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Purification and identification of subunit structure of the human mitochondrial DNA polymerase
1Department of Biochemistry, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, Piscataway 08854.
Abstract:
The mitochondrial DNA polymerase of HeLa cells was purified 18,000-fold to near homogeneity. The purified polymerase cofractionated with two polypeptides that had molecular mass of 140 and 54 kDa. The 140-kDa subunit was specifically radiolabeled in a photoaffinity cross-linking assay and is most likely the catalytic subunit of the mitochondrial DNA polymerase. The purified enzyme exhibited properties that have been attributed to DNA polymerase gamma and shows a preference for replicating primed poly(pyrimidine) DNA templates in the presence of 0.5 mM MgCl2. As in the case of mitochondrial DNA polymerases from other animal cells, human DNA polymerase gamma cofractionated with a 3'----5' exonuclease activity. However, it has not been possible to determine if the two enzymatic activities reside in the same polypeptide. The exonuclease activity preferentially removes mismatched nucleotides from the 3' end of a duplex DNA and is not active toward DNA with matched 3' ends. These properties are consistent with the notion that the exonuclease activity plays a proofreading function in the replication of the organelle genome.
Insights
Researchers purified HeLa cell mitochondrial DNA polymerase, identifying a 140-kDa catalytic subunit and a 3'–5' exonuclease proofreading activity essential for organelle genome replication.
Area of Science:
- Molecular Biology
- Genetics
- Biochemistry
Background:
- Mitochondrial DNA (mtDNA) replication is crucial for cellular energy production.
- Understanding the enzymes involved in mtDNA replication, like DNA polymerase gamma, is key to studying mitochondrial function and disease.
Purpose of the Study:
- To purify and characterize the mitochondrial DNA polymerase from HeLa cells.
- To identify the catalytic subunit and associated enzymatic activities, such as proofreading exonuclease function.
Main Methods:
- Extensive purification of HeLa cell mitochondrial DNA polymerase (18,000-fold).
- Photoaffinity cross-linking assays to identify the catalytic subunit.
- Cofractionation analysis to detect associated enzymatic activities.
Main Results:
- Purified mitochondrial DNA polymerase consisted of 140-kDa and 54-kDa polypeptides; the 140-kDa subunit is likely catalytic.
- The enzyme exhibited properties of DNA polymerase gamma, preferring primed poly(pyrimidine) templates.
- A 3'–5' exonuclease activity, capable of removing mismatched nucleotides, was identified and cofractionated with the polymerase.
Conclusions:
- The purified enzyme represents human DNA polymerase gamma.
- The associated 3'–5' exonuclease activity likely functions as a proofreading mechanism during mtDNA replication.
- Further studies are needed to determine if polymerase and exonuclease activities reside on the same polypeptide.