex from the 5-FU-modified strand. Using surface plasmon resonance, we demonstrate greater binding between hMutS alpha and 5-FU-modified DNA compared with complementary DNA or DNA containing a C/T mismatch. CONCLUSIONS: The MMR complex hMutS alpha specifically recognizes and binds to 5-FU-modified DNA. Because MMR components are required for the induction of apoptosis by many DNA-damaging agents, the chemosensitivity of 5-FU for patients with advanced colorectal cancer may be in part due to recognition of 5-FU incorporated into tumor DNA by the MMR proteins.

Trapping of DNA nucleotide excision repair factors by nonrepairable carcinogen adducts.

Nucleotide excision repair is part of a cellular defense system that protects genome integrity.Here, this versatile repair system was challenged with mixtures of DNA adducts that were generated to mimic the wide spectrum of bulky lesions produced by complex genotoxic insults. Probing human excision activity with substrate combinations instead of single lesions resulted in a strong bias for particular base adducts, such that the repair factors were immobilized on a small fraction of damaged DNA, whereas the simultaneous excision of other sites was suppressed. Immobilization of excision factors was also induced by nonrepairable decoy adducts, thereby revealing a mechanism of repair inhibition because of hijacking of critical subunits. Thus, the efficiency of excision repair in response to bulky carcinogen-DNA damage is dependent on an antagonistic interaction with both substrate and decoy adducts.

Dominant Saccharomyces cerevisiae msh6 mutations cause increased mispair binding and decreased dissociation from mispairs by Msh2-Msh6 in the presence of ATP.

A previous study described four dominant msh6 mutations that interfere with both the Msh2-Msh6 and Msh2-Msh3 mismatch recognition complexes (Das Gupta, R., and Kolodner, R. D. (2000) Nat. Genet. 24, 53-56). Modeling predicted that two of the amino acid substitutions (G1067D and G1142D) interfere with protein-protein interactions at the ATP-binding site-associated dimer interface, one (S1036P) similarly interferes with protein-protein interactions and affects the Msh2 ATP-binding site, and one (H1096A) affects the Msh6 ATP-binding site. The ATPase activity of the Msh2-Msh6-G1067D and Msh2-Msh6-G1142D complexes was inhibited by GT, +A, and +AT mispairs, and these complexes showed increased binding to GT and +A mispairs in the presence of ATP. The ATPase activity of the Msh2-Msh6-S1036P complex was inhibited by a GT mispair, and it bound the GT mispair in the presence of ATP, whereas its interaction with insertion mispairs was unchanged compared with the wild-type complex. The ATPase activity of the Msh2-Msh6-H1096A complex was generally attenuated, and its mispair-binding behavior was unaffected. These results are in contrast to those obtained with the wild-type Msh2-Msh6 complex, which showed mispair-stimulated ATPase activity and ATP inhibition of mispair binding. These results indicate that the dominant msh6 mutations cause more stable binding to mispairs and suggest that there may be differences in how base base and insertion mispairs are recognized.