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Abstract

The crystal structure of the c-ring from the proton-coupled F₁F_o ATP synthase from Spirulina platensis is shown at 2.1-Å resolution. The ring includes 15 membrane-embedded c subunits forming an hourglass-shaped assembly. The structure demonstrates that proton translocation across the membrane entails protonation of a conserved glutamate located near the membrane center in the c subunit outer helix. The proton is locked in this site by a precise hydrogen bond network reminiscent of that in Na⁺-dependent ATP synthases. However, the structure suggests that the different coordination chemistry of the bound proton and the smaller curvature of the outer helix drastically enhance the selectivity of the H⁺ site against other cations, including H₃O⁺. We propose a model for proton translocation whereby the c subunits remain in this proton-locked state when facing the membrane lipid. Proton exchange would occur in a more hydrophilic and electrostatically distinct environment upon contact with the a subunit interface.