Studies on the immunological properties of complex V (mitochondrial ATP synthetase complex)

Membrane topography and near-neighbor relationships of the mitochondrial ATP synthase subunits e, f, and g

The well characterized subunits of the bovine ATP synthase complex are the alpha, beta, gamma, delta, and epsilon subunits of the catalytic sector, F1; the ATPase inhibitor protein; and subunits a, b, c, and d, OSCP (oligomycin sensitivity-conferring protein), F6, and A6L, which are present in the membrane sector, F0, and the 45-A-long stalk that connects F1 to F0. It has been shown recently that bovine ATP synthase preparations also contain three small polypeptides, designated e, f, and g, with respective molecular masses of 8.2, 10. 2, and 11.3 kDa. To ascertain their involvement as bona fide subunits of the ATP synthase and to investigate their membrane topography and proximity to the above ATP synthase subunits, polyclonal antipeptide antibodies were raised in the rabbit to the COOH-terminal amino acid residues 57-70 of e, 75-86 of f, and 91-102 of g. It was shown that (i) e, f, and g could be immunoprecipitated with anti-OSCP IgG from a fraction of bovine submitochondrial particles enriched in oligomycin-sensitive ATPase; (ii) the NH2 termini of f and g are exposed on the matrix side of the mitochondrial inner membrane and can be curtailed by proteolysis; (iii) the COOH termini of all three polypeptides are exposed on the cytosolic side of the inner membrane; and (iv) f cross-links to A6L and to g, and e cross-links to g and appears to form an e-e dimer. Thus, the bovine ATP synthase complex appears to have 16 unlike subunits, twice as many as its counterpart in Escherichia coli.

Evidence from immunological studies of structure-mechanism relationship of F1 and F1F0

Monoclonal and polyclonal antibodies directed against peptides of F1-ATPase of F1F0-ATPase synthase provide new and efficient tools to study structure-function relationships and mechanisms of such complex membrane enzymes. This review summarizes the main results obtained using this approach. Antibodies have permitted the determination of the nature of subunits involved in the complex, their stoichiometry, their organization, neighboring interactions, and vectorial distribution within or on either face of the membrane. Moreover, in a few cases, amino acid sequences exposed on a face of the membrane or buried inside the complex have been identified. Antibodies are very useful for detecting the role of each subunit, especially for those subunits which appear to have no direct involvement in the catalytic mechanism. Concerning the mechanisms, the availability of monoclonal antibodies which inhibit (or activate) ATP hydrolysis or ATP synthesis, which modify nucleotide binding or regulation of activities, which detect specific conformations, etc. brings many new ways of understanding the precise functions. The specific recognition by monoclonal antibodies on the beta subunit of epitopes in the proximity of, or in the catalytic site, gives information on this site. The use of anti-alpha monoclonal antibodies has shown asymmetry of alpha in the complex as already shown for beta. In addition, the involvement of alpha with respect to nucleotide site cooperativity has been detected. Finally, the formation of F1F0-antibody complexes of various masses, seems to exclude the functional rotation of F1 around F0 during catalysis.

Topological and functional characterization of the F0I subunit of the membrane moiety of the mitochondrial H+-ATP synthase

Using isolated polypeptides of the F0 sector of bovine heart mitochondrial H+-ATPase, antisera were developed detecting specifically two components of F0. These two components were identified as F0I and oligomycin-sensitivity-conferring protein (OSCP) respectively. Both F0I and OSCP were digested by mild trypsin treatment of submitochondrial particles depleted of the catalytic part of H+-ATPase (USMP). Proteolysis was largely prevented by binding of F1 to F0. Proteolysis of F0I resulted in the formation of three immunoreactive, membrane-bound fragments of apparently 26 kDa, 25.5 kDa and 18 kDa, respectively, indicating that F0I contains trypsin-accessible Arg or Lys residues located close to the end and the middle part of the protein, respectively, which are in intimate contact with F1. Digestion of USMP with trypsin resulted in depression of passive H+ conduction through F0 which could be ascribed to proteolysis of F0I.

Accessibility of F0 subunits from Escherichia coli ATP synthase. A study with subunit specific antisera

Antisera have been raised against denatured and non-denatured subunits a, b and c of the F0 complex of the ATP synthase from Escherichia coli. The subunit specificity of the antibodies has been established with immunoblot analysis or enzyme-linked immunosorbent assay (ELISA). In inside-out oriented membrane vesicles the binding avidities of both sets of antisera, against denatured and non-denatured subunits of F0, were similar in the presence as well as in the absence of the F1 part. F1-depleted everted membrane vesicles always produced an efficient binding of the different antisera. In the presence of F1 no antibody recognition could be observed with the anti-a antisera, while anti-b and anti-c antisera showed strong binding. However, a higher membrane protein concentration was necessary for the same antibody binding as in F1-stripped vesicles. In membrane vesicles with right-side-out orientation the recognition of the three F0 subunits was dependent on the antisera set used. Antisera raised against denatured subunits showed no binding to the membrane vesicles, only in case of anti-(dodecylsulfate-denatured b) antiserum could a slight affinity be detected. An antigen-antibody recognition with all three F0 subunits occurred when the antisera against non-denatured subunits were incubated with membrane vesicles of right-side-out orientation. The membrane protein concentration which was necessary to produce a significant binding was 10-100-fold higher compared to that of F1-depleted everted membrane vesicles.

Monoclonal antibodies against subunits of yeast mitochondrial H+-ATPase

Fourteen stable lines of myeloma-spleen cell hybrids producing antibodies against the mitochondrial H+-ATPase have been isolated. One reacted with the alpha-subunit of the enzyme complex (Mr 56000), nine with the beta-subunit (Mr 54000), and four with a 25 kDa subunit which has not been previously characterized. These antibodies are inhibitory or stimulatory or have no effect upon the enzyme activity. Two of the monoclonal anti-beta-subunit antibodies were found to be particularly effective in immunoprecipitating intact H+-ATPase complex.

Membrane topology of ATP synthase from bovine heart mitochondria and Escherichia coli

The polypeptides exposed to lipids in the membranous F0 sector of the mitochondrial and Escherichia coli ATP synthases were labelled with radioactive photoreactive lipids. Highly resolving gel electrophoretic conditions were used in order to separate all the eighteen components forming the bovine heart mitochondrial enzyme. The hydrophobic labelling was performed on fully active and inhibitor-sensitive ATP synthases. In the mitochondrial enzyme prepared according to Serrano et al. (1976) [J. Biol. Chem. 251, 2453-2461] seven polypeptides of Mr 30500; 11500; 10500; 10000; 9500; 8500 and 4500 were labelled. The major amount of radioactivity was associated with the 30500-Mr component, which is thought to be the adenine nucleotide carrier. In the preparation of Galante et al., (1979) which almost completely lacks this component [J. Biol. Chem. 254, 12372-12378] nine polypeptides of Mr 25000; 21000; 11500; 10500; 10000; 9500; 9200; 8500 and 4500 were labelled. In the ATPase synthase from E. coli the major amount of labelling was associated with subunit b and only a minor portion with subunit c.