Fig. 1. Model of lipid-raft structure and function in biological membranes. (A) Rafts are membrane microdomains formed by high concentrations of sphingolipids (dark-brown-headed structures) and cholesterol (red bean-shaped structures) immersed in a phospholipid-rich (light-brown-headed structures) environment. Glycolipids and sphingomyelin are restricted to the outer leaflet of the bilayer, whereas cholesterol and phospholipids are in both leaflets. Note that lipids in the rafts usually have long and saturated fatty acyl chains (red two-legged shapes), whereas those in lipids excluded from these microdomains are shorter and unsaturated (green two-legged shapes). (B) Principles of selective recruitment of proteins in rafts. Recruitment of membrane proteins in phospholipid-rich membrane regions takes place through protein-protein interactions. However, in rafts this process takes place through interactions between the lipids within the rafts and the transmembrane domain of integral membrane proteins (lipid-protein interaction) or the lipid moiety of proteins attached to the membrane by a lipid modification (lipid-lipid interaction). The recruitment of cytosolic proteins by protein-protein interactions through modular domains (SH2 domains, SH3 domains, etc.) can take place in both raft and non-raft membranes. Proteins excluded from rafts are in yellow; proteins included in rafts are in blue (integral membrane proteins), light brown (GPI-anchored proteins) or pink (acylated, cytosolically-oriented, proteins such as Src family kinases, Ras and heterotrimeric G proteins).
