The oligomeric subunit c rotor in the Fo sector of ATP synthase:: Unresolved questions in our understanding of function

We have proposed a model for the oligomeric c-rotor of the F-o sector of ATP synthase and its interaction with subunit a during Hi-transport driven rotation. The model is based upon the solution structure of monomeric subunit c, determined by NMR, and an extensive series of cross-linking distance constraints between c subunits and between subunits c and a. To explain the complete set of cross-linking data, we have suggested that the second transmembrane helix rotates during its interaction with subunit a in the course of the H+-translocation cycle. The H+-transport coupled rotation of this helix is proposed to drive the stepwise movement of the c-oligomeric rotor. The model is testable and provides a useful framework for addressing questions raised by other experiments.