heterogeneous regions on biomembranes. Phase separation in lipid bilayer membranes was first observed as Ca2
+
-induced phase separation of acidic phospholipid
by phospholipid spin probe [3] and then phase separation of acidic phospholipid by
basic protein was also observed [4]. Phase separations in lipid bilayer membranes
were observed in various systems such as proton-induced phase separation [5],
ternary lipid system containing phosphatidic acid [6] and diacylglycerol promoting
Ca
2+ -induced phase in lipid bilayers mimicked for cellular signaling [7].
6.3 Micro Domain of Biomembranes (Lipid Raft)
Biological functions of biomembranes are directly responsible for proteins, products
of genetic information, but interaction between lipids and proteins is essential for the
function because most of membrane proteins require lipid for their function. Physical
properties of lipid membranes such as membrane fluidity, phase transition and phase
separation participate in regulation of the function through interaction between lipid
and membrane protein. A cell has various functions in micro regions and the
heterogeneous regions of biomembranes play key role for realizing the functions.
Membrane fractions insoluble in detergent were found in cell membrane, and they
were named lipid rafts from an image of ‘raft’ in sea of lipid bilayers. Raft is a region,
rich in sphingolipids and cholesterol [8] and proteins related to cell signaling were
gathered [9] and cholesterol play a key role for formation of liquid-ordered phase
[10]. The reported phase separated region of liquid ordered phase [10] is recognized
as closely related to formation of raft. Schematic image of lipid raft is shown in
Fig. 6.1. In an experiment for examining preferential interaction of cholesterol with
various phospholipids, selectivity of cholesterol to various phospholipids was
reported as partition coefficient for each phospholipid [11]. Interaction between
cholesterol and sphingomyelin was investigated at each atom of these molecules
by NMR and specific strong interaction was not found between these molecules
Fig. 6.1 Lipid raft as micro domain in biomembranes
Raft is a micro domain as phase separated liquid-ordered phase composed by interaction of
cholesterol with sphingolipids in biomembranes. Proteins related to signaling of cell are assembled
in the micro domain. Violet circles of signaling proteins are gathered into large circles of micro
domains (raft)
6.3 Micro Domain of Biomembranes (Lipid Raft)
83
+
-induced phase separation of acidic phospholipid
by phospholipid spin probe [3] and then phase separation of acidic phospholipid by
basic protein was also observed [4]. Phase separations in lipid bilayer membranes
were observed in various systems such as proton-induced phase separation [5],
ternary lipid system containing phosphatidic acid [6] and diacylglycerol promoting
Ca
2+ -induced phase in lipid bilayers mimicked for cellular signaling [7].
6.3 Micro Domain of Biomembranes (Lipid Raft)
Biological functions of biomembranes are directly responsible for proteins, products
of genetic information, but interaction between lipids and proteins is essential for the
function because most of membrane proteins require lipid for their function. Physical
properties of lipid membranes such as membrane fluidity, phase transition and phase
separation participate in regulation of the function through interaction between lipid
and membrane protein. A cell has various functions in micro regions and the
heterogeneous regions of biomembranes play key role for realizing the functions.
Membrane fractions insoluble in detergent were found in cell membrane, and they
were named lipid rafts from an image of ‘raft’ in sea of lipid bilayers. Raft is a region,
rich in sphingolipids and cholesterol [8] and proteins related to cell signaling were
gathered [9] and cholesterol play a key role for formation of liquid-ordered phase
[10]. The reported phase separated region of liquid ordered phase [10] is recognized
as closely related to formation of raft. Schematic image of lipid raft is shown in
Fig. 6.1. In an experiment for examining preferential interaction of cholesterol with
various phospholipids, selectivity of cholesterol to various phospholipids was
reported as partition coefficient for each phospholipid [11]. Interaction between
cholesterol and sphingomyelin was investigated at each atom of these molecules
by NMR and specific strong interaction was not found between these molecules
Fig. 6.1 Lipid raft as micro domain in biomembranes
Raft is a micro domain as phase separated liquid-ordered phase composed by interaction of
cholesterol with sphingolipids in biomembranes. Proteins related to signaling of cell are assembled
in the micro domain. Violet circles of signaling proteins are gathered into large circles of micro
domains (raft)
6.3 Micro Domain of Biomembranes (Lipid Raft)
83