The DNA repair enzyme MutY plays an important role in the prevention of DNA mutations resulting from the presence of the oxidatively damaged lesion 7,8-dihydro-8-oxo-2'-deoxyguanosine (OG) in DNA by the removal of misincorporated adenine residues in OG:A mispairs. MutY also exhibits adenine glycosylase activity toward adenine in G:A and C:A mismatches, although the importance of this activity in vivo has not been established. We have investigated the kinetic properties of MutY's glycosylase activity with OG:A and G:A containing DNA duplexes. Our results indicate that MutY's processing of these two substrates is distinctly different. By using single-turnover experiments, the intrinsic rate for adenine removal by MutY from an OG:A substrate was found to be at least 6-fold faster than that from the corresponding G:A substrate. However, under conditions where [MutY] << [DNA], OG:A substrates are not quantitatively converted to product due to the inefficient turnover resulting from slow product release. In contrast, with a G:A substrate MutY's dissociation from the corresponding product is more facile, such that complete conversion of the substrate to product can be achieved under similar conditions. The kinetic results illustrate that the glycosylase reaction catalyzed by MutY has significant differences depending on the characteristics of the substrate. The lingering of MutY with the product of its reaction with OG:A mispairs may be biologically significant to prevent premature removal of OG. Thus, this approach is providing insight into factors that may be influencing the repair of damaged and mismatched DNA in vivo by base-excision repair glycosylases.

The DNA repair enzyme MutY plays an important role in the prevention of DNA mutations caused by the oxidatively damaged lesion 7,8-dihydro-8-oxo-2′-deoxyguanosine (OG) by removal of misincorporated adenine residues in OG:A mismatched base pairs using N-glycosylase activity. MutY also has glycosylase activity toward adenine in the mismatched base-pairs G:A and C:A. We have investigated the interaction of MutY with DNA duplexes containing the 2′-deoxyadenosine (A) analogs 2′-deoxytubercidin (7-deaza-2′-deoxyadenosine, Z) and 2′-deoxyformycin A (F). Both F and Z should effectively mimic the recognition properties of A but be resistant to the glycosylase activity of MutY, owing to their structural properties. Thus, these derivatives will provide a method for forming a stable MutY-substrate analog complex amenable to structural and biochemical investigation. We find that oligonucleotide duplexes containing OG/G:F and OG/G:Z base-pairs are not substrates for MutY as expected. Using a gel retardation method to measure relevant K d values, we determined that MutY has an increased association with duplexes containing OG/G:F and OG/G:Z base-pairs over their OG/G:C counterparts. Interestingly, MutY has a higher affinity for the F-containing duplexes than the Z counterparts. Additionally, MutY binds to the OG:F and G:F duplexes with a similar, albeit lower, affinity as the substrate OG:A and G:A duplexes. In footprinting experiments using methidiumpropyl-EDTA-Fe(II), a region of the duplex surrounding the OG:F base-pair is observed which is protected by MutY from hydroxyl radical cleavage. These results provide additional evidence for specific recognition of the OG:F base-pair within the DNA duplex. Furthermore, these results also illustrate the utility of OG:F duplexes for providing information regarding the MutY-mismatched DNA complex which could not be obtained with the normal OG:A substrate since a footprint on both strands of the duplex could only be observed with the OG:Fcontaining duplex. These substrate analog duplexes will provide avenues for structural analysis of the MutYmismatched DNA complex and for investigating the properties of the unusual [4Fe-4S] center in MutY.

This paper reports a phenotypic characterization of ggpl mutants. The cloned GGPI (GAS)) gene, which encodes a mawjor GPI-anchored glycoprotein (gpllS)

The Escherichia coli DNA repair enzyme MutY plays an important role in the recognition and repair of 7, 8-dihydro-8-oxo-2'-deoxyguanosine: 2'-deoxyadenosine (OG:A) mismatches in DNA [Michaels et al. (1992) Proc. Natl. Acad. Sci. U.S. A. 89, 7022-7025]. MutY prevents mutations due to misincorporation of A opposite OG during DNA replication by removing the adenine base. This enzyme has significant sequence homology with the [4Fe-4S]2+ cluster-containing DNA repair enzyme, endonuclease III [Michaels et al. (1990) Nucleic AcidsRes. 18, 3841-3845]. In the present study, we have investigated the importance of cluster assembly in folding of MutY. MutY was denatured and then refolded in the presence or absence of ferrous and sulfide ions. Denatured MutY can refold in the presence of ferrous and sulfide ions to provide active enzyme. This suggests the cluster can self-assemble and that this process is facile in vitro. Interestingly, CD spectra and Tm measurements of MutY refolded with and without ferrous and sulfide ions are essentially identical, implying that assembly of the cluster is not required for MutY folding. Additionally, Tm measurements indicated that the [4Fe-4S]2+ cluster does not contribute significantly to the overall thermal stability of MutY. Refolded forms of MutY which lack the cluster are unable to perform the adenine glycosylase function and bind to DNA. However, these inactive folded forms regain activity by addition of ferrous and sulfide ions. This indicates that the Fe-S cluster may have a superficial location, allowing for its assembly after folding. More importantly, these results provide evidence that the presence of the [4Fe-4S]2+ cluster is critical for the specific recognition of substrate DNA necessary for the adenine glycosylase activity of MutY.

