NADH:Ubiquinone Oxidoreductase (Complex I) is the largest of the mitochondrial respiratory chain complexes, consisting of 44 subunits encoded by both mitochondrial and nuclear DNA. The accessory subunit NDUFA13 also known as GRIM-19 is not only involved in complex I biogenesis, but also has been reported to be a negative regulator of STAT3, a nuclear transcription factor that is also present in mitochondria and thought to interact with the mitochondrial apoptosis machinery. In some human tumors NDUFA13 is suppressed or mutated, leading to constitutive STAT3 activity and indicating the tumor-suppressive function of NDUFA13. However, the molecular mechanisms of how NDUFA13 fulfils both roles remain elusive. Earlier attempts to generate NDUFA13 knock-out mice were unsuccessful and demonstrated that loss of NDUFA13 is embryonically lethal.
To investigate the molecular mechanisms of NDUFA13, a human cell line harbouring disruption to this gene was generated using ‘transcription activator-like effector nucleases’ (TALENs). Analysis of this cell line by Blue-native (BN) PAGE revealed a severe complex I assembly defect, indicating NDUFA13 is critical at an early stage of complex I biogenesis. Pulse-chase analysis of mitochondrial encoded subunits, as well as quantitative mass spectrometry (SILAC-MS) also demonstrated NDUFA13 to be important at an early stage of complex I assembly. Furthermore, Fluorescence Activated Cell Sorting (FACS) analysis of cells lacking NDUFA13 showed increased resistance to cell death induction possibly through activation of STAT3. This work has revealed the importance of NDUFA13 in complex I biogenesis and apoptosis, further indicating its unique role in life and death.