Lipid rafts are described as specialized structures, or as dynamic cell membrane microdomains. They are believed to be glycolipoprotein lipid microdomains containing a combination of glycosphingolipids, cholesterol, and protein that is recruited, including receptors. Although initially described in the plasma membrane and Golgi membrane, lipid rafts have also been reported in other endomembranes such as lysosomes. However, it is questionable whether it is appropriate to label any such structures lipid rafts, given that their composition and properties deviate considerably from the original lipid raft theory. The lipid raft theory has a history of being redefined in response to contradictory evidence.
Cholesterol is an intrinsic component of the cell membrane where it maintains the stability and fluidity of the membrane. Cholesterol regulates protein signaling as a major organiser of lipid rafts. Lipid rafts serve as organising centers for assembling signalling molecules, and influence membrane protein trafficking, to regulate different cell processes such as neurotransmission and receptor trafficking.
Lipid rafts are more ordered and tightly packed than the surrounding bilayer, but float freely within the membrane bilayer.
Some researchers propose that lipid rafts are an incidental feature of only incidental physiological significance.
Properties
One key difference between lipid rafts and the plasma membranes from which they are derived is lipid composition. Research has shown that lipid rafts contain 3 to 5-fold the amount of cholesterol found in the surrounding bilayer. Also, lipid rafts are enriched in sphingolipids such as sphingomyelin, which is typically elevated by 50% compared to the plasma membrane. To offset the elevated sphingolipid levels, phosphatidylcholine levels are decreased which results in similar choline-containing lipid levels between the rafts and the surrounding plasma membrane. Cholesterol interacts preferentially, although not exclusively, with sphingolipids due to their structure and the saturation of the hydrocarbon chains. Although not all of the phospholipids within the raft are fully saturated, the hydrophobic chains of the lipids contained in the rafts are more saturated and tightly packed than the surrounding bilayer. Cholesterol is the dynamic "glue" that holds the raft together. Due to the rigid nature of the sterol group, cholesterol partitions preferentially into the lipid rafts where acyl chains of the lipids tend to be more rigid and in a less fluid state. One important property of membrane lipids is their amphipathic character. Amphipathic lipids have a polar, hydrophilic head group and a non-polar, hydrophobic region. The figure to the right shows the inverted cone-like shape of sphingomyelin and the cone-like shape of cholesterol based on the area of space occupied by the hydrophobic and hydrophilic regions. Cholesterol can pack in between the lipids in rafts, serving as a molecular spacer and filling any voids between associated sphingolipids.
Rietveld & Simons related lipid rafts in model membranes to the immiscibility of ordered (Lo phase) and disordered (Ld or Lα phase) liquid phases. The cause of this immiscibility is uncertain, but is thought to minimize the free energy between the two phases. Studies have shown there is a difference in thickness of the lipid rafts and the surrounding membrane which results in hydrophobic mismatch at the boundary between the two phases. This phase height mismatch has been shown to increase line tension which may lead to the formation of larger and more circular raft platforms to minimize the energetic cost of maintaining the rafts as a separate phase. Other spontaneous events, such as curvature of the membrane and fusing of small rafts into larger rafts, can also minimize line tension.
By one early definition of lipid rafts, lipid rafts differ from the rest of the plasma membrane. In fact, researchers have hypothesized that the lipid rafts can be extracted from a plasma membrane. The extraction would take advantage of lipid raft resistance to non-ionic detergents, such as Triton X-100 or Brij-98 at low temperatures (e.g., 4 °C). When such a detergent is added to cells, the fluid membrane will dissolve while the lipid rafts may remain intact and could be extracted.
Because of their composition and detergent resistance, lipid rafts are also called detergent-insoluble glycolipid-enriched membrane (GEM) complexes or DIGs or Detergent Resistant Membranes (DRMs). However the validity of the detergent resistance methodology of membranes has recently been called into question due to ambiguities in the lipids and proteins recovered and the observation that they can also cause solid areas to form where there were none previously.
Function Mediation of substrate presentation. Lipid rafts localize palmitoylated proteins away from the disordered region of the plasma membrane. Disruption of palmitate mediated localization then allows for exposure of a protein to its binding partner or substrate in the disordered region, an activation mechanism termed substrate presentation. For example, a protein is often palmitoylated and binds phosphatidylinositol 4,5-biphosphate (PIP2). PIP2 is polyunsaturated and does not reside in lipid rafts. When the levels of PIP2 increase in the plasma membrane, the protein trafficks to PIP2 clusters where it can be activated directly by PIP2 (or another molecule that associates with PIP2).
It is probable that other functions exist.