The Escherichia coli adenine glycosylase MutY is involved in the repair of 7,8-dihydro-8-oxo-2"-deoxyguanosine (OG):A and G:A mispairs in DNA. Our approach toward understanding recognition and processing of DNA damage by MutY has been to use substrate analogs that retain the recognition properties of the substrate mispair but are resistant to the glycosylase activity of MutY. This approach provides stable MutY-DNA complexes that are amenable to structural and biochemical characterization. In this work, the interaction of MutY with the 2"-deoxyadenosine analogs 2"-deoxy-2"-fluoroadenosine (FA), 2"-deoxyaristeromycin (R) and 2"-deoxyformycin A (F) was investigated. MutY binds to duplexes containing the FA, R or F analogs opposite G and OG within DNA with high affinity; however, no enzymatic processing of these duplexes is observed. The specific nature of the interaction of MutY with an OG:FA duplex was demonstrated by MPE-Fe(II) hydroxyl radical footprinting experiments which showed a nine base pair region of protection by MutY surrounding the mispair. DMS footprinting experiments with an OG:A duplex revealed that a specific G residue located on the OG-containing strand was protected from DMS in the presence of MutY. In contrast, a G residue flanking the substrate analogs R, F or FA was observed to be hypersensitive to DMS in the presence of MutY. These results suggest a major conformational change in the DNA helix upon binding of MutY that exposes the substrate analog-containing strand. This finding is consistent with a nucleotide flipping mechanism for damage recognition by MutY. This work demonstrates that duplex substrates for MutY containing FA, R or F instead of A are excellent substrate mimics that may be used to provide insight into the recognition by MutY of damaged and mismatched base pairs within DNA.

Gas1p is a glucan-elongase that plays a crucial role in yeast morphogenesis. It is predominantly anchored to the plasma membrane through a glycosylphosphatidylinositol, but a fraction was also found covalently bound to the cell wall. We have used fusions with the green fluorescent protein or red fluorescent protein (RFP) to determine its localization. Gas1p was present in microdomains of the plasma membrane, at the mother-bud neck and in the bud scars. By exploiting the instability of RFP-Gas1p, we identified mobile and immobile pools of Gas1p. Moreover, in chs3⌬ cells the chitin ring and the cross-linked Gas1p were missing, but this unveiled an additional unexpected localization of Gas1p along the septum line in cells at cytokinesis. Localization of Gas1p was also perturbed in a chs2⌬ mutant where a remedial septum is produced. Phenotypic analysis of cells expressing a fusion of Gas1p to a transmembrane domain unmasked new roles of the cell wall-bound Gas1p in the maintenance of the bud neck size and in cell separation. We present evidence that Crh1p and Crh2p are required for tethering Gas1p to the chitin ring and bud scar. These results reveal a new mechanism of protein immobilization at specific sites of the cell envelope.

The DNA repair enzyme MutY plays an important role in the prevention of DNA mutations resulting from the presence of the oxidatively damaged lesion 7,8-dihydro-8-oxo-2-deoxyguanosine (OG). MutY is a base excision repair (BER) glycosylase that removes misincorporated adenine residues from OG:A mispairs, as well as G:A and C:A mispairs. We have previously shown that, under conditions of low MutY concentrations relative to an OG:A or G:A substrate, the time course of the adenine glycosylase reaction exhibits biphasic kinetic behavior due to slow release of the DNA product by MutY. The dissociation of MutY from its product may require the recruitment of other proteins from the BER pathway, such as an apurinic-apyrimidinic (AP) endonuclease, as turnover-enhancing cofactors. The effect of the AP endonucleases endonuclease IV (Endo IV), exonuclease III (Exo III), and Ape1 on the reaction kinetics of MutY with G:A-and OG:A-containing substrates was investigated. The effect of the glycosylases UDG and MutM and the DNA polymerase pol I was also characterized. Endo IV and Exo III, unlike Ape1, UDG, and pol I, greatly enhance the rate of product release with a G:A substrate, whereas the rate constant for the adenine removal step remains unchanged. Furthermore, the turnover rate with a truncated form of MutY, Stop 225, which lacks 125 amino acids of the C terminus, is unaffected by the presence of Endo IV or Exo III. These results constitute the first evidence of an interaction between the MutY-product DNA complex and Endo IV or Exo III. Furthermore, they suggest a role for the C-terminal domain of MutY in mediating this interaction. The base excision repair (BER)1 pathway is the primary cellular mechanism charged with the task of removing DNA bases modified via hydrolysis, oxidation, and alkylation (1). BER relies on the action of damage-specific DNA glycosylases that recognize various types of modified or inappropriate bases within the context of normal Watson-Crick DNA (2). The repair process is initiated by hydrolytic removal of the target base by the relevant DNA glycosylase and proceeds by incision at the abasic site, generation of a gap, reparative DNA synthesis, and ligation of the nicked DNA (1, 3). The BER pathway has been reconstituted in vitro with cell-free extracts or purified protein components, and these experiments have established the minimal requirements for restoration of the damaged DNA (4 -11). For example, repair of uracil in DNA was achieved by use of five Escherichia coli proteins: uracil-DNA glycosylase (UDG), endonuclease IV (an AP endonuclease), RecJ protein, DNA polymerase I, and DNA ligase (7). Both "short patch" and "long patch" BER pathways have been observed, which differ in the number of nucleotides and the various protein components involved (12, 13). The type of BER pathway utilized depends on the organism and the type of DNA damage; however, minimally the BER pathway requires a damage-specific glycosylase, an AP endonuclease, DNA polymerase, and DNA ligase.The DNA glycosylases of BER may ...