Tembusu virus infection in Cherry Valley ducks: the effect of age at infection.

Three groups of Cherry Valley ducks at 5 day, 2 week and 5 week of age were intranasally infected with the WFCL strain of Tembusu virus (TMUV) to investigate the effect of host age on the outcome of TMUV infection. For each age group, clinical signs, gross and microscopic lesions, viral copy numbers in tissues and serum neutralizing antibody titers were recorded. Age-related differences in the resistance to TMUV infection were observed with younger ducks being more susceptible. Some ducks infected at 5 day and 2 week of age developed severe clinical signs, including severe neurological dysfunction and death. However, subclinical signs and no mortality were observed in ducks infected at 5 week of age. A decline in the severity of gross and microscopic lesions was observed as ducks mature. Systemic infections were established in the three age groups post challenge. Higher viral copy numbers in the tissues, especially in vital organs such as the brain and the heart, were developed in the ducks infected at 5 day of age than older ducks, correlating with the severity of clinical signs and lesions in the tissues. Furthermore, ducks infected at 5 week of age developed significantly higher serum neutralizing antibody titers than ducks infected at 5 day of age as determined by serum neutralization test. Therefore, age-related differences in the resistance to TMUV infection should be considered when studying the pathogenicity, pathogenesis, formulation of the vaccination and therapy strategies of TMUV infection in ducks.

Kashin-Beck disease and Sayiwak disease in China: prevalence and a comparison of the clinical manifestations, familial aggregation, and heritability.

OBJECTIVE: To compare the prevalence, the clinical manifestations, familial aggregation and heritability of Kashin-Beck disease (KBD) and Sayiwak disease (SD) in China. METHODS: 10,823 people from 1361 families in 14 villages in Linyou County, Shaanxi Province, were examined for KBD, and 2264 people from 552 families in Sayiwak village, Kashi city, Xinjiang, were examined for SD. The investigation included documentation of individual information and clinical manifestations. Patients were subject to radiographic imaging of the right hand. t-Tests and chi-square tests were used to examine correlations of the diseases with age and gender in each of the two groups. Analysis of familial aggregation was conducted with the chi-square distribution analysis of goodness of fit using the SAS8.0 program. The Li-Mantel-Gart method was employed for the segregation analysis. The Falconer regression method11 was employed to estimate heritability (h²). RESULTS: The prevalence of KBD in Linyou County was 10.90%, and of SD in Sayiwak village was 0.57%. Of the 21 clinical signs examined, KBD cases exhibited 19 signs (90.48%) and SD cases exhibited 18 signs (85.71%), which indicate similarities between the two diseases. However, different clinical signs were evident between the KBD and SD cases, with different impairment rates among joints of limbs in KBD and similar rates in SD. A comparison of radiological features of limb arthropathy between the two diseases showed differences in several characteristics between the two diseases. In addition, measurements of stature and sitting height showed significant differences in bone development between the two diseases. For KBD cases, the values of h² in the first-degree and the second-degree relatives were 41.76% and 37.20% (P<0.05). The CI of h² was 31.17-52.38 and 19.86-54.55, with a segregation ratio of P=0.12, SE(P)=0.014, 95%CI 0.09-0.15, less than 0.25(χ²=42.36, df=1, P<0.001). For SD cases, the values of h² were 155.61%, 273.63% and 236.83%. The 95% CIs of h² were 133.20-178.12, 229.83-317.42 and 145.83-327.81, respectively, with a segregation ratio of P=0.34, SE(P)=0.059, and CIs between 0.22 and 0.45(χ²=4.9817, df=1, P>0.05). CONCLUSION: The results indicate both similarities and differences in the clinical manifestations of KBD and SD. However, environmental factors appear to play a major role in KBD, while hereditability is a major factor in SD.

Cowpea bruchid Callosobruchus maculatus counteracts dietary protease inhibitors by modulating propeptides of major digestive enzymes.

