Characterization of a membrane fragment of respiratory chain complex I from Neurospora crassa. Insights on the topology of the ubiquinone-binding site.

1. A membrane fragment of complex I from the fungus Neurospora crassa was isolated by immunoprecipitation from alkaline-extracted mitochondrial membranes. 2. Analysis of the polypeptide composition of this hydrophobic domain of complex I has brought insights on the topology of two subunits of the enzyme, namely the 20.8 and 9.3 kDa components. 3. Our results indicate that the ubiquinone-binding site of complex I resides in the interface of the peripheral and membrane arms of the enzymes. The significance of these findings are discussed.