Structure of the c(10) Ring of the Yeast Mitochondrial ATP Synthase in the Open Conformation

The proton pores of F(1)F(o)-ATP synthases consist of a ring of c-subunits, which rotates driven by downhill proton diffusion. An essential carboxylate side chain in the c-subunit provides a proton-binding site. In all the structures of c-rings reported to date, these sites are in a closed, ion-locked state. Structures are presented of the c(10) ring from Saccharomyces cerevisiae determined at pH 8.3, 6.1 and 5.5, at resolutions of 2.0 Å, 2.5 Å and 2.0 Å, respectively. The overall structure of this mitochondrial c-ring is similar to known homologues, except that the essential carboxylate, Glu59, adopts an open extended conformation. Molecular dynamics simulations reveal that opening of the essential carboxylate is a consequence of the amphiphilic nature of the crystallization buffer. We propose that this new structure represents the functionally open form of the c-subunit, which facilitates proton loading and release.