Mitochondria contain a single DNA polymerase, pol γ, responsible for replication and repair of mtDNA. Human pol γ is isolated from mitochondria as a complex containing a catalytic subunit, pol γA, of 139 kDa and an accessory subunit, pol γB, of 53 kDa. The accessory factor is itself a homodimer with remarkable structural similarity to prokaryotic tRNA synthetases (1). Our studies using gel filtration and isothermal calorimetry show that that the functional human holoenzyme contains two subunits of the processivity factor and one catalytic subunit, thereby forming a heterotrimer (3). Pol γ is mutated in individuals with genetic disorders such as chronic external opthalmoplegia and Alpers syndrome, it is an adventitious target for toxic side effects of several antiviral compounds and mutation of its proofreading exonuclease leads to accelerated aging in mouse models (2).

We used electron microscopy to examine the structure of the human DNA pol γ heterotrimer. Separate analysis of negatively stained preparations of the catalytic subunit, pol γA, and of the holoenzyme including the dimeric accessory factor, pol γB₂, permitted unambiguous identification of the position of the accessory factor within the holoenzyme. One pol γB₂ subunit dominates contacts with the catalytic subunit while the second B subunit is largely exposed to solvent. These EM studies of pol γ provide a clear view of residues in the accessory subunit that interact with the catalytic subunit and suggest a model for the interaction of the enzyme with DNA.

References and Footnotes

1. Carrodeguas, J. A., Theis, K., Bogenhagen, D. F., and Kisker, C. Mol Cell 7, 43-54 (2001).
2. Trifunovic, A., Wredenberg, A., Falkenberg, M., Spelbrink, J., Rovio, A., Bruder, C., Bohlooly-Y, M., Gidlof, S., Oldfors, A., Wibom, R., et al. Nature 429, 417-423 (2004).
3. Yakubovskaya, E., Chen, Z., Carrodeguas, J. A., Kisker, C., and Bogenhagen, D. F. J Biol Chem 281, 374-382 (2006).