Interactions of DNA Helicases with Damaged DNA: Possible Biological Consequences

nucleotide excision repair high mobility group human DNA helicase Herpes simplex virus translesion synthesis Werner's syndrome DNA helicases are a class of enzymes able to unwind the two complementary DNA strands in a reaction dependent on energy derived from nucleoside 5′-triphosphate hydrolysis. Their action is required in fundamental cellular processes such as DNA replication, repair, recombination, and transcription. From the known crystal structures it appears that the enzymatic machinery of DNA helicases has been highly conserved; nevertheless their functions can be distinguished by co-factor utilization, substrate preference, directionality of unwinding, processivity, and interaction with other proteins. These aspects of DNA helicase biochemistry have recently been reviewed (1Hall M.C. Matson S.W. Mol. Microbiol. 1999; 34: 867-877Crossref PubMed Scopus (274) Google Scholar, 2Chakraverty R.K. Hickson I.D. Bioessays. 1999; 21: 286-294Crossref PubMed Scopus (197) Google Scholar, 3Ellis N.A. Curr. Opin. Genet. Dev. 1997; 7: 354-363Crossref PubMed Scopus (115) Google Scholar, 4Lohman T.M. Bjornson K.P. Annu. Rev. Biochem. 1996; 65: 169-214Crossref PubMed Scopus (672) Google Scholar). Unrepaired DNA lesions can affect the outcome of DNA replication, repair, recombination, and transcription. Thus, because DNA helicases are among the first proteins that would encounter DNA damage during these processes, a complete understanding of the metabolism of damaged DNA requires a biochemical analysis of the interaction between helicases and lesions. Clearly, chemical or physical cross-links between opposite DNA strands (interstrand lesions) will represent a mechanical block to DNA helicase action. However,in vivo the functions of DNA helicases are also expected to be severely impaired by bulky, helix-distorting intrastrand DNA lesions like those produced by ultraviolet irradiation or by chemical agents such as cisplatin, acetylaminofluorene, and benzopyrene. (For a general description of DNA lesions, see Ref. 5Friedberg E.C. Walker G.C. Siede W. DNA Repair and Mutagenesis. American Society for Microbiology, Washington, D.C.1995: 1-58Google Scholar.). Although helicases unwind duplex DNA, most known helicases associate and translocate unidirectionally along one strand of the DNA. Accordingly, except for interstrand lesions, the position of a lesion may differentially influence the action of a DNA helicase, depending on whether it is located on the DNA strand along which the enzyme translocates or on the opposite one. In this minireview we will focus mainly on in vitro studies aimed at investigating the effects of helix-distorting DNA lesions on the action of DNA helicases implicated in DNA repair, recombination, and replication; the possible biological consequences of these interactions will also be discussed. To facilitate organization of this review, helicases have been categorized depending on whether they function in repair, recombination, or replication. However, the reader should be aware that there is substantial overlap between these processes. To overcome the deleterious effects of DNA lesions, a variety of DNA repair systems is present in all living organisms (5Friedberg E.C. Walker G.C. Siede W. DNA Repair and Mutagenesis. American Society for Microbiology, Washington, D.C.1995: 1-58Google Scholar). Nucleotide excision repair (NER)1 is one of the most versatile systems and shows remarkable similarities in both prokaryotes and eukaryotes (for review see Refs. 6Batty D.P. Wood R.D. Gene ( Amst. ). 2000; 241: 193-204Crossref PubMed Scopus (252) Google Scholar and 7Petit C. Sancar A. Biochimie ( Paris ). 1999; 81: 15-25Crossref PubMed Scopus (200) Google Scholar). Among the six proteins necessary for NER in Escherichia coli, the UvrA2B protein complex and the UvrD protein have been shown to possess DNA helicase activities. The 5′ to 3′ helicase activity of UvrA2B is thought to allow limited ATP-dependent scanning of DNA to detect damaged bases. For example, UV-induced lesions have been shown to inhibit translocation of UvrA2B, suggesting that arrest of the helicase may be a signal for repair (8Oh E.Y. Grossman L. J. Biol. Chem. 1989; 264: 1336-1343Abstract Full Text PDF PubMed Google Scholar). Subsequently, a preincision complex is formed, and incision occurs on both sides of the lesion. UvrD then loads onto the free 3′ end of the damaged oligonucleotide and displaces it using its intrinsic 3′ to 5′ helicase activity (6Batty D.P. Wood R.D. Gene ( Amst. ). 