Mechanisms in Eukaryotic Mismatch Repair

Inactivation of the human mismatch repair system confers a large increase in spontaneous mutability and a strong predisposition to tumor development. Mismatch repair provides several genetic stabilization functions; it corrects DNA biosynthetic errors, ensures the fidelity of genetic recombination, and participates in the earliest steps of checkpoint and apoptotic responses to several classes of DNA damage (see Refs. 1Kunkel T.A. Erie D.A. Annu. Rev. Biochem. 2005; 74: 681-710Crossref PubMed Scopus (1051) Google Scholar, 2Iyer R.R. Pluciennik A. Burdett V. Modrich P.L. Chem. Rev. 2006; 106: 302-323Crossref PubMed Scopus (708) Google Scholar, 3Jiricny J. Nat. Rev. Mol. Cell Biol. 2006; 7: 335-346Crossref PubMed Scopus (995) Google Scholar for recent reviews). Defects in this pathway are the cause of typical and atypical hereditary nonpolyposis colon cancer (4Peltomaki P. Fam. Cancer. 2005; 4: 227-232Crossref PubMed Scopus (204) Google Scholar) but may also play a role in the development of 15–25% of sporadic tumors that occur in a number of tissues (5Peltomaki P. J. Clin. Oncol. 2003; 21: 1174-1179Crossref PubMed Scopus (603) Google Scholar). The system is also of biomedical interest because mismatch repair-deficient tumor cells are resistant to certain cytotoxic chemotherapeutic drugs (2Iyer R.R. Pluciennik A. Burdett V. Modrich P.L. Chem. Rev. 2006; 106: 302-323Crossref PubMed Scopus (708) Google Scholar, 3Jiricny J. Nat. Rev. Mol. Cell Biol. 2006; 7: 335-346Crossref PubMed Scopus (995) Google Scholar), a manifestation of its involvement in the DNA damage response. Of the several mutation avoidance functions of mismatch repair, the reaction responsible for replication error correction has been the most thoroughly studied, and the discussion that follows is restricted to this pathway. Correction of DNA biosynthetic errors requires targeting of mismatch repair to the newly synthesized strand at the replication fork. In contrast to Escherichia coli, where mismatch repair is directed by the transient absence of adenine methylation at d(GATC) sites within newly synthesized DNA, the strand signals that direct replication error correction in eukaryotes have not been identified. However, the function of the hemimethylated d(GATC) strand signal in E. coli mismatch repair is provision of a nick on the unmethylated strand, which serves as the actual signal that directs the reaction (2Iyer R.R. Pluciennik A. Burdett V. Modrich P.L. Chem. Rev. 2006; 106: 302-323Crossref PubMed Scopus (708) Google Scholar). Similarly, a strand-specific nick or gap is sufficient to direct mismatch repair in extracts of mammalian and Drosophila cells as well as Xenopus egg extracts (1Kunkel T.A. Erie D.A. Annu. Rev. Biochem. 2005; 74: 681-710Crossref PubMed Scopus (1051) Google Scholar, 2Iyer R.R. Pluciennik A. Burdett V. Modrich P.L. Chem. Rev. 2006; 106: 302-323Crossref PubMed Scopus (708) Google Scholar, 3Jiricny J. Nat. Rev. Mol. Cell Biol. 2006; 7: 335-346Crossref PubMed Scopus (995) Google Scholar). These findings, coupled with the observation that mismatch repair is more efficient on the lagging strand at the replication fork (6Pavlov Y.I. Mian I.M. Kunkel T.A. Curr. Biol. 2003; 13: 744-748Abstract Full Text Full Text PDF PubMed Scopus (125) Google Scholar), suggest that DNA termini that occur as natural intermediates during replication (3′ terminus on the leading strand; 3′ and 5′ termini on the lagging strand) may suffice as strand signals to direct the correction of DNA biosynthetic errors in eukaryotic cells. Available information on the mechanism of eukaryotic mismatch repair is largely derived from analysis of the nick-directed repair of circular heteroduplexes in mammalian cell extracts. As shown in Fig. 1, the strand break that directs repair may reside either 3′ or 5′ to the mispair as viewed along the shorter path linking the two sites in the circular substrate, and processing of such molecules in extracts is largely restricted to this region. Examination of intermediates produced in HeLa nuclear extracts when repair DNA synthesis is blocked has demonstrated that mismatch-provoked excision removes that portion of the incised strand spanning the shorter path between the nick and the mismatch (Fig. 1) with excision tracts extending from the strand break to a number of sites within a region ≈90–170 nucleotides beyond the mispair (7Fang W.