Human Exonuclease 5 Is a Novel Sliding Exonuclease Required for Genome Stability

Abstract Previously, we characterized S. cerevisiae exonuclease 5 (EXO5) which is required for mitochondrial genome maintenance. Here, we identify the human homologue (C1orf176, EXO5) that functions in the repair of nuclear DNA damage. Human Exo5 (hExo5) contains an iron-sulfur cluster. It is a single-stranded DNA (ssDNA) specific bidirectional exonuclease with a preference for 5′-ends. After loading at a ssDNA end, Exo5 slides extensively along the ssDNA prior to cutting, hence the designation: sliding exonuclease. However, human single-stranded binding protein (hRPA) restricts sliding and enforces a unique, species-specific 5'-directionality onto hExo5. This specificity is lost with a mutant form of hRPA (hRPA-t11) that fails to interact with hExo5. hExo5 localizes to nuclear repair foci in response to DNA damage, and its depletion in human cells leads to an increased sensitivity to DNA damaging agents, in particular interstrand cross linking-inducing agents. Depletion of hExo5 also results in an increase in spontaneous and damage-induced chromosome abnormalities, including the frequency of triradial chromosomes, suggesting an additional defect in the resolution of stalled DNA replication forks in hExo5-depleted cells.