title: Activation and inhibition of the Escherichia coli F1-ATPase by monoclonal antibodies which recognize the epsilon subunit.
abstract: The properties of two monoclonal antibodies which recognize the epsilon subunit of Escherichia coli F1-ATPase were studied in detail. The epsilon subunit is a tightly bound but dissociable inhibitor of the ATPase activity of soluble F1-ATPase. Antibody epsilon-1 binds free epsilon with a dissociation constant of 2.4 nM but cannot bind epsilon when it is associated with F1-ATPase. Likewise epsilon cannot associate with F1-ATPase in the presence of high concentrations of epsilon-1. Thus epsilon-1 activates F1-ATPase which contains the epsilon subunit, and prevents added epsilon from inhibiting the enzyme. Epsilon-1 cannot bind to membrane-bound F1-ATPase. The epsilon-4 antibody binds free epsilon with a dissociation constant of 26 nM. Epsilon-4 can bind to the F1-ATPase complex, but, like epsilon-1, it reverses the inhibition of F1-ATPase by the epsilon subunit. The epsilon subunit remains crosslinkable to both the beta and gamma subunits in the presence of epsilon-4, indicating that it is not grossly displaced from its normal position by the antibody. Presumably the activation arises from more subtle conformational effects. Antibodies epsilon-4 and delta-2, which recognizes the delta subunit, both bind to F1F0 in E. coli membrane vesicles, indicating that these subunits are substantially exposed in the membrane-bound complex. Epsilon-4 inhibits the ATPase activity of the membrane-bound enzyme by about 50%, and Fab prepared from epsilon-4 inhibits by about 40%. This inhibition is not associated with any substantial change in the major apparent Km for ATP. These results suggest that inhibition of membrane-bound F1-ATPase arises from steric effects of the antibody.
mesh_terms: D000911:Antibodies, Monoclonal / Q000276:immunology*; D000915:Antibody Affinity; D000918:Antibody Specificity; D001693:Biological Transport, Active; D002462:Cell Membrane / Q000201:enzymology / Q000276:immunology; D000939:Epitopes; D004926:Escherichia coli / Q000201:enzymology; D006863:Hydrogen-Ion Concentration; D046911:Macromolecular Substances; D008564:Membrane Potentials; D006180:Proton-Translocating ATPases / Q000037:antagonists & inhibitors / Q000276:immunology*
chemical_list: D000911:Antibodies, Monoclonal; D000939:Epitopes; D046911:Macromolecular Substances; D006180:Proton-Translocating ATPases