Mitochondrial DNA ligase III function is independent of Xrcc1.
Hamster EM9 cells, which lack Xrcc1 protein, have reduced levels of DNA ligase III and are defective in nuclear base excision repair. The Xrcc1 protein stabilizes DNA ligase III and may even play a direct role in catalyzing base excision repair. Since DNA ligase III is also thought to function in mitochondrial base excision repair, it seemed likely that mitochondrial DNA ligase III function would also be dependent upon Xrcc1. However, several lines of evidence indicate that this is not the case. First, western blot analysis failed to detect Xrcc1 protein in mitochondrial extracts. Second, DNA ligase III levels present in mitochondrial protein extracts from EM9 cells were indistinguishable from those seen in similar extracts from wild-type (AA8) cells. Third, the mitochondrial DNA content of both cell lines was identical. Fourth, EM9 cells displayed no defect in their ability to repair spontaneous mitochondrial DNA damage. Fifth, while EM9 cells were far more sensitive to the cytotoxic effects of ionizing radiation due to a defect in nuclear DNA repair, there was no apparent difference in the ability of EM9 and AA8 cells to restore their mitochondrial DNA to pre-irradiation levels. Thus, mitochondrial DNA ligase III function is independent of the Xrcc1 protein.
Lakshmipathy
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University of Minnesota Medical School, Department of Pharmacology, 6-120 Jackson Hall, 321 Church Street SE, Minneapolis, MN 55455, USA.
Campbell
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D000818:Animals; D015139:Blotting, Southern; D015153:Blotting, Western; D002457:Cell Extracts / Q000737:chemistry; D002460:Cell Line; D002467:Cell Nucleus / Q000235:genetics / Q000528:radiation effects; D006224:Cricetinae; D004249:DNA Damage / Q000235:genetics / Q000528:radiation effects; D000072481:DNA Ligase ATP; D011088:DNA Ligases / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D004260:DNA Repair / Q000235:genetics / Q000528:radiation effects; D004272:DNA, Mitochondrial / Q000235:genetics / Q000528:radiation effects; D004268:DNA-Binding Proteins / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D004307:Dose-Response Relationship, Radiation; D005720:Gamma Rays; D007700:Kinetics; D008928:Mitochondria / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism / Q000528:radiation effects; D009154:Mutation / Q000235:genetics; D009687:Nuclear Proteins / Q000032:analysis; D000075223:Poly-ADP-Ribose Binding Proteins; D011836:Radiation Tolerance; D000076105:X-ray Repair Cross Complementing Protein 1; D029867:Xenopus Proteins
D002457:Cell Extracts; D004272:DNA, Mitochondrial; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; D000075223:Poly-ADP-Ribose Binding Proteins; D000076105:X-ray Repair Cross Complementing Protein 1; D029867:Xenopus Proteins; D011088:DNA Ligases; D000072481:DNA Ligase ATP; C000618816:DNA ligase III alpha protein, Xenopus