Energetics of stalk intermediates in membrane fusion are controlled by lipid composition

TLDR

X‑ray diffraction of rhombohedral phospholipid phases and electron‑density isosurfaces were used to reconstruct and analyze stalk geometries, curvatures, and hydration interactions across pure lipids and mixtures. The stalk structure proved universal across lipid systems, with curvature varying subtly with composition, and dehydration—occurring at a constant interbilayer spacing of ~9.0 Å—identified as the key driver of stalk formation.

Abstract

We have used X-ray diffraction on the rhombohedral phospholipid phase to reconstruct stalk structures in different pure lipids and lipid mixtures with unprecedented resolution, enabling a quantitative analysis of geometry, as well as curvature and hydration energies. Electron density isosurfaces are used to study shape and curvature properties of the bent lipid monolayers. We observe that the stalk structure is highly universal in different lipid systems. The associated curvatures change in a subtle, but systematic fashion upon changes in lipid composition. In addition, we have studied the hydration interaction prior to the transition from the lamellar to the stalk phase. The results indicate that facilitating dehydration is the key to promote stalk formation, which becomes favorable at an approximately constant interbilayer separation of 9.0 ± 0.5 Å for the investigated lipid compositions.