F₁F₀ ATP synthase subunit c is a substrate of the novel YidC pathway for membrane protein biogenesis

The Escherichia coli YidC protein belongs to the Oxa1 family of membrane proteins that have been suggested to facilitate the insertion and assembly of membrane proteins either in cooperation with the Sec translocase or as a separate entity. Recently, we have shown that depletion of YidC causes a specific defect in the functional assembly of F₁F₀ ATP synthase and cytochrome o oxidase. We now demonstrate that the insertion of in vitro–synthesized F₁F₀ ATP synthase subunit c (F₀c) into inner membrane vesicles requires YidC. Insertion is independent of the proton motive force, and proteoliposomes containing only YidC catalyze the membrane insertion of F₀c in its native transmembrane topology whereupon it assembles into large oligomers. Co-reconstituted SecYEG has no significant effect on the insertion efficiency. Remarkably, signal recognition particle and its membrane-bound receptor FtsY are not required for the membrane insertion of F₀c. In conclusion, a novel membrane protein insertion pathway in E. coli is described in which YidC plays an exclusive role.