Resolution of the membrane domain of bovine complex I into subcomplexes: Implications for the structural organization of the enzyme

Complex I (NADH:ubiquinone oxidoreductase) purified from bovine heart mitochondria was treated with the detergent N,N-dimethyldodecylamine N-oxide (LDAO), The enzyme dissociated into two known subcomplexes, I alpha and I beta, containing mostly hydrophilic and hydrophobic subunits, and a previously undetected fragment referred to as I gamma. Subcomplex I gamma contains the hydrophobic subunits ND1, ND2, ND3, and ND4L which are encoded in the mitochondrial genome, and the nuclear-encoded subunit KFYI. During size-exclusion chromatography in the presence of LDAO, subcomplex I alpha lost several subunits and formed another characterized subcomplex known as I lambda, Similarly, subcomplex I beta dissociated into two smaller subcomplexes, one of which contains the hydrophobic subunits ND4 and ND5; subcomplex I gamma released a fragment containing ND1 and ND2. These results suggest that in the intact complex subunits ND1 and ND2 are likely to be in a different region of the membrane domain than subunits ND4 and ND5. The compositions of the various subcomplexes and fragments of complex I provide an organization of the subunits of the enzyme in the framework of the known low resolution structure of the enzyme.