Structural organization of complex I from bovine mitochondria.

NADHxbiquinone oxidoreductase (complex I), the first component of the mitochondrial electron transport chain, transfers electrons from NADH to ubiquinone, with concomitant translocation of four protons per pair of electrons across the inner membrane (reviewed in [ 1-31). It contains one molecule of FMN and at least four iron-sulphur centres. Complex I from bovine heart mitochondria is now known to be a complex of around 41 different proteins. It consists of seven hydrophobic subunits encoded in mitochondrial DNA, and 34 nuclearencoded proteins which have been characterized by sequence analysis [ 1, 4-61. Assuming that one copy of each protein is present in the complex, its subunits contain a total of over 7955 amino acids making it more complex than the Escherichia coli ribosome, which has 7336 amino acid residues. The calculated molecular mass of the complex is greater than 900 kDa. This is close to values (670-890 kDa) determined from the flavin content [7]. The only other well characterized complex I is that from Neurosporu crasu mitochondria [8]. It has a molecular mass of 610 kDa [9] and contains at least 30 proteins [ lo]. Nine nuclear-encoded subunits have so far been identified as homologues of bovine complex I subunits [ 11, and homologues of at least six of the seven complex I gene products (ND1-ND6 and ND4L) encoded in bovine mitochondria] DNA are also found in the N. crussu mitochondria] genome [ 10121. As summarized in Table 1, some of the subunits of bovine complex I are covalently modified post-translationally. The most unexpected of these modifications is the pantethenic acid moiety attached to subunit SDAP, which is related in sequence to acyl carrier proteins [13]. The role of this subunit in complex I has not been elucidated, but it could be involved in phospholipid biosynthesis in mitochondria [ 141. An acyl carrier protein has also been found to be associated with the N crassu complex I [ 151. If the mechanism of complex I is to be understood, a more detailed picture of its structure, and particularly of the electron pathway through the enzyme, are required. Given its size, the solution of