A Comparative Study of the Effects of Cholesterol and Lanosterol on Hydrated Phosphatidylethanolamine Assemblies: Focusing on Physical Parameters Related to Membrane Fusion

Cholesterol (Chol) plays a crucial role in regulating membrane properties and biological processes such as membrane fusion, yet the molecular mechanisms underlying its function remain incompletely understood. In order to elucidate how sterol structure influences phospholipid organization relevant to membrane fusion, we compared the effects of Chol and its biosynthetic precursor lanosterol (Lan) on hydrated phosphatidylethanolamine (PE) assemblies using X-ray diffraction, the neutral flotation method, and osmotic stress measurements. Volumetric analyses revealed that Lan has a larger occupied molecular volume than Chol in the bilayers. These values were largely independent of differences between phospholipids (phosphatidylcholine and PE), indicating that sterols are deeply embedded within the bilayer. In palmitoyl-oleoyl-PE lamellar membranes, both sterols increased bilayer thickness. They both enhanced short-range repulsive hydration forces, but Chol suppressed fluctuation-induced repulsion more effectively, reflecting its greater stiffening effect. In bacterial PE systems forming the inverted hexagonal (H_II) phase, increasing sterol concentration decreased the lattice constant, with a more substantial effect for Lan, which also induced greater curvature of the water columns. These results suggest that while Chol enhances mechanical rigidity and membrane cohesion, Lan promotes molecular flexibility and curvature, properties associated with fusion intermediates.