Mitochondrial complex I is an intricate 1MDa membrane protein complex with a central role in aerobic energy metabolism. The minimal form of complex I consists of fourteen central subunits that are conserved from bacteria to man. In addition, eukaryotic complex I comprises some 30 accessory subunits of largely unknown function. The gene for the accessory NDUFS4 subunit of human complex I is a hot spot for fatal pathogenic mutations in humans. We have deleted the gene for the orthologous NUYM subunit in the aerobic yeast Yarrowia lipolytica, an established model system to study eukaryotic complex I and complex I linked diseases. We observed assembly of complex I which lacked only subunit NUYM and retained weak interaction with assembly factor N7BML (human NDUFAF2). Absence of NUYM caused distortion of iron sulfur clusters of the electron input domain leading to decreased complex I activity and increased release of reactive oxygen species. We conclude that NUYM has an important stabilizing function for the electron input module of complex I and is essential for proper complex I function.

NADH:ubiquinone-oxidoreductase (complex I) is the largest membrane protein complex of the respiratory chain. Complex I couples electron transfer to vectorial proton translocation across the inner mitochondrial membrane. The L shaped structure of complex I is divided into a membrane arm and a matrix arm. Fourteen central subunits are conserved throughout species, while some 30 accessory subunits are typically found in eukaryotes. Complex I dysfunction is associated with mutations in the nuclear and mitochondrial genome, resulting in a broad spectrum of neuromuscular and neurodegenerative diseases. Accessory subunit NDUFS4 in the matrix arm is a hot spot for mutations causing Leigh or Leigh-like syndrome. In this review, we focus on accessory subunits of the matrix arm and discuss recent reports on the function of accessory subunit NDUFS4 and its interplay with NDUFS6, NDUFA12, and assembly factor NDUFAF2 in complex I assembly.