Control Systems in the Living World - Building the Most Complex Structure on Earth - page 26

MT iMTOCs

Nucleus

Figure 1.14 A model for nuclear positioning and interphase MT architecture in S. pombe .
Microtubules (MTs) are organized from medial organizing centers (MTOCs) in multiple
bundles with an antiparallel configuration and dynamic plus ends facing the cell tips and
minus ends in medial-bundled regions. One MT bundle is attached to the nuclear envelope at
the spindle pole body (SPB), and other MT bundles may be attached at additional sites. When
an MT end contacts the cell tip, MT polymerization produces a transient pushing force that
pushes the MT lattice and attached nucleus away from that cell tip. A balance of these pushing
forces from these MTs may position the nucleus in the middle of the cell ( Tran et al., 2001 ).

Cytoskeleton Regulates the Directed Release of Golgi Units from Nurse
Cells via Ring Canals
In multicellulars, the transport of Golgi units from nurse cells to the oocyte is an
active transport regulated by the cytoskeleton rather than a random diffusion process;
Golgi units “move in a direct path toward the ring canals” that connect nurse cells
with the oocyte and a conical actin basket at the nurse cell side of ring canals regu-
lates their directed transport to the oocyte ( Nicolas et al., 2009 ).

Cytoskeleton Determines the Position of the Nucleus in the Cell
The depolymerization of microtubules in yeast displaces the cell nucleus from its
central position in the cell. Because the nucleus shifts its central position the sep-
tum is misplaced and cell division is unequal; that is, the resulting cells are different
( Tran et al., 2000, 2001 ) ( Figure 1.14 ).

Cytoskeleton Determines the Position of Organelles Within the Cell
Intracellular transport of molecules and organelles is responsible for their delivery to
destination sites. Since the transport takes place primarily along microtubules, it is
important that the free (plus) end of microtubules finds the correct destination site.
Microtubules are dynamic polymers that continually growing or shortening in length
to probe and explore many regions of the cell at random. This dynamic structure is
conserved throughout the living world. In unicellulars, this allows cells to adapt their
shape and form appendages (pseudopods, cilia, and flagella) and, in multicellulars,
to determine the shape of tens to hundreds of different types of cells in the process
of cell differentiation. It is interesting to observe that in unicellulars with permanent
cell shapes depolymerizing drugs have little effect on microtubules ( Heidemann
et al., 1985 ). These cells can stabilize the polymer structure of their microtu-
bules to a high degree when necessary. The process of differential stabilization of