7.19.2 Change in Liposome Shape Accompanying Severing
of Actin Filaments
There are shape changes of liposomes, which is driven by a different mechanism
[186]. As described above, liposome assumed disk-like shape as a result of slow
polymerization, which remained unchanged over a long period (at least 20 min after
the final shapes were reached). However, when cytochalasin was administered to the
disk-shaped liposome, further shape change occurred: the liposome transformed to a
rugby ball-like shape. This was accompanied by a re-arrangement of actin filaments
from the original circular distribution to a rather uniform alignment along the longer
axis of the rugby ball. The change in the alignment was deduced by the polarization
microscopy of the encapsulated actin labeled with acrylodan [40]. Cytochalasin
inhibits the actin polymerization, but it also severs the actin filaments. Since
cytochalasin is membrane-permeable, it probably entered the liposome and severed
the encapsulated filaments. Perhaps, this altered the force balance between the actin
filaments and the lipid membrane, leading to the transformation. This is another
example of sculpting of the lipid membrane by the structure of actin filaments, rather
than polymerization force. To summarize, actin has an ability to mold the shape of
lipid membranes, and actin cross-linking and bundling proteins should strongly
enhance this ability by stabilizing various types of actin-based structures.
7.19.3 Liposomes Encapsulating Tubulin
The polymerization-coupled force has been demonstrated with liposomes containing
tubulin, not actin [187, 188]. Tubulin dimer was encapsulated in liposomes on ice
and was allowed to polymerize at 37
C. The shape change of the tubulin-containing
liposome follows considerably different path than the actin-containing liposome.
Thus in the former case, small projections first appear at the two opposite ends of a
liposome, altering the liposome shape to a lemon-like shape. The protrusions further
grew in the opposite directions, and the main part of the liposome again became
spherical. The remarkable difference from the case of actin is likely to stem from the
large difference in the rigidity of two polymer systems; a single microtubule is about
one hundred times higher than that of a single actin filament [189]. Hence, even a
single microtubule was sufficient for the generation of membrane projection.
It has been shown that a thin tube (tether) grows by simply pulling glass bead that
had been attached to the membrane of a liposome [87] or a red blood cell [190]. In
another study, the end-to-end distance of the lemon-shaped liposome has been
measured as a function of the axial force [191]. Micron-sized plastic beads were
encapsulated in a giant liposome and were manipulated by the double-optical trap
system: two beads were captured in each trap and were brought apart by deflection of the
laser beam. As a result, the beads exerted forces on the lipid membrane at the opposite
locations. An increase in the inter-bead distance first induced a lemon-shape and
142
7 Moving Life
of Actin Filaments
There are shape changes of liposomes, which is driven by a different mechanism
[186]. As described above, liposome assumed disk-like shape as a result of slow
polymerization, which remained unchanged over a long period (at least 20 min after
the final shapes were reached). However, when cytochalasin was administered to the
disk-shaped liposome, further shape change occurred: the liposome transformed to a
rugby ball-like shape. This was accompanied by a re-arrangement of actin filaments
from the original circular distribution to a rather uniform alignment along the longer
axis of the rugby ball. The change in the alignment was deduced by the polarization
microscopy of the encapsulated actin labeled with acrylodan [40]. Cytochalasin
inhibits the actin polymerization, but it also severs the actin filaments. Since
cytochalasin is membrane-permeable, it probably entered the liposome and severed
the encapsulated filaments. Perhaps, this altered the force balance between the actin
filaments and the lipid membrane, leading to the transformation. This is another
example of sculpting of the lipid membrane by the structure of actin filaments, rather
than polymerization force. To summarize, actin has an ability to mold the shape of
lipid membranes, and actin cross-linking and bundling proteins should strongly
enhance this ability by stabilizing various types of actin-based structures.
7.19.3 Liposomes Encapsulating Tubulin
The polymerization-coupled force has been demonstrated with liposomes containing
tubulin, not actin [187, 188]. Tubulin dimer was encapsulated in liposomes on ice
and was allowed to polymerize at 37
C. The shape change of the tubulin-containing
liposome follows considerably different path than the actin-containing liposome.
Thus in the former case, small projections first appear at the two opposite ends of a
liposome, altering the liposome shape to a lemon-like shape. The protrusions further
grew in the opposite directions, and the main part of the liposome again became
spherical. The remarkable difference from the case of actin is likely to stem from the
large difference in the rigidity of two polymer systems; a single microtubule is about
one hundred times higher than that of a single actin filament [189]. Hence, even a
single microtubule was sufficient for the generation of membrane projection.
It has been shown that a thin tube (tether) grows by simply pulling glass bead that
had been attached to the membrane of a liposome [87] or a red blood cell [190]. In
another study, the end-to-end distance of the lemon-shaped liposome has been
measured as a function of the axial force [191]. Micron-sized plastic beads were
encapsulated in a giant liposome and were manipulated by the double-optical trap
system: two beads were captured in each trap and were brought apart by deflection of the
laser beam. As a result, the beads exerted forces on the lipid membrane at the opposite
locations. An increase in the inter-bead distance first induced a lemon-shape and
142
7 Moving Life