Nuclear DNA origin of mitochondrial complex I deficiency in fatal infantile lactic acidosis evidenced by transnuclear complementation of cultured fibroblasts.

We have studied complex I (NADH-ubiquinone reductase) defects of the mitochondrial respiratory chain in 2 infants who died in the neonatal period from 2 different neurological forms of severe neonatal lactic acidosis. Specific and marked decrease in complex I activity was documented in muscle, liver, and cultured skin fibroblasts. Biochemical characterization and study of the genetic origin of this defect were performed using cultured fibroblasts. Immunodetection of 6 nuclear DNA-encoded (20, 23, 24, 30, 49, and 51 kDa) and 1 mitochondrial DNA-encoded (ND1) complex I subunits in fibroblast mitochondria revealed 2 distinct patterns. In 1 patient, complex I contained reduced amounts of the 24- and 51-kDa subunits and normal amounts of all the other investigated subunits. In the second patient, amounts of all the investigated subunits were severely decreased. The data suggest partial or extensive impairment of complex I assembly in both patients. Cell fusion experiments between 143B206 rho degrees cells, fully depleted of mitochondrial DNA, and fibroblasts from both patients led to phenotypic complementation of the complex I defects in mitochondria of the resulting cybrid cells. These results indicate that the complex I defects in the 2 reported cases are due to nuclear gene mutations.

cDNA sequence and chromosomal localization of the NDUFS8 human gene coding for the 23 kDa subunit of the mitochondrial complex I.

We have sequenced the cDNA for the 23 kDa subunit of the human mitochondrial respiratory complex I. The deduced protein consists of 210 amino acids (Mr = 23705 Da) with a 34 amino acid N terminus presumably acting as a presequence for mitochondrial import. The predicted mature protein (Mr = 20290 Da) is 92% identical to the bovine mitochondrial subunit and 72% to the Rhodobacter capsulatus NUOI counterpart. Two clusters of four cysteine residues are conserved among these proteins. The gene (NDUFS8) coding for the human subunit has been mapped to chromosome 11q13.

Mapping of nucleotide-depleted mitochondrial F1-ATPase with 2-azido-[alpha-32P]adenosine diphosphate. Evidence for two nucleotide binding sites in the beta subunit.

Photolabeling of nucleotide binding sites in nucleotide-depleted mitochondrial F1 has been explored with 2-azido [alpha-32P]adenosine diphosphate (2-N3[alpha-32P] ADP). Control experiments carried out in the absence of photoirradiation in a Mg2+-supplemented medium indicated the presence of one high affinity binding site and five lower affinity binding sites per F1. Similar titration curves were obtained with [3H]ADP and the photoprobe 3'-arylazido-[3H]butyryl ADP [( 3H]NAP4-ADP). Photolabeling of nucleotide-depleted F1 with 2-N3[alpha-32P]ADP resulted in ATPase inactivation, half inactivation corresponding to 0.6-0.7 mol of photoprobe covalently bound per mol F1. Only the beta subunit was photolabeled, even under conditions of high loading with 2-N3[alpha-32P]ADP. The identification of the sequences labeled with the photoprobe was achieved by chemical cleavage with cyanogen bromide and enzymatic cleavage by trypsin. Under conditions of low loading with 2-N3[alpha-32P]ADP, resulting in photolabeling of only one vacant site in F1, covalently bound radioactivity was located in a peptide fragment of the beta subunit spanning Pro-320-Met-358 identical to the fragment photolabeled in native F1 (Garin, J., Boulay, F., Issartel, J.-P., Lunardi, J., and Vignais, P. V. (1986) Biochemistry 25, 4431-4437). With a heavier load of photoprobe, leading to nearly 4 mol of photoprobe covalently bound per mol F1, an additional region of the beta subunit was specifically labeled, corresponding to a sequence extending from Gly-72 to Arg-83. The isolated beta subunit also displayed two binding sites for 2-N3-[alpha-32P]ADP. When F1 was first photolabeled with a low concentration of NAP4-ADP, leading to the covalent binding of 1.5 mol of NAP4-ADP/mol F1, with the bound NAP4-ADP distributed equally between the alpha and beta subunits, a subsequent photoirradiation in the presence of 2-N3[alpha-32P]ADP resulted in covalent binding of the 2-N3[alpha-32P]ADP to both alpha and beta subunits. It is concluded that each beta subunit in mitochondrial F1 contains two nucleotide binding regions, one of which belongs to the beta subunit per se, and the other to a subsite shared with a subsite located on a juxtaposed alpha subunit. Depending on the experimental conditions, the subsite located on the alpha subunit is either accessible or masked. Unmasking of the subsite in the three alpha subunits of mitochondrial F1 appears to proceed by a concerted mechanism.

Characterization of exchangeable and nonexchangeable bound adenine nucleotides in F1-ATPase from Escherichia coli.

The total amount of bound exchangeable and nonexchangeable adenine nucleotides in Escherichia coli F1-ATPase (BF1) was determined; three exchangeable nucleotides were assessed by equilibrium dialysis in a [14C]ADP-supplemented medium. When BF1 was purified in a medium supplemented with ATP, a stoichiometry of nearly 6 mol of bound nucleotides/mol of enzyme was found; three of the bound nucleotides were ATP and the others ADP. When BF1 was filtered on Sephadex G-50 in a glycerol medium (Garrett, N.E., and Penefsky, H.S. (1975) J. Biol. Chem. 250, 6640-6647), bound ADP was rapidly released, in contrast to bound ATP which remained firmly attached to the enzyme. Upon incubation of BF1 with [14C]ADP, the bound ADP rather than the bound ATP was exchanged. Of the three [14C]ADPs which have bound to BF1 by exchange after equilibrium dialysis, one was readily lost by gel filtration on Sephadex G-50; the loss of bound [14C]ADP was markedly reduced by incubation of BF1 with aurovertin, a specific ligand of the beta subunit which is known to increase the affinity of the beta subunit for nucleotides (Issartel, J.-P., and Vignais, P. V. (1984) Biochemistry 23, 6591-6595). Upon photoirradiation of BF1 with [alpha-32P]2-azido-ADP, only the beta subunit was labeled; concomitantly, bound ADP was released, but the content in bound ATP remained stable. These results suggest that specific sites located on the three beta subunits bind nucleotides in a reversible manner. Consequently, the tightly bound ATP of native BF1 would be located on the alpha subunits.