The a subunit of F1Fo (F1Fo-ATP synthase) is a highly hydrophobic protein with five putative transmembrane helices which plays a central role in H+-translocation coupled with ATP synthesis/hydrolysis. In the present paper, we show that the a subunit produced by the in vitro protease-free protein synthesis system (the PURE system) is integrated into a preformed Foa-less F1Fo complex in Escherichia coli membrane vesicles and liposomes. The resulting F1Fo has a H+-coupled ATP synthesis/hydrolysis activity that is approximately half that of the native F1Fo. By using this procedure, we analysed five mutations of F1Fo, where the conserved residues in the a subunit (Asn90, Asp112, Arg169, Asn173 and Gln217) were individually replaced with alanine. All of the mutant Foa subunits were successfully incorporated into F1Fo, showing the advantage over conventional expression in E. coli by which three (N90A, D112A, and Q217A) mutant a subunits were not found in F1Fo. The N173A mutant retained full activity and the mutants D112A and Q217A had weak, but detectable, activity. No activity was observed for the R169A and N90A mutants. Asn90 is located in the middle of putative second transmembrane helix and likely to play an important role in H+-translocation. The present study exemplifies that the PURE system provides an alternative approach when in vivo expression of membranous components in protein complexes turns out to be difficult.

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