Common types Two types of lipid rafts have been proposed: planar lipid rafts (also referred to as non-caveolar, or glycolipid, rafts) and caveolae. Planar rafts are defined as being continuous with the plane of the plasma membrane (not invaginated) and by their lack of distinguishing morphological features. Caveolae, on the other hand, are flask shaped invaginations of the plasma membrane that contain caveolin proteins and are the most readily observed structures in lipid rafts. Caveolins are widely expressed in the brain, micro-vessels of the nervous system, endothelial cells, astrocytes, oligodendrocytes, Schwann cells, dorsal root ganglia and hippocampal neurons. Planar rafts contain flotillin proteins and are found in neurons where caveolae are absent. Both types have similar lipid composition (enriched in cholesterol and sphingolipids). Flotillin and caveolins can recruit signaling molecules into lipid rafts, thus playing an important role in neurotransmitter signal transduction. It has been proposed that these microdomains spatially organize signaling molecules to promote kinetically favorable interactions which are necessary for signal transduction. Conversely, these microdomains can also separate signaling molecules, inhibiting interactions and dampening signaling responses.
Role in signal transduction The specificity and fidelity of signal transduction are essential for cells to respond efficiently to changes in their environment. This is achieved in part by the differential localization of proteins that participate in signalling pathways. In the plasma membrane, one approach of compartmentalization utilizes lipid rafts.
One reasonable way to consider lipid rafts is that small rafts can form concentrating platforms after ligand binding activation for individual receptors. Lipid rafts have been found to be involved in a number of signal transduction processes, such as Immunoglobulin E signalling, T cell antigen receptor signalling, B cell antigen receptor signalling, EGF receptor signalling, insulin receptor signalling and so on. In order to illustrate these principles, detailed examples of signalling pathways that involve lipid rafts are described below.
Epidermal growth factor signaling Epidermal growth factor (EGF) binds to EGF receptor, also known as HER-1 or ErbB1, to initiate transmembrane signaling. Lipid rafts have been suggested to play a bipartite role in this process. Certain aspects of lipid rafts inhibit EGF receptor function: the ganglioside component of lipid rafts was shown to inhibit receptor activation the membrane dipole potential, which was shown to be higher in lipid rafts than in the rest of the membrane, was demonstrated to inhibit EGF binding to its receptor EGF binding was shown to be inhibited by non-caveolar lipid rafts due to a decrease in the number of receptors available for ligand binding EGF and ErbB2 (HER-2) were shown to migrate out of lipid rafts or caveolae during or after activation disruption of lipid rafts was shown to induce ligand-independent activation of EGF receptor At the same time lipid rafts seem to be necessary for or potentiate transmembrane signaling: sequestration of ErbB2 from lipid rafts have been shown to inhibit EGF-induced signaling the membrane dipole potential, which is higher in lipid rafts than in the rest of the membrane, potentiates EGF-induced signaling EGF was shown to bring about coalescence of individual lipid rafts, similar to what has been suggested to play a role in the activation of the T-cell receptor localization of EGF receptor to lipid rafts induces resistance to tyrosine kinase inhibitors
Immunoglobulin E signaling
Immunoglobulin E (IgE) signaling is the first convincingly demonstrated signaling process which involves lipid rafts. Evidence for this fact includes decreased solubility of Fc-epsilon receptors (FcεR) in Triton X-100 from steady state to crosslinking state, formation of patches large enough to be visualized by fluorescence microscopy from gangliosides and GPI-anchored proteins, abolition of IgE signaling by surface cholesterol depletion with methyl-β-cyclodextrin and so on.
This signaling pathway can be described as follows: IgE first binds to Fc-epsilon receptors (FcεR) residing in the plasma membrane of mast cells and basophils through its Fc segment. FcεR is a tetramer consist of one α, one β and two γ chains. It is monomeric and binds one IgE molecule. The α chain binds IgE and the other three chains contain immune receptor tyrosine-based activation motifs (ITAM). Then oligomeric antigens bind to receptor-bound IgE to crosslink two or more of these receptors. This crosslinking then recruits doubly acylated non-receptor Src-like tyrosine kinase Lyn to phosphorylate ITAMs. After that, Syk family tyrosine kinases bind these phosphotyrosine residues of ITAMs to initiate the signaling cascade. Syk can, in turn, activate other proteins such as LAT. Through crosslinking, LAT can recruit other proteins into the raft and further amplify the signal.
T-cell antigen receptor signaling
T cell antigen receptor (TCR) is a molecule found on the surface of T lymphocytes (T cells). It is composed of αβ-heterodimers, CD3 (γδε) complex and ξ-homodimer. The α- and β- subunits contain extracellular binding sites for peptides that are presented by the major histocompatibility complex (MHC) class I and class II proteins on the surface of antigen presenting cells (APCs). The CD3 and ξ- subunits contain cytoplasmic ITAM motifs. During the signaling process, MHCs binding to TCRs brings two or more receptors together. This crosslinking, similar to IgE signaling, then recruits doubly acylated non-receptor Src-like tyrosine kinases to phosphorylate ITAM tyrosine residues. In addition to recruiting Lyn, TCR signaling also recruits Fyn.
Following this procedure, ZAP-70 (which is also different with IgE signalling) binds to phosphorylated ITAMs, which leads to its own activation and LAT activation. LAT activation is the source of signal amplification. Another difference between IgE and T cell antigen receptor signalling is that Lck activation by TCR could result in more severe raft clustering thus more signal amplification. One possible mechanism of down-regulating this signaling involves the binding of cytosolic kinase Csk to the raft associated protein CBP. Csk may then suppress the Src-family kinases through phosphorylation.