-h. Modrich P. J. Biol. Chem. 1993; 268: 11838-11844Abstract Full Text PDF PubMed Google Scholar, 8Wang H. Hays J.B. J. Biol. Chem. 2002; 277: 26136-26142Abstract Full Text Full Text PDF PubMed Scopus (50) Google Scholar). Radiolabeling of repair DNA synthesis tracts is also consistent with this view (9Thomas D.C. Roberts J.D. Kunkel T.A. J. Biol. Chem. 1991; 266: 3744-3751Abstract Full Text PDF PubMed Google Scholar). The mammalian repair system thus displays a bidirectional capability in the sense that it responds to both 3′- and 5′-heteroduplex orientations, and functionality is retained at nick mismatch separation distances as large as 1000 bp (7Fang W.-h. Modrich P. J. Biol. Chem. 1993; 268: 11838-11844Abstract Full Text PDF PubMed Google Scholar). The nicks that direct the E. coli mismatch repair also serve as sites for initiation of excision (2Iyer R.R. Pluciennik A. Burdett V. Modrich P.L. Chem. Rev. 2006; 106: 302-323Crossref PubMed Scopus (708) Google Scholar), and function of the strand break in the eukaryotic reaction has generally been interpreted in a similar manner (1Kunkel T.A. Erie D.A. Annu. Rev. Biochem. 2005; 74: 681-710Crossref PubMed Scopus (1051) Google Scholar, 2Iyer R.R. Pluciennik A. Burdett V. Modrich P.L. Chem. Rev. 2006; 106: 302-323Crossref PubMed Scopus (708) Google Scholar, 3Jiricny J. Nat. Rev. Mol. Cell Biol. 2006; 7: 335-346Crossref PubMed Scopus (995) Google Scholar). However, a distinct mechanism for mismatch-provoked excision has been proposed based on radiolabeling of repair DNA synthesis tracts in Xenopus egg extracts. In contrast to results obtained with the human system (9Thomas D.C. Roberts J.D. Kunkel T.A. J. Biol. Chem. 1991; 266: 3744-3751Abstract Full Text PDF PubMed Google Scholar), radiolabeling of repair products in Xenopus extracts was significantly higher near the mismatch than the strand break (10Varlet I. Canard B. Brooks P. Cerovic G. Radman M. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 10156-10161Crossref PubMed Scopus (25) Google Scholar). Based on this analysis, Varlet et al. (10Varlet I. Canard B. Brooks P. Cerovic G. Radman M. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 10156-10161Crossref PubMed Scopus (25) Google Scholar) suggested that the nick that directs repair does not correspond to the site of excision initiation. Rather, a mismatch-activated strand-specific endonuclease is postulated to introduce a second nick near the mispair, with this nick serving as an entry site for the excision system. As described below, recent experiments suggest that the mammalian repair system supports both of these modes of excision. The activities responsible for initiation of E. coli mismatch repair are MutS and MutL, which function as homo-oligomers (2Iyer R.R. Pluciennik A. Burdett V. Modrich P.L. Chem. Rev. 2006; 106: 302-323Crossref PubMed Scopus (708) Google Scholar). MutS is responsible for mismatch recognition and MutL serves to interface mismatch recognition by MutS to activation of downstream activities. Mammalian cells possess two MutS activities that function as heterodimers and share MSH2 as a common subunit (11Drummond J.T. Li G.-M. Longley M.J. Modrich P. Science. 1995; 268: 1909-1912Crossref PubMed Scopus (541) Google Scholar, 12Palombo F. Gallinari P. Iaccarino I. Lettieri T. Hughes M. D'Arrigo A. Truong O. Hsuan J.J. Jiricny J. Science. 1995; 268: 1912-1914Crossref PubMed Scopus (483) Google Scholar, 13Palombo F. Iaccarino I. Nakajima E. Ikejima M. Shimada T. Jiricny J. Curr. Biol. 1996; 6: 1181-1184Abstract Full Text Full Text PDF PubMed Scopus (304) Google Scholar, 14Acharya S. Wilson T. Gradia S. Kane M.F. Guerrette S. Marsischky G.T. Kolodner R. Fishel R. