Crystal structure and deletion analysis show that the accessory subunit of mammalian DNA polymerase γ, PolγB, functions as a homodimer

Polymerase gamma, which replicates and repairs mitochondrial DNA, requires the Pol gammaB subunit for processivity. We determined the crystal structure of mouse Pol gammaB, a core component of the mitochondrial replication machinery. Pol gammaB shows high similarity to glycyl-tRNA synthetase and dimerizes through an unusual intermolecular four-helix bundle. A human Pol gammaB mutant lacking the four-helix bundle failed to dimerize in solution or to stimulate the catalytic subunit Pol gammaA, but retained the ability to bind with Pol gammaA to a primer-template construct, indicating that the functional holoenzyme contains two Pol gammaB molecules. Other mutants retained stimulatory activity but lost the ability to bind folded ssDNA. These results suggest that the Pol gammaB dimer contains distinct sites for Pol gammaA binding, dimerization, and DNA binding.