BioPHYSICAL chemistry

regions were rich in cholesterol, glycos-

phingolipids, and lipid-anchored proteins.

When the cell cholesterol was depleted,

the low-density fraction was not observed.

In model membrane systems, mixtures of

these lipids have been found to organize

domains. The simple interpretation of these

experiments was that the low-density region

exists in the cell membrane as a so-called

lipid raft. However, the complexity of the

membrane allows for the possibility that

this simple interpretation is not correct due

to unforseen artifacts. Support for such

localized regions is provided by antibody

patching and immunofluorescence micro-

scopy but variation in the results among

cells makes quantification difficult. Other

experiments, such as tracking the location

of single fluorophores to monitor the dif-

fusion of individual raft proteins or lipids,

would potentially be very revealing but such measurements are technic-

ally very challenging.

Interest in lipid rafts rose when it was recognized that many critical

membrane-associated proteins could be part of lipid rafts (Simons & Toomre

2000; Kenworthy 2002). It appears that the rafts are centers for receptor-

mediated signaling involving various membrane receptors such as the

T-cell, B-cell, epidermal growth factor, and insulin receptors. In many cases,

the receptors belong to a class of proteins called glycosylphosphatidyli-

nositol (GPI)-anchored proteins. GPI, is a hydrophobic molecule that is

attached to the C-terminus of certain proteins. The GPI molecule allows the

protein to be closely associated with the cell membrane through a flexible

tether while having the ability to interact with different proteins located

on the extracellular side of the membrane. Enrichment of GPI-anchored

proteins is observed in the low-density fractions, and treatments that dis-

rupt the proposed rafts also disrupt the receptor function. A simple model

of lipid rafts is that the rafts segregate in the lipid bilayer and sequester

proteins attached by the GPI anchors. Several proteins are known to depend

upon cholesterol that is enriched in the raft. Making analysis of these

domains difficult is both the small size of the rafts and the presence of

invaginations called caveolae that are rich in specific lipids. Although

separate from rafts, the presence of the caveolae may be regulated by

lipid rafts. Adding to the complexity of rafts is the observation that the

distribution of lipid rafts is not uniform. This is presumably due to the

function of the rafts, such as a proposed role in cellular trafficking. For

example, the rafts may serve as sorting platforms to which vesicular

integral membrane proteins associate as stabilizers for other proteins in

78 PARTI THERMODYNAMICS AND KINETICS

Low density

High density

(a)

(b)

?



?

Normal cell Cholesterol depleted

Lipid rafts on the

unperturbed cell membrane

GPI-anchored

and other raft

proteins

Cholesterol Glycosphingolipids



Figure 4.7Density
determinations of
cell membrane
fractions show the
presence of a low-
density component
assigned to a lipid-
raft contribution in
(a) normal cells but
not (b) cholestrol-
depleted cells.
Modified from
(Edidin 2001).