The polymerization and depolymerization rates are modified greatly by the
binding of microtubule associated proteins (MAPs). For example, the presence of
MAP-2 stabilizes the polymer by decreasing the frequency of disassembly [75].
Dark-field microscopy combined with epi-fluorescence microscopy of microtubules
that was decorated with fluorescently labeled MAP-2 demonstrated that at a low
concentration of MAP-2 formed discrete clusters along the microtubule; based on
this characteristic distribution of bound MAP-2, it has been suggested the rescue
probability significantly increases at the positions of MAP-2 clusters [76]. Thus, the
clusters will decrease the shortening length. The GTP-bound tubulin specifically
labeled with the antibody demonstrated that GTP-tubulin exists not only at the plus
end but also in the middle of microtubule. It has been hypothesized that the
GTP-bound tubulins in the middle of microtubule participate in the rescue
event [70].
7.7 Cell Motility
The movement of the cell has been a central theme in cell biology. Cell movement is
important for all living organisms. For example, amoeba cells move in the soil to
search for food; nerve cell extends a long process called axon during the formation of
neuronal network; wound such as a cut in a skin will be repaired by the migration of
cells of the skin to fill the wound. Cells move in a controlled manner during the
development of multicellular organisms; the uncontrolled movements (e.g., loss of
contact inhibition) of cancer cells are the essence of metastasis. In vitro, cells adhere
to surfaces of culture dish or microscope slide and crawl on these substrates. How
cells crawl can be easily observed from the top by light microscopy (Fig. 7.17a). The
observation of the side view of amoeba cells crawling on the surface of glass rod has
demonstrated that in the crawling movement the front edge is first extended and then,
it adheres to the surface. This is followed by the retraction of the rear of the cell as a
result of the contraction of cell body while the front part remains attached. The cell
moves forward by repeating these three steps (Fig. 7.17b; [77]).
The speed of the movement of the cell varies from < 1 μm/h to a millimeter per
hour, depending on the cell type [78, 79]. Fibroblast usually moves slowly and the
event of extension and that of the rear retraction occur with different timings,
whereas in fast moving fish keratocyte the three activities is probably coordinated
so that the cell moves without appreciable change in its shape [79]. However, as
shown in Fig. 7.17a, even fibroblast moves over a short period of time with little
change in the shape. In this case, the coordinated movements of the protrusion and
the retraction can occur.
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7 Moving Life
binding of microtubule associated proteins (MAPs). For example, the presence of
MAP-2 stabilizes the polymer by decreasing the frequency of disassembly [75].
Dark-field microscopy combined with epi-fluorescence microscopy of microtubules
that was decorated with fluorescently labeled MAP-2 demonstrated that at a low
concentration of MAP-2 formed discrete clusters along the microtubule; based on
this characteristic distribution of bound MAP-2, it has been suggested the rescue
probability significantly increases at the positions of MAP-2 clusters [76]. Thus, the
clusters will decrease the shortening length. The GTP-bound tubulin specifically
labeled with the antibody demonstrated that GTP-tubulin exists not only at the plus
end but also in the middle of microtubule. It has been hypothesized that the
GTP-bound tubulins in the middle of microtubule participate in the rescue
event [70].
7.7 Cell Motility
The movement of the cell has been a central theme in cell biology. Cell movement is
important for all living organisms. For example, amoeba cells move in the soil to
search for food; nerve cell extends a long process called axon during the formation of
neuronal network; wound such as a cut in a skin will be repaired by the migration of
cells of the skin to fill the wound. Cells move in a controlled manner during the
development of multicellular organisms; the uncontrolled movements (e.g., loss of
contact inhibition) of cancer cells are the essence of metastasis. In vitro, cells adhere
to surfaces of culture dish or microscope slide and crawl on these substrates. How
cells crawl can be easily observed from the top by light microscopy (Fig. 7.17a). The
observation of the side view of amoeba cells crawling on the surface of glass rod has
demonstrated that in the crawling movement the front edge is first extended and then,
it adheres to the surface. This is followed by the retraction of the rear of the cell as a
result of the contraction of cell body while the front part remains attached. The cell
moves forward by repeating these three steps (Fig. 7.17b; [77]).
The speed of the movement of the cell varies from < 1 μm/h to a millimeter per
hour, depending on the cell type [78, 79]. Fibroblast usually moves slowly and the
event of extension and that of the rear retraction occur with different timings,
whereas in fast moving fish keratocyte the three activities is probably coordinated
so that the cell moves without appreciable change in its shape [79]. However, as
shown in Fig. 7.17a, even fibroblast moves over a short period of time with little
change in the shape. In this case, the coordinated movements of the protrusion and
the retraction can occur.
120
7 Moving Life