membrane skeleton + membrane-bound myosin; [152]). The bleb is created by the
increase in the intracellular pressure due to the contraction of the cell cortex
(Fig. 7.24d represents the contraction of the tail of the cell). In this case, polymerizing actin will fill the space in the bleb to form a protrusion (pseudopod in ameba;
[153, 154]). The blebbing-based protrusive growth seems to be suitable for quick
determination of the direction of the cell in response to the change in the distribution
of extracellular signaling molecules.
It seems that living organisms have achieved the protrusive activity by utilizing
every possible mechanism that probably had been explored in the process of
evolution.
7.17 Other Motility Based on Actin Polymerization
The motility depending on the actin polymerization is often called “actin-based
motility”. There seem to be a number of modifications in coupling of actin polymerization to the biological work. Acrosomal reaction, in which the elongation of
the structure called acrosomal process from the head of the sperm of sea cucumber,
Thyone, in the initial stage of fertilization, is thought to be an example of the
polymerization-coupled force generation [155, 156]. At the base of the future
acrosomal process in the sperm head, actin is complexed with an actin binding
protein that prevents the polymerization of monomer and hence is stored as a
non-polymerized form. In the acrosomal reaction, the tip of the head comes into
contact with the jelly coat layer of the egg, and actin is freed from the binding protein
that had prohibited the polymerization. Like in lamellipodium, the freed actin rapidly
polymerizes toward the tip to form the acrosomal process filled with actin bundle;
there is no myosin II in the acrsosomal process. The rate of elongation, ~10 μm/s,
seemed to be unable to be caught up by diffusion of actin from the base of the
acrosome [155], but later, the discrepancy has been interpreted as the increased rate
of actin transport by decrease in the volume of the actin reservoir and the increase in
the chemical potential of actin monomer due to the high actin concentration
[156]. This model has suggested that the speed of the extension is limited by the
rate of delivery; the mechanism of the generation of the work may be the same as
what is operating in lamellipodia, although it was not specified in the acrosomal
study.
Another example of the actin-based motility is the propulsion of bacteria such as
Listeria and Shigella in cytoplasm [90]. These bacteria enter the cell through
phagocytosis (ingestion of small particles by invagination of the cell membrane).
After entering the cell it moves around in the cytoplasm at 1 to 10 μm/s, depending
on bacteria species, evading the host defense mechanism. At the end of the intracellular stage of its life cycle, the bacterium pushes the cell membrane and a long
process is formed. This membranous process is phagocytosed by the surrounding
cells and the bacteria further spread. Electron microscopy demonstrated that the
intracellular trail of the moving bacterium (called comet due to the morphological
similarity) contains actin of the host cell [157], and that the barbed end of each
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7 Moving Life
increase in the intracellular pressure due to the contraction of the cell cortex
(Fig. 7.24d represents the contraction of the tail of the cell). In this case, polymerizing actin will fill the space in the bleb to form a protrusion (pseudopod in ameba;
[153, 154]). The blebbing-based protrusive growth seems to be suitable for quick
determination of the direction of the cell in response to the change in the distribution
of extracellular signaling molecules.
It seems that living organisms have achieved the protrusive activity by utilizing
every possible mechanism that probably had been explored in the process of
evolution.
7.17 Other Motility Based on Actin Polymerization
The motility depending on the actin polymerization is often called “actin-based
motility”. There seem to be a number of modifications in coupling of actin polymerization to the biological work. Acrosomal reaction, in which the elongation of
the structure called acrosomal process from the head of the sperm of sea cucumber,
Thyone, in the initial stage of fertilization, is thought to be an example of the
polymerization-coupled force generation [155, 156]. At the base of the future
acrosomal process in the sperm head, actin is complexed with an actin binding
protein that prevents the polymerization of monomer and hence is stored as a
non-polymerized form. In the acrosomal reaction, the tip of the head comes into
contact with the jelly coat layer of the egg, and actin is freed from the binding protein
that had prohibited the polymerization. Like in lamellipodium, the freed actin rapidly
polymerizes toward the tip to form the acrosomal process filled with actin bundle;
there is no myosin II in the acrsosomal process. The rate of elongation, ~10 μm/s,
seemed to be unable to be caught up by diffusion of actin from the base of the
acrosome [155], but later, the discrepancy has been interpreted as the increased rate
of actin transport by decrease in the volume of the actin reservoir and the increase in
the chemical potential of actin monomer due to the high actin concentration
[156]. This model has suggested that the speed of the extension is limited by the
rate of delivery; the mechanism of the generation of the work may be the same as
what is operating in lamellipodia, although it was not specified in the acrosomal
study.
Another example of the actin-based motility is the propulsion of bacteria such as
Listeria and Shigella in cytoplasm [90]. These bacteria enter the cell through
phagocytosis (ingestion of small particles by invagination of the cell membrane).
After entering the cell it moves around in the cytoplasm at 1 to 10 μm/s, depending
on bacteria species, evading the host defense mechanism. At the end of the intracellular stage of its life cycle, the bacterium pushes the cell membrane and a long
process is formed. This membranous process is phagocytosed by the surrounding
cells and the bacteria further spread. Electron microscopy demonstrated that the
intracellular trail of the moving bacterium (called comet due to the morphological
similarity) contains actin of the host cell [157], and that the barbed end of each
134
7 Moving Life