Wetting Transitions in the ATP Synthase C-Subunit Ring, a Large-Conductance Ion Channel

Abstract

Apart from its known role in proton transport, the mammalian ATP synthase c-subunit ring (Fo domain) has been observed in recent electrophysiology experiments to form a high-conductance voltage-gated ion channel (∼1.5 nS). Yet, the ring's hydrophobic central lumen has been assumed to be dry, precluding a physical explanation for the observed conductance. We present an atomistic molecular dynamics simulation study of lumen wetting for mammalian and yeast c-subunit rings. Consistent with prior work, we find that the ∼12 Å diameter yeast ring remains dry on the several-hundred-nanosecond timescale of the simulations. In contrast, the narrower, ∼8 Å diameter, mammalian ring undergoes rapid (∼50 ns) liquid-vapor transitions, forming a transient but continuous water column across the lumen. Modifying the charge on the glutamate residues of the proton-transport pathway at the membrane-protein interface affects the duration and extent of lumen wetting, as do in silico mutations at various sites. The wetting variations are qualitatively consistent with electrophysiology experimental findings for wild-type and mutant forms of the mammalian c-subunit ring. We are encouraged to think that the simulations will serve as a valuable “computational microscope” for understanding how charge conductance can occur in the ATP synthase c-subunit ring. To determine the mechanism of wetting and its modulation, we are pursuing analysis of the surface hydrophobicity of the lumen lining and its response to the mutations. The simulation finding of reversible lumen wetting is supported by published work indicating c-subunit ring wetting in the intact ATP synthase. New experiments and published data additionally indicate that the Fo domain may exist at least partly dissociated from its F1 counterpart. ATP synthase c-subunit ring wetting, therefore, has implications with respect to osmotic swelling and the enigmatic mitochondrial permeability transition.