Lipid raft

Indeed, Kervin and Overduin imply that lipid rafts are misconstrued protein islands, which they propose form through a proteolipid code.

Cholesterol can pack in between the lipids in rafts, serving as a molecular spacer and filling any voids between associated sphingolipids.

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.

When such a detergent is added to cells, the fluid membrane will dissolve while the lipid rafts may remain intact and could be extracted.

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.

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).

In 1978, X-Ray diffraction studies led to further development of the "cluster" idea defining the microdomains as "lipids in a more ordered state".

Karnovsky's studies showed heterogeneity in the lifetime decay of 1,6-diphenyl-1,3,5-hexatriene, which indicated that there were multiple phases in the lipid environment of the membrane.

The original concept of rafts was used as an explanation for the transport of cholesterol from the trans Golgi network to the plasma membrane.

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.

[40][41][42] Evidence for this fact includes decreased solubility of Fc-epsilon receptors (FcεR) in Triton X-100 from steady state to crosslinking state,[40] formation of patches large enough to be visualized by fluorescence microscopy from gangliosides and GPI-anchored proteins,[43][44] abolition of IgE signaling by surface cholesterol depletion with methyl-β-cyclodextrin[45] 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.

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.

One possible mechanism of down-regulating this signaling involves the binding of cytosolic kinase Csk to the raft associated protein CBP.

[50] B cell antigen receptor (BCR) is a complex between a membrane bound Ig (mIg) molecule and a disulfide-linked Igα- Igβ heterodimer of two polypeptides.

Accumulated evidence supports that viruses enter cells via penetration of specific membrane microdomains, including lipid rafts.

The best studied models of lipid rafts-related nonenveloped viral entry are simian virus 40 (SV40, Papovaviridae) and echovirus type 1 (EV1, Picornaviridae).

[52][53] SV40 utilizes two different receptors to bind onto cell surface: ganglioside GM1 located in lipid rafts and major histocompatibility (MHC) class I molecule.

[53] Similar to SV40, attachment and binding with cells triggers clustering and relocation of integrin molecules from lipid rafts to the caveolae-like structures.

[53] Influenza viruses bind to the cellular receptor sialic acid, which links to glycoconjugate on the cell surface, to initiate endocytosis.

An alternative receptor for HIV-1 envelope glycoprotein on epithelial cells is glycosphingolipid galactosyl-ceramide (GalCer), which enriches at lipid raft.

[6] Due to their size being below the classical diffraction limit of a light microscope, lipid rafts have proved difficult to visualize directly.

[62] To combat the problems of small size and dynamic nature, single particle and molecule tracking using cooled, sensitive CCD cameras and total internal reflection (TIRF) microscopy is coming to prominence.

[63] Other optical techniques are also used: Fluorescence Correlation and Cross-Correlation Spectroscopy (FCS/FCCS) can be used to gain information of fluorophore mobility in the membrane, Fluorescence Resonance Energy Transfer (FRET) can detect when fluorophores are in close proximity and optical tweezer techniques can give information on membrane viscosity.

[26] Not only optical techniques, but also scanning probe techniques like atomic force microscopy (AFM) or Scanning Ion Conductance Microscopy (SICM) can be used to detect the topological and mechanical properties of synthetic lipids[64] or native cell membranes[65] isolated by cell unroofing.

Also used are dual polarisation interferometry, Nuclear Magnetic Resonance (NMR) although fluorescence microscopy remains the dominant technique.

[67][68][1] The role of rafts in cellular signaling, trafficking, and structure has yet to be determined despite many experiments involving several different methods, and their very existence is controversial despite all the above.

Pike and Miller discuss potential pitfalls of using cholesterol depletion to determine lipid raft function.