The pyruvate oxidase of Escherichia coli is a peripheral membrane flavoprotein that is dra matically activated by lipids. The enzyme strongly binds to phospholipid vesicles in vitro. In vivo, in addition to enzyme activation, binding is thought to be important to provide access of the enzyme to ubiquinone dissolved in the lipid bilayer. It was unclear if both or either of these attributes is needed for enzyme function in vivo. To differentiate between activation and lipid binding, we have constructed, using recombinant DNA techniques, a mutant gene that produces a truncated protein. The truncated protein lacks the last 24 amino acids of the C-terminus of the oxidase (due to introduction of a translation termination codon) and thus is closely analogous to the activated species produced in vitro by limited chymotrypsin cleavage [Recny, M. A., Grabau, C., Cronan, J. E., Jr., & Hager, L. P. (1985) J. Biol. Chem. 260, 14287–14291]. The truncated protein (like the protease-derived species) is fully active in vitro in the absence of lipid, and its activity is not further increased by addition of lipid activators. Moreover, the truncated enzyme fails to bind Triton X-114, a detergent that binds to and activates the wild-type oxidase. Strains producing the truncated protein were devoid of oxidase activity in vivo. This result indicates that binding to membrane lipids is specifically required for function of the oxidase in vivo; activation alone does not suffice.
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