Structural and Single-Molecule Insights into the Core Human Mitochondrial DNA Replisome

Replication of human mitochondrial DNA (mtDNA) is essential for the maintenance of oxidative phosphorylation and cellular energy homeostasis. Impairment of this process leads to mtDNA deletions, depletion, and point mutations that underlie a broad spectrum of mitochondrial diseases, as well as contributing to neurodegeneration, aging, and cancer. The core human mitochondrial replisome, composed of DNA polymerase γ (Polγ), the replicative helicase Twinkle, and the mitochondrial single-stranded DNA-binding protein (mtSSB), is the main complex responsible for replicating the mitochondrial genome through a highly coordinated yet still incompletely understood mechanism. Mutations in the nuclear genes encoding these proteins represent the most common cause of inherited disorders affecting mtDNA maintenance, underscoring the importance of understanding their coordinated molecular function. Recent advances in cryo-electron microscopy and single-molecule approaches have provided unprecedented insight into the structural organization and dynamic operation of the core components of the mitochondrial replisome. These complementary methods are establishing a quantitative mechanistic framework for understanding how the mitochondrial replisome initiates, progresses, and regulates the replication of the light and heavy strands of mtDNA. In the present review, we integrate recent structural and single-molecule findings to describe the mechanisms governing the activity of Polγ, Twinkle, and mtSSB at the mitochondrial replication fork, and discuss remaining challenges toward reconstructing a complete mechanistic model of human mtDNA replication.