MutS recognition: multiple mismatches and sequence context effects

MutS recognition of mismatches within primed DNA replication intermediates

MutS initiates mismatch repair by recognizing mismatches in newly replicated DNA. Specific interactions between MutS and mismatches within double-stranded DNA promote ADP-ATP exchange and a conformational change into a sliding clamp. Here, we demonstrated that MutS from Pseudomonas aeruginosa associates with primed DNA replication intermediates. The predicted structure of this MutS-DNA complex revealed a new DNA binding site, in which Asn 279 and Arg 272 appeared to directly interact with the 3'-OH terminus of primed DNA. Mutation of these residues resulted in a noticeable defect in the interaction of MutS with primed DNA substrates. Remarkably, MutS interaction with a mismatch within primed DNA induced a compaction of the protein structure and impaired the formation of an ATP-bound sliding clamp. Our findings reveal a novel DNA binding mode, conformational change and intramolecular signaling for MutS recognition of mismatches within primed DNA structures.

Melting temperature measurement and mesoscopic evaluation of single, double and triple DNA mismatches

Unlike the canonical base pairs AT and GC, the molecular properties of mismatches such as hydrogen bonding and stacking interactions are strongly dependent on the identity of the neighbouring base pairs. As a result, due to the sheer number of possible combinations of mismatches and flanking base pairs, only a fraction of these have been studied in varying experiments or theoretical models. Here, we report on the melting temperature measurement and mesoscopic analysis of contiguous DNA mismatches in nearest-neighbours and next-nearest neighbour contexts. A total of 4032 different mismatch combinations, including single, double and triple mismatches were covered. These were compared with 64 sequences containing all combinations of canonical base pairs in the same location under the same conditions. For a substantial number of single mismatch configurations, 15%, the measured melting temperatures were higher than the least stable AT base pair. The mesoscopic calculation, using the Peyrard-Bishop model, was performed on the set of 4096 sequences, and resulted in estimates of on-site and nearest-neighbour interactions that can be correlated to hydrogen bonding and base stacking. Our results confirm many of the known properties of mismatches, including the peculiar sheared stacking of tandem GA mismatches. More intriguingly, it also reveals that a number of mismatches present strong hydrogen bonding when flanked on both sites by other mismatches. To highlight the applicability of our results, we discuss a number of practical situations such as enzyme binding affinities, thymine DNA glycosylase repair activity, and trinucleotide repeat expansions.

In vitro affinity of Deinococcus radiodurans MutS towards mismatched DNA exceeds that of its orthologues from Escherichia coli and Thermus thermophilus

The mismatch binding protein MutS is responsible for the recognition of mispaired and unpaired bases, which is the initial step in DNA repair. Among the MutS proteins most extensively studied in vitro are those derived from Thermus thermophilus, Thermus aquaticus and Escherichia coli. Here, we present the first report on the in vitro examination of DNA mismatch binding activity of MutS protein from Deinococcus radiodurans and confront this with the properties of those from E. coli and T. thermophilus. The analyses which included mobility gel-shift assay, colorimetric and qPCR estimation of MutS-bound DNA clearly showed that D. radiodurans MutS exhibited much higher affinity towards mismatched DNA in vitro than its counterparts from E. coli and T. thermophilus. In addition, D. radiodurans MutS displayed a significantly higher specificity of DNA mismatch binding than the two other orthologues. The specificity expressed as the ratio of mismatched to fully complementary DNA bound reached over 4 and 20-fold higher values for D. radiodurans than for T. thermophilus and E. coli MutS, respectively. The results demonstrate mainly the biotechnological potential of D. radiodurans MutS but the in vitro characteristics of the MutS orthologues could reflect substantial differences in DNA mismatch binding activities existing in vivo.

