Figure 1.
Possible physico-chemical mechanisms for lipid sorting. (A) Lipid sorting based on the raft hypothesis. The composition and destination of newly formed vesicles are determined by the type of raft from which the vesicle originates. (B) Different lipid species and their shape. Lipids are characterized by a shape based on the size of the head group, the number of chains, and the degree of chain saturation. From left to right the shapes are inverted-conical for Lyso-Phosphatidic acid (LPA), cylindrical for unsaturated 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and conical for 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE). Bottom: saturated cylindrical Sphingomyelin (SM) and cholesterol. (C) Two physical states relevant for cellular membranes. Top: drawing of a liquid-disordered (Ld) membrane, which is rich in unsaturated lipids (with little chain order); bottom: drawing of a liquid-ordered (Lo) membrane, which is rich in saturated lipids (with high degree of chain ordering) and cholesterol. (D) Lipid sorting based on spontaneous curvature. The membrane consists of cylindrical, inverted-conical. and conical lipids. The outer leaflet of the budding region is enriched in cylindrical and inverted conical lipids, whereas the inner leaflet is enriched in conical ones. (E) Lipid sorting based on composition dependence of the bending stiffness. The highly curved budding region is enriched in unsaturated lipids, so that the bending stiffness of this region is lower than the surrounding membrane.