Lipid rafts and membrane dynamics

Abstract

Cell membranes contain a variety of lipid species that differ in their physico-chemical properties. Lipid-lipid immiscibility gives rise to lateral heterogeneities in the membrane plane, a subset of which are termed lipid rafts (Simons and Vaz, 2004). Originally defined biochemically as detergent-resistant membrane (DRM) fractions, lipid rafts are proposed to be highly dynamic, submicroscopic assemblies that float freely within the liquid disordered bilayer in cell membranes and can coalesce upon clustering of their components. Sphingolipids and cholesterol in the outer exoplasmic leaflet play a crucial role in the assembly of these domains.Rafts are liquid-ordered domains that are more tightly packed than the surrounding non-raft phase of the bilayer. The tighter packing is due to the saturated hydrocarbon chains in raft sphingolipids and phospholipids compared with the unsaturated fatty acids of phospholipids in the non-raft phase (Simons and Vaz, 2004). Recent studies have suggested that an equivalent domain organisation could be present in the cytoplasmic leaflet as well. However, the properties of this inner leaflet have not been adequately defined (Parton and Richards, 2003). Theoretical considerations predict that a liquid-ordered packing of the outer leaflet leads to a more ordered packing also of the inner leaflet (Israelachvili, 1973).FIG1Although a large fraction of the cell surface proteins are found in the liquid disordered regions, some proteins preferentially partition into the ordered raft domains. Typical examples include the glycosylphosphatidylinositol (GPI)-anchored proteins, which are attached to the outer leaflet of the membrane via the GPI anchor (Chatterjee and Mayor, 2001); the Src-family tyrosine kinases (e.g. Lck, Fyn and Lyn), which are anchored to the inner leaflet via their dual acylation modification (Simons and Toomre, 2000); palmitoylated and myristoylated proteins such as flotillins (Rajendran et al., 2003); cholesterol-binding proteins such as caveolins (Kurzchalia and Parton, 1999) and hedgehog (Karpen et al., 2001); heterotrimeric G proteins; and phospholipid-binding proteins such as annexins (Babiychuk et al., 2002). One morphologically identifiable raft structure is the caveola (Kurzchalia and Parton, 1999). Caveolae are flask-shaped membrane invaginations found in the plasma membranes of several types of cell enriched in their scaffolding proteins, caveolins. Overexpression of these proteins in cells lacking caveolae, such as lymphocytes and neuronal cells, can induce the formation of caveolae (Fra et al., 1995), and targeted disruption of caveolin 1 in mice leads to the disappearance of morphologically recognisable caveolae. Flotillins are non-caveolar proteins that localise to microdomains and probably function as raft organisers. Annexins have also been reported to organise rafts in a calcium-dependent fashion.Rafts are dynamic and this means that both proteins and lipids can move in and out of raft domains with different partitioning kinetics. Despite much evidence supporting the existence of raft domains, the size and the functions of these domains are debated (Edidin, 2003). The controversy mainly arises because these domains are too small to be optically resolved. However, recent advances in imaging are now providing insights into their behaviour (Gaus et al., 2003; Parton and Richards, 2003; Pralle et al., 2000). Depending on the time-resolution of the technique used, different properties can be revealed (Kusumi et al., 2004).The first method to biochemically define lipid rafts was based on the resistance of lipid rafts to extraction by Triton X-100 at 4°C (Brown and Rose, 1992). These DRM fractions are aggregates of raft domains and thus do not represent the native state of lipid rafts in cell membranes (Munro, 2003). One confusion in this field was caused by the equation of lipid rafts to caveolae (Anderson, 1998). Caveolins are clearly a part of DRMs in cells that express these proteins but form a subclass of rafts (as explained above). A number of new methods are being introduced to study rafts in cells and this field needs better methodology if we are to come to grips with these elusive membrane domains.At steady state, rafts are too small to engage in raft-associated processes. Whatever their size is, researchers agree that these domains contain only a few proteins. To engage in membrane function, they usually have to cluster together. There is increasing evidence that the outer leaflet domains and the inner leaflet domains are coupled in raft clusters (Gri et al., 2004). Raft clustering can be accomplished from both sides of the plasma membrane. Antibodies, antigens or raft-lipid-binding proteins such as cholera toxin B, cluster rafts on the extracellular side of the membrane whereas raft-clustering proteins such annexins, flotillins or other scaffolding proteins could serve as clustering agents for the rafts in the cytoplasmic leaflet. Clustered rafts