Cowpea bruchids, when challenged by consumption of the soybean cysteine protease inhibitor scN, reconfigure expression of their major CmCP digestive proteases and resume normal feeding and development. Previous evidence indicated that insects selectively induced CmCPs from subfamily B, that were more efficient in autoprocessing and possessed not only higher proteolytic, but also scN-degrading activities. In contrast, dietary scN only marginally up-regulated genes from the more predominant CmCP subfamily A that were inferior to subfamily B. To gain further molecular insight into this adaptive adjustment, we performed domain swapping between the two respective subfamily members B1 and A16, the latter unable to autoprocess or degrade scN even after intermolecular processing. Swapping the propeptides did not qualitatively alter autoprocessing in either protease isoform. Incorporation of either the N- (pAmBA) or C-terminal (pAmAB) mature B1 segment into A16, however, was sufficient to prime autoprocessing of A16 to its mature form. Further, the swap at the N-terminal mature A16 protein region (pAmBA) resulted in four amino acid changes. Replacement of these amino acid residues by the corresponding B1 residues, singly and pair-wise, revealed that autoprocessing activation in pAmBA resulted from cumulative and/or coordinated individual effects. Bacterially expressed isolated propeptides (pA16 and pB1) differed in their ability to inhibit mature B1 enzyme. Lower inhibitory activity in pB1 is likely attributable to its lack of protein stability. This instability in the cleaved propeptide is necessary, although insufficient by itself, for scN-degradation by the mature B1 enzyme. Taken together, cowpea bruchids modulate proteolysis of their digestive enzymes by controlling proCmCP cleavage and propeptide stability, which explains at least in part the plasticity cowpea bruchids demonstrate in response to protease inhibitors.

Molecular cloning and functional analysis of the MutY homolog of Deinococcus radiodurans.

The mutY homolog gene (mutY(Dr)) from Deinococcus radiodurans encodes a 39.4-kDa protein consisting of 363 amino acids that displays 35% identity to the Escherichia coli MutY (MutY(Ec)) protein. Expressed MutY(Dr) is able to complement E. coli mutY mutants but not mutM mutants to reduce the mutation frequency. The glycosylase and binding activities of MutY(Dr) with an A/G-containing substrate are more sensitive to high salt and EDTA concentrations than the activities with an A/7,8-dihydro-8-oxoguanine (GO)-containing substrate are. Like the MutY(Ec) protein, purified recombinant MutY(Dr) expressed in E. coli has adenine glycosylase activity with A/G, A/C, and A/GO mismatches and weak guanine glycosylase activity with a G/GO mismatch. However, MutY(Dr) exhibits limited apurinic/apyrimidinic lyase activity and can form only weak covalent protein-DNA complexes in the presence of sodium borohydride. This may be due to an arginine residue that is present in MutY(Dr) at the position corresponding to the position of MutY(Ec) Lys142, which forms the Schiff base with DNA. The kinetic parameters of MutY(Dr) are similar to those of MutY(Ec). Although MutY(Dr) has similar substrate specificity and a binding preference for an A/GO mismatch over an A/G mismatch, as MutY(Ec) does, the binding affinities for both mismatches are slightly lower for MutY(Dr) than for MutY(Ec). Thus, MutY(Dr) can protect the cell from GO mutational effects caused by ionizing radiation and oxidative stress.

Fission yeast (Schizosaccharomyces pombe) cells defective in the MutY-homologous glycosylase activity have a mutator phenotype and are sensitive to hydrogen peroxide.

The modified base 7,8-dihydro-8-oxo-guanine (8-oxoG) is one of the most stable deleterious products of oxidative DNA damage because it mispairs with adenine during DNA replication. In the fission yeast Schizosaccharomyces pombe, the MutY homolog (SpMYH) is responsible for removing misincorporated adenines from A/8-oxoG or A/G mismatches and thus preventing G:C to T:A mutations. In order to study the functional role of SpMYH, an SpMYH knockout strain was constructed. The SpMYH knockout strain, which does not express SpMYH and has no A/8-oxoG glycosylase activity, displays a 36-fold higher frequency of spontaneous mutations than the wild type strain. Disruption of SpMYH causes increased sensitivity to H2O2 but not to UV-irradiation. Expression of SpMYH in the mutant cells restores the adenine glycosylase activity, reduces the mutation frequency, and elevates the resistance to H2O2. Asp172 of SpMYH is conserved in a helix-hairpin-helix superfamily of glycosylases. The SpMYHA strain expressing D172N SpMYH retained the mutator phenotype. Moreover, when D172N mutant SpMYH was expressed in the wild-type cells, the mutation frequency observed was even higher than that of the parental strains. Thus, a mutant SpMYH that retains substrate-binding activity but is defective in glycosylase activity exhibits a dominant negative effect. This is the first demonstration that a MutY homolog plays an important role in protecting cells against oxidative DNA damage in eukaryotes.

