Structural Insight into Processive Human Mitochondrial DNA Synthesis and Disease-Related Polymerase Mutations

Human mitochondrial DNA polymerase (Pol γ) is the sole replicase in mitochondria. Pol γ is vulnerable to nonselective antiretroviral drugs and is increasingly associated with mutations found in patients with mitochondriopathies. We determined crystal structures of the human heterotrimeric Pol γ holoenzyme and, separately, a variant of its processivity factor, Pol γB. The holoenzyme structure reveals an unexpected assembly of the mitochondrial DNA replicase where the catalytic subunit Pol γA interacts with its processivity factor primarily via a domain that is absent in all other DNA polymerases. This domain provides a structural module for supporting both the intrinsic processivity of the catalytic subunit alone and the enhanced processivity of holoenzyme. The Pol γ structure also provides a context for interpreting the phenotypes of disease-related mutations in the polymerase and establishes a foundation for understanding the molecular basis of toxicity of anti-retroviral drugs targeting HIV reverse transcriptase.