2000; 241: 193-204Crossref PubMed Scopus (252) Google Scholar). The recent determination of the crystal structure of UvrB suggests that formation of the preincision complex requires unwinding of the DNA by UvrA2B (9Theis K. Chen P.J. Skorvaga M. Van Houten B. Kisker C. EMBO J. 1999; 18: 6899-6907Crossref PubMed Scopus (165) Google Scholar). It was postulated that stalling of the UvrA2B complex upon encountering a lesion triggers dissociation of the complex. In another study (10Gordienko I. Rupp D.W. EMBO J. 1997; 16: 889-895Crossref PubMed Scopus (37) Google Scholar), it was found that the presence of the 2-(acetylamino)fluorene lesion in either DNA strand stimulated the activity of UvrA2B, leading the authors to suggest that the strand-separating activity of UvrA2B may not play a major role in lesion recognition. The function of UvrD helicase (helicase II) in E. coli NER is in the excision of damage-containing oligonucleotides. The helicase activity of UvrD was found to be only moderately sensitive to ultraviolet radiation damage and intrastrand cisplatin adducts, whereas it was exquisitively sensitive to intercalators that position in the major groove of DNA (11Naegeli H. Modrich P. Friedberg E. J. Biol. Chem. 1993; 268: 10386-10393Abstract Full Text PDF PubMed Google Scholar, 12George J.W. Ghate S. Matson S.W. Besterman J.M. J. Biol. Chem. 1992; 267: 10683-10689Abstract Full Text PDF PubMed Google Scholar). However, in one study (11Naegeli H. Modrich P. Friedberg E. J. Biol. Chem. 1993; 268: 10386-10393Abstract Full Text PDF PubMed Google Scholar) but not in the other (12George J.W. Ghate S. Matson S.W. Besterman J.M. J. Biol. Chem. 1992; 267: 10683-10689Abstract Full Text PDF PubMed Google Scholar), UvrD helicase action was found to be affected by distamycin A, an intercalator binding to the minor grove of DNA. Generally speaking, it is possible that UV-induced lesions or other helix-distorting intrastrand adducts are better tolerated by DNA helicases than those produced by intercalating agents, which can either totally disrupt the continuity of what a helicase normally recognizes or increase the thermodynamic stability of the DNA duplex in such a way that it resists helicase action (13Banville D.L. Keniry M.A. Shafer R.H. Biochemistry. 1990; 29: 9294-9304Crossref PubMed Scopus (50) Google Scholar). In the same study (11Naegeli H. Modrich P. Friedberg E. J. Biol. Chem. 1993; 268: 10386-10393Abstract Full Text PDF PubMed Google Scholar), the activity of UvrD helicase on damaged DNA was compared with the product of the yeast Saccharomyces cerevisiae rad3gene, a helicase that is absolutely required for yeast NER. The 5′ to 3′ helicase activity of the Rad3 protein was found to be profoundly inhibited by UV damage and intrastrand cisplatin adducts located on the strand along which the enzyme translocates, whereas lesions on the opposite strand had no effect. Blockage of Rad3 by a lesion results in the formation of an abnormally stable protein-DNA complex (14Naegeli H. Bardwell L. Friedberg E. J. Biol. Chem. 1992; 267: 392-398Abstract Full Text PDF PubMed Google Scholar). These results indicate that the DNA helicase activity of Rad3 protein may have been adopted by the yeast NER machinery to locate and determine the strand specificity of DNA damage, thus establishing some functional relationship with E. coliUvrA2B. It has long been known that the recognition-incision step of NER in human cells requires ATP (15Calsou P. Salles B. Nucleic Acids Res. 1994; 22: 4937-4942Crossref PubMed Scopus (31) Google Scholar), and the recent reconstitution of NER with recombinant human proteins now provides an interesting tool to study the role of the different factors involved in this process (16Araujo S.J. Tirode F. Coin F. Pospiech H. Syvaoja J.E. Stucki M. Hubscher U. Egly J.M. Wood R. Genes Dev. 2000; 14: 349-359Crossref PubMed Google Scholar). Eukaryotic TFIIH is a multisubunit protein complex involved in RNA polymerase II transcription and nucleotide excision repair, which contains two demonstrated DNA helicases, named Rad3 and Rad25 in yeast and XPB and XPD in mammals (17Eisen A. Lucchesi J.C. Bioessays. 