DNA ligase
As mentioned above, DNA ligase III-alpha mRNA encodes nuclear and mitochondrial versions of DNA ligase III-alpha. Nuclear DNA ligase III-alpha exists and functions in a stable complex with the DNA repair protein XRCC1. These proteins interact via their C-terminal BRCT domains. XRCC1 has no enzymatic activity but instead appears to acts as a scaffold protein by interacting with a large number of proteins involved in base excision and single-strand break repair. The participation of XRCC1 in these pathways is consistent with the phenotype of xrcc1 cells. In contrast to nuclear DNA ligase III-alpha, mitochondrial DNA ligase III-alpha functions independently of XRCC1, which is not found in mitochondria. It appears that nuclear DNA ligase III-alpha forms a complex with XRCC1 in the cytoplasm and the subsequent nuclear targeting of the resultant complex is directed by the XRCC1 nuclear localization signal. While mitochondrial DNA ligase III-alpha also interacts with XRCC1, it is likely that the activity of the mitochondrial targeting sequence of DNA ligase III-alpha is greater than the activity of the XRCC1 nuclear localization signal and that the DNA ligase III-alpha/XRCC1 complex is disrupted when mitochondrial DNA ligase III-alpha passes through the mitochondrial membrane.