Lipid rafts: contentious only from simplistic standpoints

Abstract

The hypothesis that lipid rafts exist in plasma membranes and have crucial biological functions remains controversial. The lateral heterogeneity of proteins in the plasma membrane is undisputed, but the contribution of cholesterol-dependent lipid assemblies to this complex, non-random organization promotes vigorous debate. In the light of recent studies with model membranes, computational modelling and innovative cell biology, I propose an updated model of lipid rafts that readily accommodates diverse views on plasma-membrane micro-organization.

Figure 1. Liquid-ordered domains in model membranes

a Phase diagram at 23°C for ternary mixtures of cholesterol, sphingomyelin (PSM) and l-palmitoyl-2-oleoyl-phosphatidylcholine (POPC). Each vertex of the diagram corresponds to a 100% content of each lipid. Coloured regions represent membrane compositions that can form a liquid disordered (L_d) phase. Within the region of liquid ordered (L_o)–L_d co-existence (blue), varying the cholesterol percentages from ~10% to 35% progressively increases the size of L_o domains that are detected by fluorescence resonance energy transfer (FRET) imaging or standard microscopy. It is important to note that phase diagrams such as this are crucially dependent on temperature, although an extensive area of L_o–L_d phase co-existence is present at 37°C(see REFS 3,6). b Macroscopic domain formation in giant unilamellar vesicles that are composed of cholesterol, 1,2-dipalmitoylphosphatidylcholine (DPPC) and 1,2-dioleoylphosphatidylcholine (DOPC) and visualized using dyes that preferentially partition into L_o domains (orange) and L_d domains (green). When the cholesterol percentage (χ_CHOL) is >16%, macroscopic domain formation is no longer seen, but nanodomain formation is still detectable as a loss of a FRET signal between two probes that preferentially partition into L_o and L_d domains. The yellow oval marks the approximate composition of the top row of vesicles. Scale bar = 5μm. χ_DPPC = DPPC content as a fraction of DPPC and DOPC. Part a is reproduced with permission from REF. © (2005) Elsevier; part b is reproduced with permission from REF. © (2001) The Biophysical Society.

Figure 2. Revised raft model

Liquid ordered (L_o) domains in the plasma membrane are heterogeneous in size and lifetime (from >1 s to <0.1 ms, as indicated by colour). L_o domain stability or lifetime is a function of size, capture by a raft-stabilizing protein and protein–protein interactions of constituent proteins. The length of trajectories (dotted arrowed lines) and, therefore, the probability of collision with proteins or other L_o domains are proportional to lifetime. For simplicity, only sample trajectories are shown — however, all of the domains and proteins that are shown should be envisaged to diffuse laterally. The diagram illustrates the fate of different classes of L_o domain. Small, unstable L_o domains form spontaneously, diffuse laterally in the plasma membrane, but have a limited lifetime (a). If captured by a glycosylphosphatidylinisotol (GPI)-anchored protein (or other raft-stabilizing proteins) (b) the stability of the L_o domain is increased with the formation of a complex (c), which has two possible outcomes: the complex diffuses laterally but the L_o domain disassembles (d), or there is a collision with other protein-stabilized L_o domains that creates protein clusters (e). Further collisions will generate larger, more stable L_o–protein complexes (f) that are further stabilized by protein–protein interactions, or in the absence of collisions (g) the L_o–protein complex disassembles. The fate of larger, more stable L_o–protein complexes (h) is capture by endocytic pathways that disassemble the complexes and return lipid and protein constituents back to the plasma membrane (i). Intuitively, the model predicts the generation of protein clusters that is dependent on their interaction with L_o domains. Endocytosis indicates endocytosis that is specific for larger raft complexes (as shown), possibly all endocytic processes contribute to limiting the size of raft domains. Note that the larger, more stable lipid–protein complexes (f) could operate the way rafts are proposed to in the classic model, with dynamic partitioning of proteins between the domain and the surrounding disordered membrane (not shown).