1998; 20: 634-641Crossref PubMed Scopus (52) Google Scholar). Interestingly, TFIIH containing a mutant XPD with impaired DNA helicase activity was shown to be defective in incisions around the lesion (18Winkler S.G. Araujo S.J. Fiedler U. Wermulen W. Coin F. Egly J.M. Hoeijmakers J.J. Wood R. Timmers H.T. Weeda G. J. Biol. Chem. 2000; 275: 4258-4266Abstract Full Text Full Text PDF PubMed Scopus (148) Google Scholar). Consistent with the role of the yeast homologue of XPD, Rad3, these results support previous models that suggest a role for the DNA helicases of TFIIH in the identification of the damaged strand and the correct localization of the incisions (6Batty D.P. Wood R.D. Gene ( Amst. ). 2000; 241: 193-204Crossref PubMed Scopus (252) Google Scholar, 19Nageli H.P. FASEB J. 1995; 9: 1043-1050Crossref PubMed Scopus (34) Google Scholar). The strand-specific stalling of either the 5′ to 3′ XPD helicase or the 3′ to 5′ XPB helicase could result in the distortion of the DNA-TFIIH complex, leading to the identification of the damaged strand and to the formation of a fully opened preincision complex. A schematic representation of the putative roles of complexes containing prokaryotic UvrA2B and eukaryotic TFIIH in DNA damage recognition is presented in Fig. 1. Calf thymus DNA helicase E is a moderately processive 3′ to 5′ helicase active on nicked DNA that has been proposed to play a role in DNA repair (20Turchi J.J. Murante R.S. Bambara R.A. Nucleic Acids Res. 1992; 20: 6075-6080Crossref PubMed Scopus (12) Google Scholar). In vitro, helicase E can displace a primer containing an internal GpG intrastrand cross-link, the major DNA lesion generated by the antitumor drug cisplatin (Pt-d(GpG)). If the lesion was placed on the strand to which it binds, helicase E was inhibited only if the adduct was in the single strand part of the template but not if it was within the annealed region, suggesting that, in the latter case, it did not represent an absolute stop to helicase progression (21Huang L. Turchi J.J. Wahl A.F. Bambara R.A. J. Biol. Chem. 1993; 268: 26731-26737Abstract Full Text PDF PubMed Google Scholar). Subsequent work has shown that a protein which recognizes and binds cisplatin-damaged DNA, the high mobility group 1 protein (HMG-1), reduced DNA unwinding of a platinated substrate by helicase E (22Patrick S.M. Henkels K.M. Turchi J.J. Biochim. Biophys. Acta. 1997; 1354: 279-290Crossref PubMed Scopus (12) Google Scholar). HMG-1 protein has also been reported to inhibit NER of cisplatin lesions catalyzed by either cell extract or purified proteins by shielding the lesions from repair proteins (23Zamble D.B. Mu J.T. Reardon A. Sancar J.S. Lippard S.J Biochemistry. 1996; 35: 10004-10013Crossref PubMed Scopus (311) Google Scholar). DNA recombination assures exchange of information between DNA chains and contributes to the repair of DNA damage and to restart DNA replication (for review see Refs. 24Cox M. Prog. Nucleic Acid Res. Mol. Biol. 2000; 63: 311-366Crossref Google Scholar, 25Paques F. Haber J.A. Microbiol. Mol. Biol. Rev. 1999; 63: 349-404Crossref PubMed Google Scholar, 26Baumann P. West S.C. Trends Biochem. Sci. 1998; 23: 247-251Abstract Full Text Full Text PDF PubMed Scopus (463) Google Scholar). Double strand DNA break repair by homologous recombination in E. coli is initiated by the RecBCD enzyme, which unwinds and simultaneously degrades DNA from a double-stranded DNA end (27Anderson D.G. Kowalczykowski S.C. Cell. 1997; 90: 77-86Abstract Full Text Full Text PDF PubMed Scopus (257) Google Scholar). Early studies on RecBCD enzyme showed that the enzyme could translocate through DNA duplexes that contain UV adducts, although, as expected, progression of the enzyme was arrested by interstrand lesions (28Karu A.E. Linn S. Proc. Natl. Acad. Sci. U. S. A. 1972; 10: 2855-2859Crossref Scopus (30) Google