Alteration of DNA base excision repair enzymes hMYH and hOGG1 in hydrogen peroxide resistant transformed human breast cells.

BACKGROUND: Oxidative stress is a major causative agent of carcinogenesis, aging, and a number of diseases. 8-oxoG is the most stable and deleterious lesion of oxidative DNA damage. The 8-oxoG lesions can be eliminated by human repair systems consisting of three enzymes hMTH1, hOGG1, and hMYH homologous to E. coli MutT, MutM, and MutY proteins, respectively. MATERIAL AND METHODS: Human cells (P1, P2, and P3) resistant to H(2)O(2) were derived from the non-tumorigenic human breast cell line MCF10A by sequential treatment of the cells with H(2)O(2). The protein expression levels of DNA repair enzymes were analyzed by Western blotting. The DNA binding and glycosylase activities of hMYH and hOGG1 were measured in the extracts of the H(2)O(2) resistant cells. RESULTS: The H(2)O(2) resistant cells displayed tremendously greater anchorage-independent growth capability and higher expression of the anti-apoptotic protein BCL-2 than the parental cells. H(2)O(2) detoxification ability was elevated in P1 and P2 cells, but not in P3 cells, suggesting P3 cells might employ a different defense mechanism from P1 and P2 cells. In P3 cells, both hOGG1 and hMYH glycosylase activities were reduced but their protein levels increased. Two A/8-oxoG binding complexes were detected with cell extracts: the fast-migrating complex (bottom form) was dominated in MCF10A cells, and was greatly reduced in P3 cells. Interesting, the P3 cells showing the least amount of bottom form had the weakest hMYH glycosylase activity. CONCLUSIONS: Our results demonstrated, for the first time, that alteration of base excision repair pathways is correlated to cell resistance to oxidative stress.

Differential DNA recognition and glycosylase activity of the native human MutY homolog (hMYH) and recombinant hMYH expressed in bacteria.

Human MutY homolog (hMYH), an adenine DNA glycosylase, can effectively remove misincorporated adenines opposite template G or 8-oxoG bases, thereby preventing G:C-->T:A transversions. Human cell extracts possess the adenine DNA glycosylase activity of hMYH and can form protein-DNA complexes with both A/G and A/8-oxoG mismatches. hMYH in cell extracts was shown to be the primary binding protein for A/G- and A/8-oxoG-containing DNA substrates by UV cross-linking. However, recombinant hMYH expressed in bacteria has much weaker glycosylase and substrate-binding activities towards A/G mismatches than native hMYH. Moreover, the protein-DNA complex of bacterially expressed hMYH migrates much faster than that of native hMYH in a non-denaturing polyacrylamide gel. Dephosphorylation of native hMYH reduces the glycosylase activity on A/G more extensively than on A/8-oxoG mismatches but does not alter the gel mobility of the protein-DNA complex. Our results suggest that hMYH in human cell extracts may be associated with other factors in the protein-DNA complex to account for its slower mobility in the gel. hMYH and apurinic/apyrimidinic endonuclease (hAPE1) co-migrate with the protein-DNA complex formed by the extracts and A/8-oxoG-containing DNA.

Repair of oxidative DNA damage: mechanisms and functions.

