Preparation of a highly coupled H(+)-transporting ATP synthase from pig heart mitochondria

The rotation of the alpha subunit of F1 relative to minor subunits is not involved in ATP synthesis. Evidence given by using an anti-alpha subunit monoclonal antibody

To test whether ATP synthesis could occur via a mechanism of rotational catalysis in which the alpha and beta subunits of F1 would rotate with respect to the minor subunits, we have measured the rate of ATp synthesis after binding various masses of antibodies to F1. If the rotation was an essential feature of the mechanism, the rate of ATP synthesis should be inhibited either completely or proportionately to the load carried by F1. Bivalent immunoglobulins (IgG) or monovalent Fab fragments of an anti-alpha monoclonal antibody (7B3) were bound to F1 present in electron-transport particles in a ratio of 2 Fab or 2 IgG per F1. This binding similarly inhibited the rate of ATP synthesis by a maximum of about 50%. When anti-mouse immunoglobulins were added to the F1-7B3 (IgG) complex, no significant change in the rate of inhibition was observed. In conclusion, the rate of ATP synthesis was the same when F1 was loaded with 100 kDa (2 Fab), 300 kDa (2 IgG, 7B3) or 900 kDa (2 IgG + 4 ant-mouse IgG). It is concluded that the rotation of the alpha subunits is extremely unlikely to play an essential role in the mechanism of ATP synthesis.

ATP synthase from bovine mitochondria. The characterization and sequence analysis of two membrane-associated sub-units and of the corresponding cDNAs

ATP synthase from bovine mitochondria is a complex of 13 different polypeptides, whereas the Escherichia coli enzyme is simpler and contains eight subunits only. Two of the bovine subunits, b and d, which had not been characterized, have been isolated from the purified enzyme. Subunits with sizes corresponding to bovine subunits b and d are evident in preparations of the enzyme from mitochondria of other species. Partial protein sequences have been determined by direct methods. On the basis of some of this information, two oligonucleotide mixtures, 17 and 18 bases in length, have been synthesized and used as hybridization probes in the isolation of clones of the cognate cDNAs. The sequences of the two proteins have been deduced from their DNA sequences. Subunit b is 214 amino acid residues in length and has a free N terminus. Subunit d is 160 amino acid residues long. Its N-terminal alanine is blocked by an N-acetyl group, as demonstrated by fast atom bombardment mass spectrometry of N-terminal peptides. The sequence near the N terminus of the b subunit is made predominantly of hydrophobic residues, whereas the remainder of the protein is mainly hydrophilic. This N-terminal hydrophobic region may be folded into an alpha-helical structure spanning the lipid bilayer. In its distribution of hydrophobic residues, this protein resembles the b subunits of ATP synthase complexes in bacteria and chloroplasts. The b subunit in E. coli forms an important structural link between the extramembrane sector of the enzyme F1, and the intrinsic membrane domain, FO. It is proposed that the bovine mitochondrial subunit b serves a similar function. If this is so, the mitochondrial enzyme, as the chloroplast ATP synthase, contains equivalent subunits to all eight of those that constitute the E. coli enzyme. Subunit d has no extensive hydrophobic sequences, and is not apparently related to any subunit described in the simpler ATP synthases in bacteria and chloroplasts.

Efficient reconstitution of mitochondrial energy-transfer reactions from depleted membranes and F1-ATPase as a function of the amount of bound oligomycin sensitivity-conferring protein (OSCP)

Pig heart mitochondrial membranes depleted of F1 and OSCP by various treatments were analyzed for their content in alpha and beta subunits of F1 and in OSCP using monoclonal antibodies. Membrane treatments and conditions of rebinding of F1 and OSCP were optimized to reconstitute efficient NADH- and ATP-dependent proton fluxes, ATP synthesis and oligomycin-sensitive ATPase activity. F1 and OSCP can be rebound independently to depleted membranes but to avoid unspecific binding of F1 to depleted membranes (ASUA) which is not efficient for ATP synthesis, F1 must be rebound before the addition of OSCP. The rebinding of OSCP to depleted membranes reconstituted with F1 inhibits the ATPase activity of rebound F1, while it restores the ATP-driven proton flux measured by the quenching of ACMA fluorescence. The rebinding of OSCP also renders the ATPase activity of bound F1 sensitive to uncouplers. The rebinding of OSCP alone or F1 alone, does not modify the NADH-dependent proton flux, while the rebinding of both F1 and OSCP controls this flux, inducing an inhibition of the rate of NADH oxidation. Similarly, oligomycin, which seals the F0 channel even in the absence of F1 and OSCP, inhibits the rate of NADH oxidation. OSCP is required to adjust the fitting of F1 to F0 for a correct channelling of protons efficient for ATP synthesis. All reconstituted energy-transfer reactions reach their optimal value for the same amount of OSCP. This amount is consistent with a stoichiometry of two OSCP per F1 in the F0-F1 complex.