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 13629-13634Crossref PubMed Scopus (476) Google Scholar): MutSα (MSH2·MSH6 heterodimer) and MutSβ (MSH2·MSH3 heterodimer). MutSα, which represents 80–90% of the cellular MSH2, preferentially recognizes base-base mismatches and insertion/deletion (ID) 2The abbreviations used are: ID, insertion/deletion; PCNA, proliferating cell nuclear antigen; RPA, replication protein A; RFC, replication factor C. mispairs in which one strand contains 1 or 2 unpaired nucleotides but is also capable of recognition of larger ID heterologies with reduced affinity (11Drummond J.T. Li G.-M. Longley M.J. Modrich P. Science. 1995; 268: 1909-1912Crossref PubMed Scopus (541) Google Scholar, 12Palombo F. Gallinari P. Iaccarino I. Lettieri T. Hughes M. D'Arrigo A. Truong O. Hsuan J.J. Jiricny J. Science. 1995; 268: 1912-1914Crossref PubMed Scopus (483) Google Scholar, 13Palombo F. Iaccarino I. Nakajima E. Ikejima M. Shimada T. Jiricny J. Curr. Biol. 1996; 6: 1181-1184Abstract Full Text Full Text PDF PubMed Scopus (304) Google Scholar, 15Genschel J. Littman S.J. Drummond J.T. Modrich P. J. Biol. Chem. 1998; 273: 19895-19901Abstract Full Text Full Text PDF PubMed Scopus (342) Google Scholar). MutSβ recognizes ID mismatches of 2 to about 10 nucleotides, weakly recognizes single-nucleotide ID mispairs, and is essentially inert on base-base mismatches (12Palombo F. Gallinari P. Iaccarino I. Lettieri T. Hughes M. D'Arrigo A. Truong O. Hsuan J.J. Jiricny J. Science. 1995; 268: 1912-1914Crossref PubMed Scopus (483) Google Scholar, 15Genschel J. Littman S.J. Drummond J.T. Modrich P. J. Biol. Chem. 1998; 273: 19895-19901Abstract Full Text Full Text PDF PubMed Scopus (342) Google Scholar). MSH2 and MSH6 defects have been implicated in tumor development, but the cancer predisposition conferred by MSH6 inactivation is less severe (4Peltomaki P. Fam. Cancer. 2005; 4: 227-232Crossref PubMed Scopus (204) Google Scholar, 16Edelmann W. Umar A. Yang K. Heyer J. Kucherlapati M. Lia M. Kneitz B. Avdievich E. Fan K. Wong E. Crouse G. Kunkel T. Lipkin M. Kolodner R.D. Kucherlapati R. Cancer Res. 2000; 60: 803-807PubMed Google Scholar). The association of MSH3 defects with tumor development appears to be limited (4Peltomaki P. Fam. Cancer. 2005; 4: 227-232Crossref PubMed Scopus (204) Google Scholar, 5Peltomaki P. J. Clin. Oncol. 2003; 21: 1174-1179Crossref PubMed Scopus (603) Google Scholar, 16Edelmann W. Umar A. Yang K. Heyer J. Kucherlapati M. Lia M. Kneitz B. Avdievich E. Fan K. Wong E. Crouse G. Kunkel T. Lipkin M. Kolodner R.D. Kucherlapati R. Cancer Res. 2000; 60: 803-807PubMed Google Scholar). Three eukaryotic MutL activities have been identified and, like eukaryotic MutS activities, function as heterodimeric complexes with MLH1 serving as a common subunit. MutLα, a heterodimer of MLH1 and PMS2, is the primary MutL activity in human mitotic cells and supports repair initiated by either MutSα or MutSβ (17Li G.-M. Modrich P. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 1950-1954Crossref PubMed Scopus (357) Google Scholar). MutLα accounts for ≈90% of the MLH1 in human cells (18Raschle M. Marra G. Nystrom-Lahti M. Schar P. Jiricny J. J. Biol. Chem. 1999; 274: 32368-32375Abstract Full Text Full Text PDF PubMed Scopus (149) Google Scholar, 19Cannavo E. Marra G. Sabates-Bellver J. Menigatti M. Lipkin S.M. Fischer F. Cejka P. Jiricny J. Cancer Res. 2005; 65: 10759-10766Crossref PubMed Scopus (97) Google Scholar), but two low abundance complexes involving MLH1 have also been identified. A human MLH1·PMS1 heterodimer (MutLβ) has been isolated, but involvement in mismatch repair has not been demonstrated (18Raschle M. Marra G. Nystrom-Lahti M. Schar P. Jiricny J. J. Biol. Chem. 1999; 274: 32368-32375Abstract Full Text Full Text PDF PubMed Scopus (149) Google Scholar). However, the MutLγ MLH1·MLH3 complex has been reported to support modest levels of base-base and single-nucleotide ID mismatch repair in vitro, events that are presumably initiated by MutSα (19Cannavo E. Marra G. Sabates-Bellver J. Menigatti M. Lipkin S.M. Fischer F. Cejka P. Jiricny J. Cancer Res. 2005; 65: 10759-10766Crossref PubMed Scopus (97) Google Scholar). Genetic inactivation of MLH1 or PMS2 confers cancer predisposition, but mutations in PMS1 do not (4Peltomaki P. Fam. Cancer. 