Cellular genomes suffer extensive damage from exogenous agents and reactive oxygen species formed during normal metabolism. The MutT homologs (MutT/MTH) remove oxidized nucleotide precursors so that they cannot be incorporated into DNA during replication. Among many repair pathways, the base excision repair (BER) pathway is the most important cellular protection mechanism responding to oxidative DNA damage. The 8-oxoG glycosylases (Fpg or MutM/OGG) and the MutY homologs (MutY/MYH) glycosylases along with MutT/MTH protect cells from the mutagenic effects of 8-oxoG, the most stable and deleterious product known caused by oxidative damage to DNA. The key enzymes in the BER process are DNA glycosylases, which remove different damaged bases by cleavage of the N-glycosylic bonds between the bases and the deoxyribose moieties of the nucleotide residues. Biochemical and structural studies have demonstrated the substrate recognition and reaction mechanism of BER enzymes. Cocrystal structures of several glycosylases show that the substrate base flips out of the sharply bent DNA helix and the minor groove is widened to be accessed by the glycosylases. To complete the repair after glycosylase action, the apurinic/apyrimidinic (AP) site is further processed by an incision step, DNA synthesis, an excision step, and DNA ligation through two alternative pathways. The short-patch BER (1-nucleotide patch size) and long-patch BER (2-6-nucleotide patch size) pathways need AP endonuclease to generate a 3' hydroxyl group but require different sets of enzymes for DNA synthesis and ligation. Protein-protein interactions have been reported among the enzymes involved in BER. It is possible that the successive players in the repair pathway are assembled in a complex to perform concerted actions. The BER pathways are proposed to protect cells and organisms from mutagenesis and carcinogenesis.

Human homolog of the MutY repair protein (hMYH) physically interacts with proteins involved in long patch DNA base excision repair.

The human MutY homolog (hMYH) is a DNA glycosylase involved in the removal of adenines or 2-hydroxyadenines misincorporated with template guanines or 7,8-dihydro-8-oxodeoxyguanines. hMYH is associated in vivo with apurinic/apyrimidinic endonuclease (APE1), proliferating cell nuclear antigen (PCNA), and replication protein A (RPA) in HeLa nuclear extracts as shown by immunoprecipitation and Western blotting. However, binding of hMYH to DNA polymerases beta and delta was not detected. By using constructs containing different portions of hMYH fused to glutathione S-transferase, we have demonstrated that the APE1-binding site is at a region around amino acid residue 300, that the PCNA binding activity is located at the C terminus, and that RPA binds to the N terminus of hMYH. A peptide consisting of residues 505-527 of hMYH that contains a conserved PCNA-binding motif binds PCNA, and subsequent amino acid substitution identified Phe-518 and Phe-519 as essential residues required for PCNA binding. RPA binds to a peptide that consists of residues 6-32 of hMYH and contains a conserved RPA-binding motif. The PCNA- and RPA-binding sites of hMYH are further confirmed by peptide and antibody titration. These results suggest that hMYH repair is a long patch base excision repair pathway.

[Effects of short interval successive partial hepatectomy on ACP, AKP, HSC70/HSP68 and PCNA in rat liver].

In this paper, the models of 36-4-4 SISPH and 4-36-36-36 SISPH were used to analyze the changes of activity and content of ACP, AKP, HSC70/HSP68 and PCNA in rat liver. The results showed that the activities of 140 kD ACP and AKP in SISPH were increased following the increase of SISPH number of times, but that of 160 kD ACP and AKP were decreased following the increase of SISPH number of times. The content of PCNA in 4-36-36-36 SISPH were more than that in 36-4-4 SISPH, in contrast for HSC70/HSP68 in these two models. Therefore, the content and activities of ACP, AKP, HSC70/HSP68 and PCNA could be strongly effected by SISPH number of times and SISPH methods. Its mechanisms and physiological significance were discussed.

Intact MutY and its catalytic domain differentially contact with A/8-oxoG-containing DNA.

