Energetics of stalk intermediates elucidated from hydrostatic pressure effects on membrane fusion

Membrane fusion is an essential process in cells that requires a balance of lipid composition to establish biophysical properties conducive to topology changes. During fusion, lipids of opposing membranes must invert and overcome energy barriers associated with forming highly curved stalk and pore intermediates. While theoretical work has modeled the effect of lipid intrinsic curvature on stalk formation, quantifying the relationship experimentally has proven to be a challenge due to the inability to vary lipid curvature without concomitantly changing other properties that affect fusion. Here we address this hurdle by using hydrostatic pressure to modulate lipid intrinsic curvature independently of chemical composition. Using high-pressure stopped-flow fluorimetry, we measured rates of calcium-mediated lipid mixing between populations of vesicles, a process that is strongly inhibited by pressure. We correlated mean lipid intrinsic curvature across pressure with lipid mixing rates by incorporating complementary small-angle x-ray scattering measurements for each individual lipid component. This analysis showed that lipid mixing rates, a proxy for hemifusion, correlated with mean monolayer spontaneous curvature as we varied either lipid composition or pressure. Consistent with previous theoretical models, we find a linear relation between lipid monolayer spontaneous curvature and the energy barrier to lipid mixing; the slope magnitude of this relation supports the stalk hypothesis.