Biology Reference
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barrels, lipid-linked proteins) were discovered after the Fluid Mosaic model was first formu-
lated in 1972.
Table 8.1 lists the salient features of the Fluid Mosaic model. Some of these features (e.g.
b
F. SIMONS' LIPID RAFT MODEL
In the original 1972 Fluid Mosaic model, Singer and Nicolson did not postulate lateral
heterogeneity into functional membrane domains although it was generally accepted that
membrane phospholipids and proteinswere probably heterogeneously distributed. However,
in the 1970s, reports were starting to emerge that membrane 'lipid clusters' may be common
features of biological membranes. In a 1982 paper in the Journal of Cell Biology, Karnovsky
and Klausner formalized the concept of lipid domains in membranes [14] . Interestingly, one
type of lipid cluster that kept arising was that of cholesterol and sphingolipids [15,16] . Indeed,
protein-free lipid membrane models showed that cholesterol has a higher affinity for SM
(sphingomyelin) than it does for other phospholipids [16] . Although lipid phase separations
in membranes were starting to achieve acceptance, their biological relevance awaited a link
to a biological function. The link proved to be spectacular when cholesterol- and sphingolipid-
rich microdomains were closely associated with essential cellular signaling events. However,
the real interest in these microdomains did not come until after they received the very catchy
and descriptive name 'Lipid Raft'. This critical link was made by Kai Simons ( Figure 8.7 )in
a 1997 paper in Nature [17] . Membrane studies would be changed forever.
Since 1997, lipid rafts have generated countless primary research papers and many excel-
lent reviews (e.g. [18
25] . However, one wonders if these microdomains had been more
correctly called 'phase separated cholesterol- and sphingolipid-rich, liquid ordered microdo-
mains' instead of 'Lipid Rafts' whether they would have achieved such notoriety!
Lipid rafts are cholesterol- and sphingolipid-rich, liquid ordered (l o ) state platforms
(microdomains) that float in a non-raft liquid disordered matrix ( Figure 8.8 .). The l o state
has properties midway between those of a liquid crystalline (disordered, fluid) and gel
e
FIGURE 8.7 Kai Simons (1938 e ). Courtesy of BMBF/Unternehmen Region.
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