Escherichia coli MutY is an adenine and a weak guanine DNA glycosylase active on DNA substrates containing A/G, A/8-oxoG, A/C or G/8-oxoG mismatches. A truncated form of MutY (M25, residues 1-226) retains catalytic activity; however, the C-terminal domain of MutY is required for specific binding to the 8-oxoG and is critical for mutation avoidance of oxidative damage. Using alkylation interference experiments, the determinants of the truncated and intact MutY were compared on A/8-oxoG-containing DNA. Several purines within the proximity of mismatched A/8-oxoG show differential contact by the truncated and intact MutY. Most importantly, methylation at the N7 position of the mismatched 8-oxoG and the N3 position of mismatched A interfere with intact MutY but not with M25 binding. The electrostatic contacts of MutY and M25 with the A/8-oxoG-containing DNA substrates are drastically different as shown by ethylation interference experiments. Five consecutive phosphate groups surrounding the 8-oxoG (one on the 3' side and four on the 5' side) interact with MutY but not with M25. The activities of the truncated and intact MutY are modulated differently by two minor groove-binding drugs, distamycin A and Hoechst 33258. Both distamycin A and Hoechst 33258 can inhibit, to a similar extent, the binding and glycosylase activities of MutY and M25 on A/G mismatch. However, binding and glycosylase activities on A/8-oxoG mismatch of intact MutY are inhibited to a lesser degree than those of M25. Overall, these results suggest that the C-terminal domain of MutY specifies additional contact sites on A/GO-containing DNA that are not found in MutY-A/G and M25-A/8-oxoG interactions.

Purification and characterization of a mammalian homolog of Escherichia coli MutY mismatch repair protein from calf liver mitochondria.

A protein homologous to the Escherichia coli MutY glycosylase, referred to as mtMYH, has been purified from calf liver mitochondria. SDS-polyacrylamide gel electrophoresis, western blot analysis as well as gel filtration chromatography predicted the molecular mass of the purified calf mtMYH to be 35-40 kDa. Gel mobility shift analysis showed that the purified mtMYH formed specific binding complexes with A/8-oxoG, G/8-oxoG and T/8-oxoG, weakly with C/8-oxoG, but not with A/G and A/C mismatches. The purified mtMYH exhibited DNA glycosylase activity removing adenine mispaired with G, C or 8-oxoG and weakly removing guanine mispaired with 8-oxoG. The mtMYH glycosylase activity was insensitive to high concentrations of NaCl and EDTA. The purified mtMYH cross-reacted with antibodies against both intact MutY and a peptide of human MutY homolog (hMYH). DNA glycosylase activity of mtMYH was inhibited by anti-MutY antibodies but not by anti-hMYH peptide antibodies. Together with the previously described mitochondrial MutT homolog (MTH1) and 8-oxoG glycosylase (OGG1, a functional MutM homolog), mtMYH can protect mitochondrial DNA from the mutagenic effects of 8-oxoG.

The C-terminal domain of MutY glycosylase determines the 7,8-dihydro-8-oxo-guanine specificity and is crucial for mutation avoidance.

Escherichia coli MutY is an adenine DNA glycosylase active on DNA substrates containing A/G, A/8-oxoG, or A/C mismatches and also has a weak guanine glycosylase activity on G/8-oxoG-containing DNA. The N-terminal domain of MutY, residues 1-226, has been shown to retain catalytic activity. Substrate binding, glycosylase, and Schiff base intermediate formation activities of the truncated and intact MutY were compared. MutY has high binding affinity with 8-oxoG when mispaired with A, G, T, C, or inosine. The truncated protein has more than 18-fold lower affinities for binding various 8-oxoG-containing mismatches when compared with intact MutY. MutY catalytic activity toward A/8-oxoG-containing DNA is much faster than that on A/G-containing DNA whereas deletion of the C-terminal domain reduces its catalytic preference for A/8-oxoG-DNA over A/G-DNA. MutY exerts more inhibition on the catalytic activity of MutM (Fpg) protein than does truncated MutY. The tight binding of MutY with GO mispaired with T, G, and apurinic/apyrimidinic sites may be involved in the regulation of MutM activity. An E. coli mutY strain that produces an N-terminal 249-residue truncated MutY confers a mutator phenotype. These findings strongly suggest that the C-terminal domain of MutY determines the 8-oxoG specificity and is crucial for mutation avoidance by oxidative damage.

[The effect of heat shock before rat partial hepatectomy on HSC70/HSP68 expression and phosphatase activities].