Scholar). Of the three subunits, RecB is the sole one that possesses 3′ to 5′ helicase activity (29Boehmer P.E. Emerson P.T. J. Biol. Chem. 1992; 267: 4981-4987Abstract Full Text PDF PubMed Google Scholar). The effect of a Pt-d(GpG) adduct, in the context of partially duplex synthetic oligonucleotides, on the helicase activity of RecB was examined in vitro. Both strand-separating and DNA-dependent ATPase activities of the RecB protein were inhibited by the adduct when located on the template strand (30Villani G. Cazaux C. Pillaire M. Boehmer P.E. FEBS Lett. 1993; 333: 89-95Crossref PubMed Scopus (15) Google Scholar). Although further studies are required, these results suggest that subunit interactions within RecBCD enzyme allow the enzyme to traverse regions of DNA damage whereas RecB alone is inhibited. Another protein that plays a key role in recombination in E. coli is RecA protein (24Cox M. Prog. Nucleic Acid Res. Mol. Biol. 2000; 63: 311-366Crossref Google Scholar). RecA catalyzes DNA strand exchange in vitroand has been shown to possess ATP-dependent DNA unwinding activity that is limited to the unwinding of short duplexes (31Bianchi M. Riboli B. Magni G. EMBO J. 1985; 4: 3025-3030Crossref PubMed Scopus (25) Google Scholar). The capacity of RecA to unwind a damaged template identical to the one used for the RecB helicase was examined in the same study. Contrary to RecB, it was found that neither the strand-separating nor the ATPase activities of RecA were inhibited by the Pt-d(GpG) adduct (30Villani G. Cazaux C. Pillaire M. Boehmer P.E. FEBS Lett. 1993; 333: 89-95Crossref PubMed Scopus (15) Google Scholar). Previous work had shown that RecA protein was able to bypass UV pyrimidine dimers during strand exchange in vitro (32Livneh Z. Lehman I.R. Proc. Natl. Acad. Sci. U. S. A. 1982; 79: 3171-3175Crossref PubMed Scopus (37) Google Scholar). Ku autoantigen is a DNA end binding protein that has been shown to act as a regulator of a DNA-dependent protein kinase (DNA-PK), which plays essential roles in DNA repair and recombination (for review see Ref. 33Smith G. Jackson S.P. Genes Dev. 1999; 13: 916-934Crossref PubMed Scopus (769) Google Scholar). Ku protein has been identified as human DNA helicase II (HDH II), an enzyme that preferentially unwinds partially duplex DNA proceeding in the 3′ to 5′ direction (34Tuteja N. Tuteja R. Ochem A. Taneja P. Huang N.W. Simoncsits A. Susic S. Rahman K. Marusic L. Chen J. Zhang J. Wang S. Pongor S. Falaschi A. EMBO J. 1994; 13: 4991-5001Crossref PubMed Scopus (217) Google Scholar). It was found that intercalating agents such as actinomycin, daunorubicin, or nogalamycin severely inhibited the unwinding activity of the HDH II/Ku enzyme (35Tuteja N. Phan T. Tuteja R. Ochem A. Falaschi A. Biochem. Cell Biol. 1997; 236: 636-640Google Scholar). The familiar structure of a replication fork, a site where the two strands of a duplex DNA are separated to reveal the single strands of opposite polarity, is generated through the action of replicative DNA helicases (36Baker T.A. Bell S.P. Cell. 1998; 92: 295-305Abstract Full Text Full Text PDF PubMed Scopus (274) Google Scholar). In eukaryotes, although many helicases have been characterized biochemically, their precise roles in vivoremain difficult to establish. Thus, despite the existence of several candidates, no eukaryotic DNA helicase has yet been unambiguously shown to be essential for the progression of the replication fork during chromosomal replication (37Liu Q. Choe W. Campbell J. J. Biol. Chem. 2000; 275: 1615-1624Abstract Full Text Full Text PDF PubMed Scopus (41) Google Scholar, 38You Z. Komamura Y. Ishimi Y. Mol. Cell. Biol. 1999; 19: 8003-8015Crossref PubMed Scopus (171) Google Scholar, 39Yan H. Chen C.Y. Kobayshi R. Newport J. Nat. Genet. 1998; 19: 375-378Crossref PubMed Scopus (129) Google Scholar). Such is not the case for bacteria and related phages or for eukaryotic viruses, where replicative DNA helicases have been identified and characterized (40Kornberg A. Baker T.A. DNA Replication. 