From Wikipedia,
the free encyclopedia.
A lipid raft is a
cholesterol-rich microdomain
in
cell membranes.
Since
1972, it has been believed
that, in cell membranes,
phospholipids and membrane
proteins are ubiquitously
distributed according to a
fluid mosaic model. However,
in
1988,
Kai Simons at the
European Molecular Biology
Laboratory (EMBL) in
Germany suggested the novel
idea that there exist microdomains,
which are enriched with many kinds
of lipids such as cholesterol,
glycolipids, and
sphingolipids, present in cell
membranes. He called these
microdomains "lipid rafts". The
concept of lipid rafts has been
related to the liquid-liquid
immiscibility observed within
model membranes. This
immiscibility has been reported
between two
liquid crystalline phases, the
liquid ordered phase (Lo phase)
and the liquid disordered phase
(Ld or Lalpha phase).
The evidence for lipid rafts is
still very controversial and has
yet to be confirmed despite many
experiments involving several
different methods e.g. detergent
extraction and fluorescence cell
imaging. Furthermore, recent
studies of cell membranes reported
that lipid rafts contain not only
lipid components but also numerous
signaling molecules such as
GPI-anchored proteins and
receptor or non-receptor type
tyrosine kinase, which
modulate various physiological
processes in cells. Therefore,
lipid rafts are thought to act as
a platform for protein segregation
and signaling in cells. However,
the functional role of lipid rafts
still remains very uncertain
because it has not been determined
whether they have a positive
effect or negative effect on
signal transduction in cells.
Investigating the effect of the
lipid components on cellular
signaling pathways may provide a
novel understanding of the roles
of lipid rafts in cells.