The contribution and content of the continuous heat shock protein 70/induced heat shock protein 68 (HSC70/HSP68), the contribution, variety and activity of acid phosphatases (ACP) and alkaline phosphatases (AKP) had been analysed qualitatively and quantitatively during the liver regeneration after 2/3 hepatectomy (PH) and HS (heat shock at 46 degrees C for 30 min, recovery for 8 h), which were compared with the results only by HS and only by PH. It was shown that the three kinds of treatment all can increase the activity of ACP, AKP and the expression of HSC70/HSP68, but with different change pattern. A further analysis show that after HS-PH the enhanced activity of ACP is related with that of 140 kD phosphatases, the enhanced activity of AKP is associated with that of 140 kD and 160-180 kD phosphatases. It can be reckoned from the results that ACP, AKP and HSC70/HSP68 all act on the heat shock response of hepatocyte and liver regeneration, and may take part in signal transduction in these processes, but ACP may play a dominant role in the start of hepatocyte multiplication, AKP and HSC70/HSP68 may play a dominant role in cytokineses.

The active site of the Escherichia coli MutY DNA adenine glycosylase.

Escherichia coli MutY is an adenine DNA glycosylase active on DNA substrates containing A/G, A/C, or A/8-oxoG mismatches. Although MutY can form a covalent intermediate with its DNA substrates, its possession of 3' apurinic lyase activity is controversial. To study the reaction mechanism of MutY, the conserved Asp-138 was mutated to Asn and the reactivity of this mutant MutY protein determined. The glycosylase activity was completely abolished in the D138N MutY mutant. The D138N mutant and wild-type MutY protein also possessed different DNA binding activities with various mismatches. Several lysine residues were identified in the proximity of the active site by analyzing the imino-covalent MutY-DNA intermediate. Mutation of Lys-157 and Lys-158 both individually and combined, had no effect on MutY activities but the K142A mutant protein was unable to form Schiff base intermediates with DNA substrates. However, the MutY K142A mutant could still bind DNA substrates and had adenine glycosylase activity. Surprisingly, the K142A mutant MutY, but not the wild-type enzyme, could promote a beta/delta-elimination on apurinic DNA. Our results suggest that Asp-138 acts as a general base to deprotonate either the epsilon-amine group of Lys-142 or to activate a water molecule and the resulting apurinic DNA then reacts with Lys-142 to form the Schiff base intermediate with DNA. With the K142A mutant, Asp-138 activates a water molecule to attack the C1' of the adenosine; the resulting apurinic DNA is cleaved through beta/delta-elimination without Schiff base formation.

[Changes in the content and activity of HSC70/HSP68, proteinases and phosphatases during liver regeneration].

This article reports changes in acid and alkaline phosphatases (ACP and AKP), constitutive heat shock protein 70/induced heat shock protein 68 (HSC70/HSP68) and acid and neutral proteinases during liver regeneration (0-144 h) after 2/3 hepatectomy (partial hepatectomy, PH). Both ACP and AKP had two active peaks at 4 and 48 h, 16 and 96 h, respectively, which were followed by significant decrease. The content of HSC70/HSP68 also showed two peaks (16 and 96 h), of which the content after the second peak decreased more obviously than after the first peak. Moreover a 90 kD neutral proteinase was induced at the time from 2 to 6 h and a 27 kD acid proteinase was induced at 36 h. The results suggest that the ACP and the 90 kD neutral proteinase may participate in activating hepatocytes from G0-phase into G1-phase, and AKP and HSC70/HSP68 may play a role mainly in DNA synthesis, cellular metabolism and proliferation. Furthermore 27 kD acid proteinase may be involved in re-differentiation of hepatocytes and reconstruction of liver tissue.

Characterization of the recombinant MutY homolog, an adenine DNA glycosylase, from yeast Schizosaccharomyces pombe.