2nd Ed. W. H. Freeman and Co., New York1992: 355-378Google Scholar). T7 gene 4 protein acts both as a primase and a helicase. The gene 4 protein has been shown to translocate along single-stranded DNA in the 5′ to 3′ direction, and this movement was found to be arrested in vitro by bulky DNA adducts formed by the chemical carcinogen benzo[a]pyrene. The inhibitory effects of these adducts are strand-specific in that they blocked the DNA helicase activity of gene 4 protein only if they are located on the strand along which the helicase translocates. In addition, the data presented indicate that gene 4 protein was sequestered at the site of the adduct (41Brown W.C. Romano L.J. J. Biol. Chem. 1989; 264: 6748-6754Abstract Full Text PDF PubMed Google Scholar, 42Yong Y. Romano L.J. Chem. Res. Toxicol. 1996; 9: 179-187Crossref PubMed Scopus (20) Google Scholar). InHerpes simplex virus type 1 (HSV-1) DNA replication, two helicases are required for viral origin-specific DNA replication (43Boehmer P.E. Lehman I.R. Annu. Rev. Biochem. 1997; 66: 347-384Crossref PubMed Scopus (268) Google Scholar). The first of the two helicases is the product of the UL9 gene, a 3′-5′ helicase that together with the viral single-stranded DNA binding protein, ICP8, can specifically unwind the HSV-1 origins of replication (44Lehman I.R. Boehmer P.E. J. Biol. Chem. 1999; 274: 28059-28062Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar). It was found that a Pt-d(GpG) adduct significantly reduced, but did not abolish, the helicase activity of the UL9 protein but only when it was present on the strand along which the protein translocated (45Villani G. Pillaire M.J. Boehmer P.E. J. Biol. Chem. 1994; 269: 21676-21681Abstract Full Text PDF PubMed Google Scholar). However, addition of ICP8 greatly stimulated the capacity of the helicase to unwind platinated DNA. Furthermore, the stimulation appeared to be the result of the functional and physical interaction that is known to exist between UL9 and ICP8 and not because of a preferential binding of ICP8 at the site of the adduct. Results from a subsequent study showed that ICP8 stimulated the DNA helicase activity of UL9 protein by increasing its processivity, thus facilitating its translocation along DNA and through regions of secondary structure (46Boehmer P.E. J. Biol. Chem. 1998; 273: 2676-2683Abstract Full Text Full Text PDF PubMed Scopus (36) Google Scholar). Based on the finding of this study it is tempting to speculate that ICP8 enables the UL9 protein to bypass the cisplatin Pt-d(GpG) lesion by tethering it to the DNA substrate, thereby preventing its dissociation. The second HSV-1 replicative DNA helicase is the product of the UL5, UL8, and UL52 genes. This heterotrimeric 5′ to 3′ helicase is also endowed with DNA-primase activity and is responsible for concomitant DNA unwinding and primer synthesis at the viral replication fork (44Lehman I.R. Boehmer P.E. J. Biol. Chem. 1999; 274: 28059-28062Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar). Addition of ICP8 specifically stimulated unwinding of platinated DNA by the helicase-primase, but at variance to what was found for UL9, stimulation by ICP8 appeared to enable bypass of the cisplatin intrastrand cross-link by recruiting the enzyme to the DNA rather than by increasing its processivity (47Tanguy Le Gac N. Villani G. Boehmer P.E. J. Biol. Chem. 1998; 273: 13801-13807Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar). Thus, these studies (45Villani G. Pillaire M.J. Boehmer P.E. J. Biol. Chem. 1994; 269: 21676-21681Abstract Full Text PDF PubMed Google Scholar, 47Tanguy Le Gac N. Villani G. Boehmer P.E. J. Biol. Chem. 1998; 273: 13801-13807Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar) suggest that specific protein-protein interactions between a single-stranded DNA binding protein and two replicative DNA helicases allow substantial unwinding of substrates containing a bulky intrastrand DNA lesion. In the case of simian virus 40 (SV40), the only viral protein required for viral replication is large T antigen. This protein has an intrinsic 3′ to 5′ DNA helicase activity and binds specifically to the origin, allowing it to initiate replication and functions as the replicative helicase at the fork (48Fanning E. Knippers R. Annu. Rev. Biochem. 