2005; 4: 227-232Crossref PubMed Scopus (204) Google Scholar, 16Edelmann W. Umar A. Yang K. Heyer J. Kucherlapati M. Lia M. Kneitz B. Avdievich E. Fan K. Wong E. Crouse G. Kunkel T. Lipkin M. Kolodner R.D. Kucherlapati R. Cancer Res. 2000; 60: 803-807PubMed Google Scholar). Involvement of MLH3 defects in tumor development is uncertain (4Peltomaki P. Fam. Cancer. 2005; 4: 227-232Crossref PubMed Scopus (204) Google Scholar, 5Peltomaki P. J. Clin. Oncol. 2003; 21: 1174-1179Crossref PubMed Scopus (603) Google Scholar). Yeast genetic studies and analysis of the mammalian extract reaction have implicated six additional activities in eukaryotic mismatch repair. The key finding that culminated in the reconstitution studies described below was the demonstration that exonuclease 1 (Exo1) participates in the reaction. Genetic evidence for Exo1 involvement in yeast mismatch repair (20Szankasi P. Smith G.R. Science. 1995; 267: 1166-1169Crossref PubMed Scopus (184) Google Scholar, 21Tishkoff D.X. Boerger A.L. Bertrand P. Filosi N. Gaida G.M. Kane M.F. Kolodner R.D. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 7487-7492Crossref PubMed Scopus (335) Google Scholar) led to the subsequent demonstration that Exo1 is required for repair of base-base and single-nucleotide ID mismatches in mammalian cell extracts (22Genschel J. Bazemore L.R. Modrich P. J. Biol. Chem. 2002; 277: 13302-13311Abstract Full Text Full Text PDF PubMed Scopus (203) Google Scholar, 23Wei K. Clark A.B. Wong E. Kane M.F. Mazur D.J. Parris T. Kolas N.K. Russell R. Hou Jr., H. Kneitz B. Yang G. Kunkel T.A. Kolodner R.D. Cohen P.E. Edelmann W. Genes Dev. 2003; 17: 603-614Crossref PubMed Scopus (273) Google Scholar). Because Exo1 hydrolyzes duplex DNA with 5′ to 3′ polarity (24Wilson III, D.M. Carney J.P. Coleman M.A. Adamson A.W. Christensen M. Lamerdin J.E. Nucleic Acids Res. 1998; 26: 3762-3768Crossref PubMed Scopus (97) Google Scholar, 25Lee B.I. Wilson III, D.M. J. Biol. Chem. 1999; 274: 37763-37769Abstract Full Text Full Text PDF PubMed Scopus (156) Google Scholar), the surprising feature of this requirement is that the enzyme is necessary for excision and repair directed by strand break located either 5′ or 3′ to the mismatch. This paradoxical requirement led to the suggestion that Exo1 might play a positive regulatory role in 3′ excision or that the reaction may be mediated by a cryptic Exo1 3′ to 5′ hydrolytic activity (22Genschel J. Bazemore L.R. Modrich P. J. Biol. Chem. 2002; 277: 13302-13311Abstract Full Text Full Text PDF PubMed Scopus (203) Google Scholar). As discussed below, this issue has been recently resolved, and it is not necessary to invoke either of these possibilities. Exo1–/– mice display modest cancer predisposition, and Exo1 deficiency is associated with a 30-fold elevation of hypoxanthine-guanine phosphoribosyltransferase mutability, substantially less than the 150-fold increase observed with Msh2–/– cells (23Wei K. Clark A.B. Wong E. Kane M.F. Mazur D.J. Parris T. Kolas N.K. Russell R. Hou Jr., H. Kneitz B. Yang G. Kunkel T.A. Kolodner R.D. Cohen P.E. Edelmann W. Genes Dev. 2003; 17: 603-614Crossref PubMed Scopus (273) Google Scholar). Although extracts of Exo1–/– mouse cells are virtually devoid of repair activity on base-base mismatches, they retain significant activity on one- or two-nucleotide ID mispairs (23Wei K. Clark A.B. Wong E. Kane M.F. Mazur D.J. Parris T. Kolas N.K. Russell R. Hou Jr., H. Kneitz B. Yang G. Kunkel T.A. Kolodner R.D. Cohen P.E. Edelmann W. Genes Dev. 2003; 17: 603-614Crossref PubMed Scopus (273) Google Scholar). These the of one or more excision activities, and several have been Involvement of the 3′ to 5′ exonuclease functions of DNA and in mismatch repair has been proposed on genetic and D.A. M.A. Mol. Biol. 1999; PubMed Scopus Google Scholar, H. Hays J.B. J. Biol. Chem. 2002; 277: Full Text Full Text PDF PubMed Scopus (25) Google Scholar, P. H. Kunkel T.A. M.A. D.A. Mol. Biol. 2005; PubMed Scopus Google Scholar), but this has been A. K. Kolodner R.D. Mol. 2000; 6: Full Text Full Text PDF PubMed Scopus Google Scholar, N. Modrich P. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). a et al. F. F. Li G.M. C. 2005; 6: PubMed Scopus Google Scholar) have suggested that the 3′ to participates in mismatch repair. was shown to the of repair by and repair was to levels by the of However, the involvement of activities in repair was not because the was This is of because of also to and with cell E. C. T. T. M. C. N. S. J. 1999; PubMed Scopus Google Scholar). in human cell extracts and have also involvement of several DNA in eukaryotic mismatch repair. The extract reaction is by the DNA protein C. Kolodner R. R.D. A. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar), which the gap endonuclease and repair DNA synthesis C. S. S.M. J.J. Li G.M. Mol. Biol. 2002; PubMed Scopus Google Scholar). The protein which with MutSα, may also play an role in the initiation of mismatch-provoked excision in nuclear extracts F. S. C. Li G.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). The replication and DNA have also been implicated in mismatch repair in human cell extracts A. A.B. D.C. Clark A.B. Kunkel T.A. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar, M.J. Modrich P. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar, S. H. Li G.M. Nucleic Acids Res. 1998; 26: PubMed Scopus Google Scholar, J. Modrich P. Mol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). PCNA, which confers on A. M. Annu. Rev. Biochem. 2005; 74: PubMed Scopus Google Scholar), in mismatch repair. As might be is necessary for repair DNA synthesis S. H. Li G.M. Nucleic Acids Res. 1998; 26: PubMed Scopus Google Scholar, J. Modrich P. Mol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar), but it is also required for mismatch-provoked excision A. A.B. D.C. Clark A.B. Kunkel T.A. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar). The most evidence for involvement in the excision of mismatch repair has been by a complex with PCNA, with downstream events S. B. Mol. Biol. 1998; PubMed Scopus Google Scholar). Although mismatch-provoked excision in HeLa cell of excision events are to of at two hydrolytic modes on A. A.B. D.C. Clark A.B. Kunkel T.A. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar, J. Modrich P. Mol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, S. F. Li G.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). These have led to several that on near and support mismatch-provoked excision and repair. The excision system on MutSα, MutLα, RPA, and J. Modrich P. Mol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar), and similar results have been obtained in a system that also contains F. K. Li G.M. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). As in Fig. excision in this system is mismatch-provoked and mismatch of this reaction has several of the hydrolytic MutSα Exo1 on a 5′-heteroduplex in a and In the absence of 5′ to 3′ by Exo1 by a but MutSα the enzyme in of nucleotides to J. Modrich P. Mol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar), an to of a MutSα by the complex is in by RPA, which of the complex to nucleotides, and by to of Exo1 to 5′ termini in excision J. Modrich P. Mol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). Although an gap is a for MutSα Exo1 at such sites that the contains a The of these are on a of hydrolytic which in by about nucleotides, an to of MutSα and Exo1 J. Modrich P. Mol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, F. K. Li G.M. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). is mismatch because MutSα Exo1 at the gap in the excision thus has both and positive regulatory on this by of the complex and by hydrolytic activity on excision it of the system mismatch molecules to in the reaction. MutLα is not required for and activation of but it does play a significant role in excision. in with MutSα to Exo1 on DNA that a mispair, MutLα the mismatch of the reaction J. Modrich P. Mol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, A. PubMed Scopus Google Scholar). MutLα also participates in excision in this but two have been proposed to for its function in this et al. J. Modrich P. Mol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar) have MutLα involvement in to its role in Exo1 activity on In this mechanism MutLα excision products by et al. F. K. Li G.M. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar) have that MutLα, in with RPA, an role in excision mismatch This issue has not been MutSα, MutLα, and also support mismatch-provoked excision on a As in the of a on a 5′ to 3′ from the strand break (Fig. which is the polarity for mismatch (22Genschel J. Bazemore L.R. Modrich P. J. Biol. Chem. 2002; 277: 13302-13311Abstract Full Text Full Text PDF PubMed Scopus (203) Google Scholar, N. J. R.R. Modrich P. Mol. Full Text Full Text PDF PubMed Scopus (203) Google Scholar). The 5′ to 3′ of this system has been to as a polarity (2Iyer R.R. Pluciennik A. Burdett V. Modrich P.L. Chem. Rev. 2006; 106: 302-323Crossref PubMed Scopus (708) Google Scholar). Although has significant on the restricted of this with both and the A. M. Annu. Rev. Biochem. 2005; 74: PubMed Scopus Google a system that supports mismatch from both 5′ and N. J. R.R. Modrich P. Mol. Full Text Full Text PDF PubMed Scopus (203) Google Scholar). products obtained from a 5′-heteroduplex in this system are similar to produced by MutSα, MutLα, and However, when the nick is located 3′ to the Exo1 5′ to 3′ at the nick is largely by RFC, and excision with 3′ to 5′ polarity in mismatch Although excision in this system displays to the bidirectional reaction that has been in nuclear the of excision products in the system is more than that observed in extracts. This system one or more activities that play significant in mismatch repair N. J. R.R. Modrich P. Mol. Full Text Full Text PDF PubMed Scopus (203) Google Scholar). Because an Exo1 site to support both and excision in this mismatch in both was to this exonuclease N. J. R.R. Modrich P. Mol. Full Text Full Text PDF PubMed Scopus (203) Google Scholar). was suggested that a cryptic Exo1 3′ to 5′ hydrolytic function is responsible for excision. However, the for a 3′ to in this system was by the demonstration that MutSα, RFC, and a MutLα endonuclease in an and manner N. Modrich P. 2006; Full Text Full Text PDF PubMed Scopus (483) Google Scholar). As shown in Fig. by MutLα endonuclease on both 3′- and and is to the heteroduplexes with a nick mismatch separation of to occur on the of the mismatch to the strand break but at larger separation distances between the two DNA and P. In the of a to the mismatch provides an initiation site for mismatch by the 5′ to 3′ of Exo1 (Fig. as and modes of excision have been in nuclear extracts J. Modrich P. Mol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, S. F. Li G.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar), it is that this system also N. Modrich P. 2006; Full Text Full Text PDF PubMed Scopus (483) Google Scholar). The of of this system is of the mechanism for mismatch repair in Xenopus egg extracts proposed by Varlet et al. (10Varlet I. Canard B. Brooks P. Cerovic G. Radman M. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 10156-10161Crossref PubMed Scopus (25) Google Scholar). As discussed this that the nick that directs repair serves as a strand signal but not as a site for excision which at a strand break produced by a mismatch-activated This of excision is from that used by the E. coli where at a 3′ or 5′ strand break that directs repair (2Iyer R.R. Pluciennik A. Burdett V. Modrich P.L. Chem. Rev. 2006; 106: 302-323Crossref PubMed Scopus (708) Google Scholar). The site of the MutLα endonuclease has been to a site within the PMS2 subunit that is by a N. Modrich P. 2006; Full Text Full Text PDF PubMed Scopus (483) Google Scholar). mutations within this MutLα endonuclease activity as well as the of the protein to support mismatch repair in nuclear extracts. This is in of eukaryotic PMS2 and in and MutL but is in MutL from like E. coli that on d(GATC) methylation to direct mismatch repair. The or absence of this MutL may two distinct classes of mismatch repair that support mismatch correction have also been but these in several et al. F. K. Li G.M. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar) have shown that MutSα, MutLα, RPA, and DNA are sufficient to support repair of a mismatch or a ID mispair and that repair products are obtained of these with DNA I. As observed for excision J. Modrich P. Mol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar), MutLα is not required for repair in this system F. K. Li G.M. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). The functions of and in this reaction to be largely because either protein is sufficient to support repair, and excision in the of is by the of of MutSβ for MutSα a system that supports excision and repair of a ID that like MutSα, MutSβ surprising feature of this system is that repair is of and This is because the DNA synthesis of 5′-heteroduplex repair in nuclear extracts is S. H. Li G.M. Nucleic Acids Res. 1998; 26: PubMed Scopus Google Scholar, J. Modrich P. Mol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). in contrast to its activity on a this system does not support excision or repair when with and F. K. Li G.M. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). A repair system with has been described by et al. N. Modrich P. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). In contrast to the repair system described F. K. Li G.M. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar), this system supports bidirectional mismatch repair on MutSα, MutLα, RPA, DNA RFC, and N. Modrich P. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). MutLα is for 5′ repair in this system but required for repair. The and requirement for correction is because of involvement in the repair DNA synthesis both are also required for excision on a The results obtained by et al. as with of et al. N. Modrich P. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar) and et al. N. J. R.R. Modrich P. Mol. Full Text Full Text PDF PubMed Scopus (203) Google Scholar) have been to activity between the used F. K. Li G.M. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar, N. Modrich P. 2006; Full Text Full Text PDF PubMed Scopus (483) Google Scholar). et al. F. K. Li G.M. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar) human RFC, et al. N. Modrich P. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar) used human et al. N. J. R.R. Modrich P. Mol. Full Text Full Text PDF PubMed Scopus (203) Google Scholar) obtained similar results either human or yeast The recent of that support mismatch-provoked excision and repair of this reaction. However, because the described to are it is to that they for mismatch repair as it in the eukaryotic is evidence for of hydrolytic activities in to Exo1 (23Wei K. Clark A.B. Wong E. Kane M.F. Mazur D.J. Parris T. Kolas N.K. Russell R. Hou Jr., H. Kneitz B. Yang G. Kunkel T.A. Kolodner R.D. Cohen P.E. Edelmann W. Genes Dev. 2003; 17: 603-614Crossref PubMed Scopus (273) Google Scholar), but these have not been identified. that the of MutLα endonuclease and excision have also been N. J. R.R. Modrich P. Mol. Full Text Full Text PDF PubMed Scopus (203) Google Scholar, N. Modrich P. 2006; Full Text Full Text PDF PubMed Scopus (483) Google Scholar), but these have not been The feature of the replication error correction reaction is its strand-specific an that on the of a mismatch and strand break that be by of which to the of this have been thoroughly in the (1Kunkel T.A. Erie D.A. Annu. Rev. Biochem. 2005; 74: 681-710Crossref PubMed Scopus (1051) Google Scholar, 2Iyer R.R. Pluciennik A. Burdett V. Modrich P.L. Chem. Rev. 2006; 106: 302-323Crossref PubMed Scopus (708) Google Scholar, 3Jiricny J. Nat. Rev. Mol. Cell Biol. 2006; 7: 335-346Crossref PubMed Scopus (995) Google Scholar), but the mechanism responsible for between the two DNA sites has not been However, the recent finding that by MutLα endonuclease is to the strand N. Modrich P. 2006; Full Text Full Text PDF PubMed Scopus (483) Google Scholar) that of the mismatch and strand break may of a DNA The of events during the of nick-directed mismatch repair is presumably by the of that occur on the A number of have been in this and (1Kunkel T.A. Erie D.A. Annu. Rev. Biochem. 2005; 74: 681-710Crossref PubMed Scopus (1051) Google Scholar, 2Iyer R.R. Pluciennik A. Burdett V. Modrich P.L. Chem. Rev. 2006; 106: 302-323Crossref PubMed Scopus (708) Google Scholar, 3Jiricny J. Nat. Rev. Mol. Cell Biol. 2006; 7: 335-346Crossref PubMed Scopus (995) Google Scholar, A. M. Annu. Rev. Biochem. 2005; 74: PubMed Scopus Google Scholar). the of between MutSα and Exo1 and between and the of these in nick-directed mismatch repair has not been and Pluciennik for and