Understanding Membranes and Vesicles: A Personal Recollection of the Last Two Decades

Abstract

Biomembranes consist of fluid bilayers built up from many lipid and protein components. The membrane fluidity has two important consequences. First, the molecular components can undergo fast lateral transport within the membranes, a necessary prerequisite for the formation and remodelling of intramembrane compartments. Second, the fluidity leads to unusual elastic properties of the membranes that allow them to “escape into the third dimension.” Intramembrane compartments can be formed by lipid phase separation, now observed for many ternary lipid mixtures, or by heterogeneous environments that lead to an ambience-induced segmentation of the membranes. Because of their unusual elastic properties, the membranes can attain many different shapes and undergo striking shape transformations, which reflect their ability to respond locally to external perturbations by changes in their curvature. Several molecular mechanisms for local curvature generation have been identified including membrane-anchored polymers, adsorption or depletion layers of solutes, and membrane-bound proteins. The local curvature generation is intimately related to the concept of a preferred or spontaneous curvature that describes the asymmetry between the two leaflets of the bilayer membrane. New methods to determine the spontaneous curvature in a reliable manner have been recently developed, based on spontaneous or force-induced tubulation of giant vesicles. The spontaneous curvature plays a pivotal role both for the engulfment of nanoparticles by membranes and for the wetting of membranes by aqueous droplets, two membrane processes that remain to be further elucidated. The spontaneous curvature also determines the exergonic or endergonic nature of membrane fusion and fission.

Denken ohne Erfahrung ist leer, Erfahrung ohne Denken ist blind.

(Thought without experience is empty, experience without thought is blind.)

Immanuel Kant