Conclusions

Although it is easy to lose one's way when interpreting the many biophysical approaches of analyzing the existence of lipid rafts, few conclusions could be drawn without significant dissent. Model membrane studies are vital in understanding lipid-lipid and lipid-protein interaction, but there is still a long way to go in understanding multicomponent complex biological membranes. This could be analogous to the "three-body problem" where the gravitational interaction of three masses is surprisingly difficult to solve. In the light of information that small transition energies could bring about remarkable structural changes, there may be serious problems in extending model membrane studies to biological membranes. This includes all model membrane studies which support the concept of "detergent-insoluble" complexes to describe lipid rafts. Although diffusion-based studies provide valuable live cell data, they have several drawbacks. It is not easy to interpret results from diffusion data due to a poor understanding of: (1) the diffusion of molecules in complex biological membranes; (2) the ultrastructure of biological membranes; and (3) interaction of the cytoskeleton with biological membranes. Diffusion data might provide a valuable time-kinetics, though it is difficult to deduce structural information from diffusion data with current theoretical understanding. Although homo-FRET-based studies provide an excellent source of structural information on proximity relationships derived by lipidic interactions, the relatively longer time scales of measurement mask the temporal information. In the absence of any ideal "sub-resolution ultra-fast imaging" technique, it is desirable to study chosen model raft components in a model cellular system with available biophysical techniques, and appropriate functional consequences.

Abbreviations

AP

anchored protein

CtxB

cholera toxin B

DIG

detergent-insoluble glycolipid

Dil

dialkylcarbocyanine

DLPC

dilauroyl phosphatidylcholine

DPPC

dipalmitoyl phosphatidylcholine

DRM

detergent-resistant membrane

EM

electron microscopy

ER

endoplasmic reticulum

FCS

fluorescence correlation spectroscopy

FL-DOPE

1,2-dio-leoyl-stt-glycero-3-phosphoethanolamine-fluorescein

FL-DPPE

1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-fluorescein

fl-PE

fluorescein phosphatidylethanolamine

FRAP

fluorescence recovery after photobleaching

FRET

Foerster's resonance energy transfer

GPI

glycosyl-phosphatidylinositol

GUV

giant unilamellar vesicle

HA

hemagglutinin

HCASM

human coronary artery smooth muscle

LPH

lactase-phlorizin hydrolase

mbCD

methyl- b-cyclodextrin

MDCK

Madin-Darby canine kidney

NCAM

neural cell adhesion molecule

NMR

nuclear magnetic resonance

5'-NT

5'-nucleotidase

PAGE

polyacrylamide gel electrophoresis

PC

phosphocholine

PLAP

placental alkaline phosphatase

RBL

rat basophilic leukemia

SM

sphingomyelin

SPT

single-particle tracking

SSR

surface scanning resistance

TCZ

transient confinement zone

TfR

transferrin receptor

TGN

trans-Golgi network

YFP

yellow fluorescent protein

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