The structural impact of DNA mismatches

The structure and dynamics of all the transversion and transition mismatches in three different DNA environments have been characterized by molecular dynamics simulations and NMR spectroscopy. We found that the presence of mismatches produced significant local structural alterations, especially in the case of purine transversions. Mismatched pairs often show promiscuous hydrogen bonding patterns, which interchange among each other in the nanosecond time scale. This therefore defines flexible base pairs, where breathing is frequent, and where distortions in helical parameters are strong, resulting in significant alterations in groove dimension. Even if the DNA structure is plastic enough to absorb the structural impact of the mismatch, local structural changes can be propagated far from the mismatch site, following the expected through-backbone and a previously unknown through-space mechanism. The structural changes related to the presence of mismatches help to understand the different susceptibility of mismatches to the action of repairing proteins.

Using stable MutS dimers and tetramers to quantitatively analyze DNA mismatch recognition and sliding clamp formation

The process of DNA mismatch repair is initiated when MutS recognizes mismatched DNA bases and starts the repair cascade. The Escherichia coli MutS protein exists in an equilibrium between dimers and tetramers, which has compromised biophysical analysis. To uncouple these states, we have generated stable dimers and tetramers, respectively. These proteins allowed kinetic analysis of DNA recognition and structural analysis of the full-length protein by X-ray crystallography and small angle X-ray scattering. Our structural data reveal that the tetramerization domains are flexible with respect to the body of the protein, resulting in mostly extended structures. Tetrameric MutS has a slow dissociation from DNA, which can be due to occasional bending over and binding DNA in its two binding sites. In contrast, the dimer dissociation is faster, primarily dependent on a combination of the type of mismatch and the flanking sequence. In the presence of ATP, we could distinguish two kinetic groups: DNA sequences where MutS forms sliding clamps and those where sliding clamps are not formed efficiently. Interestingly, this inability to undergo a conformational change rather than mismatch affinity is correlated with mismatch repair.

Probing cellular processes with oligo-mediated recombination and using the knowledge gained to optimize recombineering

Recombination with single-strand DNA oligonucleotides (oligos) in Escherichia coli is an efficient and rapid way to modify replicons in vivo. The generation of nucleotide alteration by oligo recombination provides novel assays for studying cellular processes. Single-strand exonucleases inhibit oligo recombination, and recombination is increased by mutating all four known exonucleases. Increasing oligo concentration or adding nonspecific carrier oligo titrates out the exonucleases. In a model for oligo recombination, λ Beta protein anneals the oligo to complementary single-strand DNA at the replication fork. Mismatches are created, and the methyl-directed mismatch repair (MMR) system acts to eliminate the mismatches inhibiting recombination. Three ways to evade MMR through oligo design include, in addition to the desired change (1) a C·C mismatch 6 bp from that change; (2) four or more adjacent mismatches; or (3) mismatches at four or more consecutive wobble positions. The latter proves useful for making high-frequency changes that alter only the target amino acid sequence and even allows modification of essential genes. Efficient uptake of DNA is important for oligo-mediated recombination. Uptake of oligos or plasmids is dependent on media and is 10,000-fold reduced for cells grown in minimal versus rich medium. Genomewide engineering technologies utilizing recombineering will benefit from both optimized recombination frequencies and a greater understanding of how biological processes such as DNA replication and cell division impact recombinants formed at multiple chromosomal loci. Recombination events at multiple loci in individual cells are described here.

Solution structure and characterization of the DNA-binding activity of the B3BP-Smr domain

