lipid bilayer membrane is on the order of 10
-3 N/m with maximum extensibility of a
few percent. Thus, the lipid bilayer is rather like a cloth than a rubber sheet. The cell
membrane is lined by a network structure called membrane skeleton. For example,
red blood cell membrane is lined by a highly regular network made of a protein
called spectrin [86]. Interestingly, the mechanical measurement has revealed that the
bending rigidity of the red blood cell membrane is similar to that of pure lipid
membrane [87]. This may be because the spectrin network is a highly flexible entity
([88]; see below).
The spectrin network is composed of spectrin and short actin filaments. Spectrin
is a heterodimer of alpha- and beta-subunit [89]. An alpha (a beta)-subunit in one
heterodimer binds to a beta (an alpha)-subunit of another heterodimer, and hence,
two heterodimers make a spectrin tetramer. The spectrin tetramer assembly is then
tied to the membrane by binding to a protein called ankyrin, which is then bound to a
transmembrane anion exchanger protein (band 3). Several heterodimers bind to a
short actin filament and a protein called band 4.1 to form spectrin/band 4.1/actin
junction. This protein complex is then tied to the cell membrane through a number of
transmembrane proteins. Thus, to the cytoplasmic side of the membrane of a red
blood cell, the spectrin-based network structure is firmly attached. The network is
most clearly seen in the plasma membrane isolated from red blood cell, which is
literally a net when it is viewed under the electron microscopy [86, 90].
In animal cells, actin filaments make another type of membrane skeleton [91]; this
skeleton is closely (~10 nm) attached to the whole plasma membrane, sandwiching
the spectrin membrane skeleton [92]. It has been demonstrated that two populations
of actin filaments exist in the membrane skeleton, which are distinguished by their
length. The growth of the longer filaments was induced by an actin-nucleating
protein called formin, while the shorter filaments were induced by a protein complex
called arp2/3 that also is an actin nucleating protein (see Sect. 7.15). The longer
filaments seem to provide the plasma membrane with more mechanical strength than
arp2/3-nucleated actin filements [93].
With the lining, the lipid membrane is expected to become stronger against
mechanical deformation. However, as mentioned above, the bending rigidity of
the red blood cell membrane is not very much higher than the pure lipid bilayer
membrane [87]. This may be because of the highly compliant nature of the spectrin
network, as revealed by direct pulling of the isolated spectrin network [88]. In
addition, the coupling of the spectrin network to the lipid bilayer may be weak in
the plane of the membrane, and the two structures may respond to the external force
almost independently. If this is the case, the bending rigidity may not be much
greater the sum of the rigidity of the two structures. Since red blood cells must pass
through narrow capillary vessels, their membrane structure might result from the
compromise between the deformability and mechanical strength.
122
7 Moving Life
-3 N/m with maximum extensibility of a
few percent. Thus, the lipid bilayer is rather like a cloth than a rubber sheet. The cell
membrane is lined by a network structure called membrane skeleton. For example,
red blood cell membrane is lined by a highly regular network made of a protein
called spectrin [86]. Interestingly, the mechanical measurement has revealed that the
bending rigidity of the red blood cell membrane is similar to that of pure lipid
membrane [87]. This may be because the spectrin network is a highly flexible entity
([88]; see below).
The spectrin network is composed of spectrin and short actin filaments. Spectrin
is a heterodimer of alpha- and beta-subunit [89]. An alpha (a beta)-subunit in one
heterodimer binds to a beta (an alpha)-subunit of another heterodimer, and hence,
two heterodimers make a spectrin tetramer. The spectrin tetramer assembly is then
tied to the membrane by binding to a protein called ankyrin, which is then bound to a
transmembrane anion exchanger protein (band 3). Several heterodimers bind to a
short actin filament and a protein called band 4.1 to form spectrin/band 4.1/actin
junction. This protein complex is then tied to the cell membrane through a number of
transmembrane proteins. Thus, to the cytoplasmic side of the membrane of a red
blood cell, the spectrin-based network structure is firmly attached. The network is
most clearly seen in the plasma membrane isolated from red blood cell, which is
literally a net when it is viewed under the electron microscopy [86, 90].
In animal cells, actin filaments make another type of membrane skeleton [91]; this
skeleton is closely (~10 nm) attached to the whole plasma membrane, sandwiching
the spectrin membrane skeleton [92]. It has been demonstrated that two populations
of actin filaments exist in the membrane skeleton, which are distinguished by their
length. The growth of the longer filaments was induced by an actin-nucleating
protein called formin, while the shorter filaments were induced by a protein complex
called arp2/3 that also is an actin nucleating protein (see Sect. 7.15). The longer
filaments seem to provide the plasma membrane with more mechanical strength than
arp2/3-nucleated actin filements [93].
With the lining, the lipid membrane is expected to become stronger against
mechanical deformation. However, as mentioned above, the bending rigidity of
the red blood cell membrane is not very much higher than the pure lipid bilayer
membrane [87]. This may be because of the highly compliant nature of the spectrin
network, as revealed by direct pulling of the isolated spectrin network [88]. In
addition, the coupling of the spectrin network to the lipid bilayer may be weak in
the plane of the membrane, and the two structures may respond to the external force
almost independently. If this is the case, the bending rigidity may not be much
greater the sum of the rigidity of the two structures. Since red blood cells must pass
through narrow capillary vessels, their membrane structure might result from the
compromise between the deformability and mechanical strength.
122
7 Moving Life