Molecular mechanism of reversed temperature-dependence of ATP synthesis in glacier ice worms.

Abstract

The F0F1 ATP synthase enzyme is highly conserved across species. The F0F1 is a reversible motor, where the clockwise rotation of the rotor portion of F0 (known as the c-ring and present in the membrane embedded F0 domain) produces ATP and the counter-clockwise rotation induces ATP hydrolysis. In a surprising contrast with temperate organisms, glacier ice worms display elevated ATP levels as temperatures decline. The increased energy expenditure is used as a strategy for survival at cold temperatures, but the mechanism is unknown. To investigate the underlying mechanism of elevated ATP levels in glacier ice worms, we investigated the effects of temperature on the dynamics of the F0F1 ATP synthase of glacier ice worms compared to their temperate counterparts. More specifically, an ice worm-specific, 18 amino acid extension with regularly spaced histidine residues was previously found to be fused to the carboxy terminal of the ATP6 subunit generating a proton shuttling domain projecting away from the F0 exit pore. The role of this C-terminal extension in the temperature dependance ATP synthesis in ice worms is undetermined. We conducted all-atomistic MD simulations of the F0 domain subunits at different temperatures and used earthworms as a control to evaluate the effect of sequence on temperature dependence. We also measured the rotational diffusion of the c-ring as a function of temperature to evaluate the change in the rotation angles of the c-ring around the z axis in clock- and counter-clockwise directions. The ATP6 subunit is attached to the c-ring at the proton exit site. This suggests that the change in the c-ring rotational diffusion, as a function of temperature, can affect the ATP6 conformation, which likely affects the rate of ATP production.