1992; 61: 55-85Crossref PubMed Scopus (458) Google Scholar). The impact of intrastrand DNA lesions on the unwinding activity of SV40 T antigen protein was recently examinedin vitro (49Veaute X. Mari-Giglia G. Lawrence C.W. Sarasin A. Mutat. Res. 2000; 459: 19-28Crossref PubMed Scopus (17) Google Scholar). Using synthetic forklike substrates containing either single UV photoproducts or a specific 2-(acetylamino)fluorene adduct, it was shown that T antigen helicase activity was not affected by the presence of the lesions on either the strand on which the enzyme translocates or on the opposite one. The capacity of T antigen to displace UV-irradiated DNA was also observed in a previous work (50Cough G. Wood R. Mutat. Res. 1989; 227: 193-197Crossref PubMed Scopus (8) Google Scholar). However, T antigen helicase was found to be blocked by DNA-intercalating drugs in vitro (51Bachur N.R. Jonhson R., F. R. N. L. Mol. 1993; Google Scholar). In intrastrand DNA lesions to affect DNA helicase activity in vitro only when placed on the strand to which the protein translocates. of the DNA helicases act as Trends Biochem. Sci. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar). helicases in to exist as A for helicase action has been proposed where one strand of the DNA through the of the helicase the second strand is the X. Nat. Biol. 1996; PubMed Scopus (115) Google Scholar). This is with the observed strand specificity if one that bulky DNA lesion will have an inhibitory role when placed on the strand by the helicase and no effect on the strand the as recently J. Biol. 1998; PubMed Scopus Google Scholar). However, in of this the unwinding activity of proteins of such as SV40 T antigen or not to be impaired in vitro by intrastrand DNA lesions on either strand (30Villani G. Cazaux C. Pillaire M. Boehmer P.E. FEBS Lett. 1993; 333: 89-95Crossref PubMed Scopus (15) Google Scholar, X. Mari-Giglia G. Lawrence C.W. Sarasin A. Mutat. Res. 2000; 459: 19-28Crossref PubMed Scopus (17) Google Scholar). the capacity of these proteins to large complexes can to their to unwind a lesion. HSV-1 UL9 helicase, action has been shown to be but not blocked by a cisplatin lesion (45Villani G. Pillaire M.J. Boehmer P.E. J. Biol. Chem. 1994; 269: 21676-21681Abstract Full Text PDF PubMed Google Scholar), may also to this UL9 helicase action appears to be a DNA-dependent of UL9 protein complex (43Boehmer P.E. Lehman I.R. Annu. Rev. Biochem. 1997; 66: 347-384Crossref PubMed Scopus (268) Google Scholar). The leading to DNA helicase by bulky DNA lesions have been in some It was found that S. cerevisiae Rad3 and T7 gene 4 helicases were sequestered on single-stranded DNA at the of damage, stable protein-DNA complexes (14Naegeli H. Bardwell L. Friedberg E. J. Biol. Chem. 1992; 267: 392-398Abstract Full Text PDF PubMed Google Scholar, W.C. Romano L.J. J. Biol. Chem. 1989; 264: 6748-6754Abstract Full Text PDF PubMed Google Scholar), whereas thymus DNA helicase E was not (21Huang L. Turchi J.J. Wahl A.F. Bambara R.A. J. Biol. Chem. 1993; 268: 26731-26737Abstract Full Text PDF PubMed Google Scholar). or not these vitro are the of different roles by enzymes in vivo to be HSV-1 ICP8 has been shown to with the HSV-1 UL9 helicase and with the subunit of the HSV-1 (43Boehmer P.E. Lehman I.R. Annu. Rev. Biochem. 1997; 66: 347-384Crossref PubMed Scopus (268) Google Scholar). ICP8 was found to specifically unwinding of platinated DNA substrates by both helicases, although the of stimulation to be different for the two enzymes (45Villani G. Pillaire M.J. Boehmer P.E. J. Biol. Chem. 1994; 269: 21676-21681Abstract Full Text PDF PubMed Google Scholar, 47Tanguy Le Gac N. Villani G. Boehmer P.E. J. Biol. Chem. 1998; 273: 13801-13807Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar). The effect of ICP8 appeared to be the result of its interaction with the helicases and not a of its preferential binding to the cisplatin adduct the addition of the HMG-1 protein, which binds the Pt-d(GpG) adducts, was found to inhibit the activity of thymus helicase E (22Patrick S.M. Henkels K.M. Turchi J.J. Biochim. Biophys. Acta. 1997; 1354: 279-290Crossref PubMed Scopus (12) Google Scholar). These data the interaction of helicases with damaged DNA can be by other proteins.