The MutS1 protein recognizes unpaired bases and initiates mismatch repair, which are essential for high-fidelity DNA replication. The homologous MutS2 protein does not contribute to mismatch repair, but suppresses homologous recombination. MutS2 lacks the damage-recognition domain of MutS1, but contains an additional C-terminal extension: the small MutS-related (Smr) domain. This domain, which is present in both prokaryotes and eukaryotes, has previously been reported to bind to DNA and to possess nicking endonuclease activity. We determine here the solution structure of the functionally active Smr domain of the Bcl3-binding protein (also known as Nedd4-binding protein 2), a protein with unknown function that lacks other domains present in MutS proteins. The Smr domain adopts a two-layer alpha-beta sandwich fold, which has a structural similarity to the C-terminal domain of IF3, the R3H domain, and the N-terminal domain of DNase I. The most conserved residues are located in three loops that form a contiguous, exposed, and positively charged surface with distinct sequence identity for prokaryotic and eukaryotic Smr domains. NMR titration experiments and DNA binding studies using Bcl3-binding protein-Smr domain mutants suggested that these most conserved loop regions participate in DNA binding to single-stranded/double-stranded DNA junctions. Based on the observed DNA-binding-induced multimerization, the structural similarity with both subdomains of DNase I, and the experimentally identified DNA-binding surface, we propose a model for DNA recognition by the Smr domain.

Altered dynamics of DNA bases adjacent to a mismatch: a cue for mismatch recognition by MutS

The structural deviations as well as the alteration in the dynamics of DNA at mismatch sites are considered to have a crucial role in mismatch recognition followed by its repair utilizing mismatch repair family proteins. To compare the dynamics at a mismatch and a non-mismatch site, we incorporated 2-aminopurine, a fluorescent analogue of adenine next to a G.T mismatch, a C.C mismatch, or an unpaired T, and at several other non-mismatch positions. Rotational diffusion of 2-aminopurine at these locations, monitored by time-resolved fluorescence anisotropy, showed distinct differences in the dynamics. This alteration in the motional dynamics is largely confined to the normally matched base-pairs that are immediately adjacent to a mismatch/ unpaired base and could be used by MutS as a cue for mismatch-specific recognition. Interestingly, the enhanced dynamics associated with base-pairs adjacent to a mismatch are significantly restricted upon MutS binding, perhaps "resetting" the cues for downstream events that follow MutS binding. Recognition of such details of motional dynamics of DNA for the first time in the current study enabled us to propose a model that integrates the details of mismatch recognition by MutS as revealed by the high-resolution crystal structure with that of observed base dynamics, and unveils a minimal composite read-out involving the base mismatch and its adjacent normal base-pairs.

Label-free voltammetric detection of single-nucleotide mismatches recognized by the protein MutS

MutS, a protein involved in DNA mismatch repair, recognizes mispaired and unpaired bases in duplex DNA. We have previously used MutS in an electrochemical double-surface technique (DST) for in-vitro detection of point mutations in DNA. The DST involved binding of unlabeled MutS to DNA heteroduplexes at the surface of magnetic beads followed by a highly sensitive electrochemical determination of the protein by measurement of a catalytic protein signal (peak H) at mercury electrodes. Detection of MutS using a peak resulting from oxidation of tyrosine and tryptophan residues of the protein at a carbon-paste electrode (CPE) was also possible but was approximately three orders of magnitude less sensitive. In this work we present an optimized technique for ex-situ voltammetric determination of MutS at a CPE. Choice of optimum experimental conditions (pH of supporting electrolyte, square-wave voltammetry settings, etc.) resulted in substantial improvement of the sensitivity of the assay, enabling detection of approximately 140 pg (1.6 fmol protein monomer) MutS in a 5-microL sample. The sensitivity was increased further by acid hydrolysis of the protein before measurement. The hydrolyzed protein was detectable down to 5 pg (approx. 56 amol) MutS in 5 microL solution. By using the DST combined with determination of the bound unlabeled MutS at the CPE we demonstrated selective interactions of the protein with single-base mismatches and discrimination among different base mispairs in 30-mer or 95-mer DNA duplexes. In agreement with previous studies, binding of the protein to the 30-mer substrates followed the trend G:T>>C:A>A:A>C:T>homoduplex. The electrochemical data were confirmed by use of an independent technique-a quartz-crystal microbalance for real-time monitoring of MutS interactions with DNA duplexes containing different base mispairs. By using the electrochemical DST a G:T mismatch was detectable in up to 1000-fold excess of homoduplex DNA.