The mutY homolog (SpMYH) gene from a cDNA library of Schizosaccharomyces pombe encodes a protein of 461 amino acids that displays 28 and 31% identity to Escherichia coli MutY and human MutY homolog (MYH), respectively. Expressed SpMYH is able to complement an E. coli mutY mutant to reduce the mutation rate. Similar to E. coli MutY protein, purified recombinant SpMYH expressed in E. coli has adenine DNA glycosylase and apurinic/apyrimidinic lyase activities on A/G- and A/7,8-dihydro-8-oxoguanine (8-oxoG)-containing DNA. However, both enzymes have different salt requirements and slightly different substrate specificities. SpMYH has greater glycosylase activity on 2-aminopurine/G and A/2-aminopurine but weaker activity on A/C than E. coli MutY. Both enzymes also have different substrate binding affinity and catalytic parameters. Although SpMYH has great affinity to A/8-oxoG-containing DNA as MutY, the binding affinity to A/G-containing DNA is substantially lower for SpMYH than MutY. SpMYH has similar reactivity to both A/G- and A/8-oxoG-containing DNA; however, MutY cleaves A/G-containing DNA about 3-fold more efficiently than it does A/8-oxoG-containing DNA. Thus, SpMYH is the functional eukaryotic MutY homolog responsible for reduction of 8-oxoG mutational effect.

Specific recognition of A/G and A/7,8-dihydro-8-oxoguanine (8-oxoG) mismatches by Escherichia coli MutY: removal of the C-terminal domain preferentially affects A/8-oxoG recognition.

Escherichia coli MutY is a 39 kDa adenine DNA glycosylase and 3' apurinic/apyrimidinic (AP) lyase that is active on DNA substrates containing A/G, A/C, or A/8-oxoG mismatches. 8-oxoG (7,8-dihydro-8-oxoguanine or GO) is a major stable product of oxidative damage, and A/GO mismatches may be particularly important biological substrates for MutY. Proteolytic digestion of MutY using thermolysin was found to produce two relatively stable fragments of 25 and 12 kDa. The 25 kDa fragment begins at the N terminus of MutY and spans the region homologous with E. coli endonuclease III, a DNA glycosylase/AP lyase that repairs oxidatively damaged pyrimidines. The 12 kDa fragment, which consists of much of the rest of MutY, had no detectable activity. The purified 25 kDa fragment (M25) had nearly wild-type binding and cleavage activities with A/G-mismatched substrates. Binding to A/GO-mismatched DNA, however, was dramatically reduced in M25 compared to that in intact protein. Borohydride-dependent enzyme-DNA cross-linking, which is a hallmark of the reaction of several DNA glycosylases that possess concomitant AP lyase activity, was also substantially reduced when M25 was allowed to react with A/GO-mismatched DNA. The significant differences in M25 recognition and reactivity with A/G and A/GO mismatches suggest that the C-terminal region of MutY, a region with no homologous counterpart in E. coli endonuclease III, plays an important role in the repair of mismatched DNA arising from oxidation damage.

Catalytic mechanism and DNA substrate recognition of Escherichia coli MutY protein.

Escherichia coli MutY protein cleaves A/G- or a/7,8-dihydro-8-oxo-guanine (A/GO)-containing DNA on the A-strand by N-glycosylase and apurinic/apyrimidinic endonuclease or lyase activities. In this paper, we show that MutY can be trapped in a stable covalent enzyme-DNA intermediate in the presence of sodium borohydride, a new finding that supports the grouping of MutY in that class of DNA glycosylases that possess concomitant apurinic/apyrimidinic lyase activity. To potentially help determine the substrate recognition site of MutY, mutant proteins were constructed. MutY proteins with a Gly116 --> Ala (G116A) or Asp (G116D) mutation had reduced binding affinities for both A/G- and A/GO-containing DNA substrates. The catalytic parameters, however, were differentially affected. While A/G- and A/GO-containing DNA were cleaved by MutY with specificity constants (kcat/Km) of 10 and 3.3 min-1 microM-1, respectively, MutY(G116D) cleaved these DNAs 2, 300- and 9-fold less efficiently. The catalytic activities of MutY(G116A) with A/G- and A/GO-containing DNA were about the same as that of wild-type MutY. Both MutY(G116A) and MutY(G116D) could be trapped in covalent intermediates with A/GO-containing DNA, but with lower efficiencies than the wild-type enzyme in the presence of sodium borohydride. MutY(G116A) also formed a covalent intermediate with A/G-containing DNA, but MutY(G116D) did not. Since Gly116 of MutY lies in a region that is highly conserved among several DNA glycosylases, it is likely this conserved region is in the proximity of the substrate binding and/or catalytic sites.