Pathogenic mutations inNUBPLaffect complex I activity and cold tolerance in the yeast modelYarrowia lipolytica

Complex I deficiency is a common cause of mitochondrial disease, resulting from mutations in genes encoding structural subunits, assembly factors or defects in mitochondrial gene expression. Advances in genetic diagnostics and sequencing have led to identification of several variants in NUBPL, an assembly factor of complex I, which are potentially pathogenic. To help assign pathogenicity and learn more about the function of NUBPL, amino acid substitutions were recreated in the homologous Ind1 protein of the yeast model Yarrowia lipolytica . L102P destabilized the Ind1 protein, leading to a null-mutant phenotype. D103Y, L191F and G285C affected complex I assembly to varying degrees, whereas the G138D variant did not impact on complex I levels or dNADH:ubiquinone activity. Blue-native PAGE and immunolabelling of the structural subunits NUBM and NUCM revealed that all Ind1 variants accumulated a Q-module intermediate of complex I. In the D103Y variant the matrix arm intermediate was virtually absent, indicating a dominant effect. Dysfunction of Ind1, but not absence of complex I, rendered Y. lipolytica sensitive to cold. The Ind1 G285C variant was able to support complex I assembly at 28°C, but not at 10°C. Our results indicate that Ind1 is required for progression of assembly from the Q module to the full matrix arm. Cold sensitivity could be developed as a phenotype assay to demonstrate pathogenicity of NUBPL mutations and other complex I defects.