Identification of Base Pairs in Single-Nucleotide Polymorphisms by MutS Protein-Mediated Capillary Electrophoresis

Single-nucleotide polymorphisms (SNPs) are widespread genomic variations, which are associated with serious health disorders and drug resistance. Multiple clinical applications and studies of global population genetics require fast and informative analysis of SNPs. Most of conventional methods sense the presence of the SNP but cannot identify the base pair in it. Here we report simple identification of base pairs in SNPs without DNA sequencing. Our approach is based on the unique ability of MutS protein to bind different single-nucleotide mismatches in DNA with different affinities. Conceptually, the DNA in question is mixed with reference DNA, melted, and reannealed. If the DNA in question has an SNP, the products of reannealing will have two different single-nucleotide mismatches, which provide a base-pair-specific signature of the SNP. The products of reannealing are mixed with MutS, equilibrated, and separated by equilibrium capillary electrophoresis of equilibrium mixtures with MutS in the run buffer. The pattern of migration times of DNAs with mismatches is used for unequivocal identification of the base pair in the SNP. In addition to its ability to identify base pairs in SNPs, the new analytical approach is fast, simple, highly sensitive, and requires no quantitation. It will find applications in studies of heterogeneity of base pairs in known SNPs in large human populations.

The construction of bifunctional fusion proteins consisting of MutS and GFP

MutS as a mismatch binding protein is a promising tool for SNP detection. Green fluorescent protein (GFP) is known as an excellent reporter domain. We constructed chimeric proteins consisting of MutS from Thermus thermophilus and GFPuv from Aequorea victoria by cloning the GFPuv gene into the plasmid vectors carrying the mutS gene. The GFPuv domain fused to the N-terminus of MutS (histag-GFP-MutS) exhibited the same level of green fluorescence as free GFPuv. To obtain the fluorescing histag-GFP-MutS protein the expression at 30 degrees C was required, while free GFPuv fluoresces when expressed both at 30 and 37 degrees C. The chimeric protein where the GFPuv domain was fused to the C-terminus of MutS exhibited much weaker green fluorescence (20-25% compared with those of histag-GFP-MutS or free GFPuv). The insertion of (ProGly)5 peptide linker between the MutS and GFP domains resulted in no significant improvement in GFP fluorescence. No shifts in the excitation and emission spectra have been observed for the GFP domain in the fusion proteins. The fusion proteins with GFP at the N- and C-terminus of MutS recognised DNA mismatches similarly like T. thermophilus MutS. The fluorescent proteins recognising DNA mismatches could be useful for SNP scanning or intracellular DNA analysis. The fusion proteins around 125 kDa were efficiently expressed in E. coli and purified in milligram amounts using metal chellate affinity chromatography.

Preliminary studies on DNA retardation by MutS applied to the detection of point mutations in clinical samples

MutS ability to bind DNA mismatches was applied to the detection of point mutations in PCR products. MutS recognized mismatches from single up to five nucleotides and retarded the electrophoretic migration of mismatched DNA. The electrophoretic detection of insertions/deletions above three nucleotides is also possible without MutS, thanks to the DNA mobility shift caused by the presence of large insertion/deletion loops in the heteroduplex DNA. Thus, the method enables the search for a broad range of mutations: from single up to several nucleotides. The mobility shift assays were carried out in polyacrylamide gels stained with SYBR-Gold. One assay required 50-200 ng of PCR product and 1-3 microg of Thermus thermophilus his6-MutS protein. The advantages of this approach are: the small amounts of DNA required for the examination, simple and fast staining, no demand for PCR product purification, no labelling and radioisotopes required. The method was tested in the detection of cancer predisposing mutations in RET, hMSH2, hMLH1, BRCA1, BRCA2 and NBS1 genes. The approach appears to be promising in screening for unknown point mutations.

