The MWFE polypeptide of mammalian complex I (the proton-translocating NADH-quinone oxidoreductase) is 70 amino acids long, and it is predicted to be a membrane protein. The NDUFA1 gene encoding the MWFE polypeptide is located on the X chromosome. This polypeptide is 1 of approximately 28 ''accessory proteins'' identified in complex I, which is composed of 42 unlike subunits. It was considered accessory, because it is not one of the 14 polypeptides making up the core complex I; a homologous set of 14 polypeptides can make a fully functional proton-translocating NADH-quinone oxidoreductase in prokaryotes. One MWFE mutant has been identified and isolated from a collection of respiration-deficient Chinese hamster cell mutants. The CCL16-B2 mutant has suffered a deletion that would produce a truncated and abnormal MWFE protein. In these mutant cells, complex I activity is reduced severely (<10%). Complementation with hamster NDUFA1 cDNA restored the rotenone-sensitive complex I activity of these mutant cells to Ϸ100% of the parent cell activity. Thus, it is established that the MWFE polypeptide is absolutely essential for an active complex I in mammals.

No abstract

The iron-sulfur subunit of succinate dehydrogenase is one of the four subunits of complex II of the mitochondrial electron transport chain. Its gene, SDH2, is one of the four nuclear-encoded genes for this complex. Reporter gene analysis of the human SDH2 promoter indicates that it is transcriptionally regulated by the nuclear respiratory factors NRF-1 and NRF-2. Their binding sites reside immediately upstream (within 90 bp) of the transcription start site. Site-directed mutagenesis of either site lowers the reporter gene activity by tenfold to a basal level. Gel shift experiments and competition experiments with the authentic NRF-1 and NRF-2 DNA oligomers from previously characterized nuclear respiratory genes strengthen the proposed role of these two transcriptional regulators. These experiments extend the proposed regulatory role of these two transcription factors to complex II of the respiratory chain.The expression of three of the four genes of complex II was also examined when mouse myoblast C2C12 cells were induced to differentiate into myotubes. Up-regulation upon differentiation in tissue culture is only modest, 2Ϫ3 fold over the myoblast cells.

No abstract