Mismatch repair ensures fidelity of replication and recombination in the radioresistant organism Deinococcus radiodurans

We have characterized the mismatch repair system (MMR) of the highly radiation-resistant type strain of Deinococcus radiodurans, ATCC 13939. We show that the MMR system is functional in this organism, where it participates in ensuring the fidelity of DNA replication and recombination. The system relies on the activity of two key proteins, MutS1 and MutL, which constitute a conserved core involved in mismatch recognition. Inactivation of MutS1 or MutL resulted in a seven-fold increase in the frequency of spontaneous RifR mutagenesis and a ten-fold increase in the efficiency of integration of a donor point-mutation marker during bacterial transformation. Inactivation of the mismatch repair-associated UvrD helicase increased the level of spontaneous mutagenesis, but had no effect on marker integration--suggesting that binding of MutS1 and MutL proteins to a mismatched heteroduplex suffices to inhibit recombination between non identical (homeologous) DNAs. In contrast, inactivation of MutS2, encoded by the second mutS -related gene present in D. radiodurans, had no effect on mutagenesis or recombination. Cells devoid of MutS1 or MutL proteins were as resistant to gamma-rays, mitomycin C and UV-irradiation as wild-type bacteria, suggesting that the mismatch repair system is not essential for the reconstitution of a functional genome after DNA damage.

Sensitive Electrochemical Determination of Unlabeled MutS Protein and Detection of Point Mutations in DNA

MutS protein plays an important role in the DNA repair system in prokaryotic and eukaryotic cells; it recognizes unpaired and mispaired bases in duplex DNA and can be used for detection of point mutations in vitro. We have shown that small amounts of this protein can be detected electrochemically at mercury and carbon electrodes without any labeling. Using constant current stripping analysis (CPSA) and mercury electrodes, tens of attomoles of this protein can be detected. The sensitivity of the determination at carbon electrodes is by more than 3 orders of magnitude lower. Using biotinylated DNA duplexes attached to magnetic beads, single-base mismatches and insertion/deletions were recognized by MutS. Picogram amounts of this protein were detected by CPSA after MutS releasing from the beads.

Insight into G[bond]T mismatch recognition using molecular dynamics with time-averaged restraints derived from NMR spectroscopy

Molecular dynamics (MD) simulations were conducted for a G[bond]T mismatch-containing DNA decamer, d(CCATGCGTGG)(2), and its Watson-Crick parent sequence, d(CCACGCGTGG)(2). Dynamics in unrestrained MD trajectories were in poor agreement with prior (13)C NMR studies. However, the accuracy of the trajectories was improved by the use of time-averaged interatomic distance restraints derived from (1)H NMR. Postprocess smoothing of the trajectories further improved accuracy. Comparison of restrained and smoothed trajectories of the two DNA molecules revealed distinct differences in dynamics. The major groove width of the mismatched oligomer was more variable over the course of the simulation compared to its parent sequence. Greater variability in helical parameters stretch and opening for the mismatches indicated less kinetically stable base pairing. Interbase helical parameters rise, roll, and tilt were also more variable in certain base steps involving mismatched bases. These dynamic differences between normal and G[bond]T mismatched DNA reflect differences in local flexibility that may play a role in mismatch recognition by the MutS. A potential alternate G[bond]T mismatch binding mode for MutS is also proposed.

Structures of Escherichia coli DNA mismatch repair enzyme MutS in complex with different mismatches: a common recognition mode for diverse substrates

We have refined a series of isomorphous crystal structures of the Escherichia coli DNA mismatch repair enzyme MutS in complex with G:T, A:A, C:A and G:G mismatches and also with a single unpaired thymidine. In all these structures, the DNA is kinked by approximately 60 degrees upon protein binding. Two residues widely conserved in the MutS family are involved in mismatch recognition. The phenylalanine, Phe 36, is seen stacking on one of the mismatched bases. The same base is also seen forming a hydrogen bond to the glutamate Glu 38. This hydrogen bond involves the N7 if the base stacking on Phe 36 is a purine and the N3 if it is a pyrimidine (thymine). Thus, MutS uses a common binding mode to recognize a wide range of mismatches.